Thesis Submitted for the Degree of Ph. D. in the University of London

THE HYDROLOGICAL EFFECTS OF URILANI SA'f ION IN ME CANON'S

BROM: CATCI1LE T, IIAIZLO14IIEW TOI;N, ESSEX.

GEORGEEDWARD HOLLIS

Thesis submitted for the degree of Ph. D.

in the University of London.

1974 2 ABSTRACT

The effect of the construction of Harlow, Essex on the hydrology

of the Canon's Brook is investigated using pre-recorded r infall and

rundff records. Similar hydrological data covering a per od of urban-

isation was found for 32 other English catchments. The 8.25 sq. mile clay

basin of the Canon's Brook had an average rainfall of 23.9 inches per year

during the study period 1950-68; a three year rural period was followed by

urban expansion resulting in 16.6% of the catchment having impervious

surfaces by 1968. Water yield was increased by urbanisation; a digital IýAAJaI simulation model of the rural catchment revealed increases in/water yield

of between 0.3 and 4.9 inches with about 16% of the basin paved. The increase in yield was smallest in wet years and greatest in dry years, confirming the results of a synthesis of data from published papers. Low flows increased with modal flow rising from 2 to 4 or 5 cusecs during the study period. The mean maximum monthly floods increased 220% because of the urbanisation and the frequency of summer floods, particularly those in the range 40-100 cusecs, increased markedly whilst the frequency of winter floods did not alter. The mean unit hydrograph for the 16% paved basin had a peak 4.6 times greater than its rural counterpart and the time of rise and width of the unit hydrograph at 50% of peak flow were 44% and 20% of the rural values respectively. However, large floods of over 150 cusecs, with a return period of perhaps 20 years, were largely unaffected by urbanisation, thus confirming the results of a synthesis of published data which showed that the effect of urbanisation on floods is inversely related to the interval recurrence of the floods. The apparent enlargement of the channel because of the changed flood conditions was not statistically significant. Sediment accumulation in a regulating reservoir gave a rate of erosion of 0.088 inches for per century a period of construction activity which when compared figures with published supports the view that construction activity increases erosion and sediment yields. '3

4 To the memory of

GEORGEPERKINS MARSH

author of

The Earth as Modified by Human Action'

(1863)

UNIVERSITY COLLEGE LONDON LIBRARY 4

CONTENTS

List of figures

List of tables N Preface

1. INTRODUCTION 15

2. THE LITERATURE ON. URBANISATION AND HYDROLOGY, 30

Precipitation Snowfall and Snowmelt Evaporation and Evapotranspiration Runoff and Catchment Water Yield Flood Frequency and Magnitude Conclusion

3. DATA SOURCES, DATA COLLECTION AND THE CONCEPTUAL FRAMEWORK 87

Hydrological Research and the Graduate Student The Choice of a Study of the Canon's Brook Catchment The Instrumentation of Canon's Brook and other Data Sources The Estimation of the Degree of Urbanisation of the Catchment The Methodological and Conceptual Basis

4. THE WATERYIELD AND FLOWREGIMEN OF THE CANON'S BROOK - 151

Double Mass Analysis Trend Analysis Multiple Regression Analysis Digital Simulation Model The Flow Regimen Conclusion

S. THE EFFECT OF URBANISATIONON FLOODSIN THE CANON'S BROOK 248

An Empirical Description of Flood Frequency and Magnitude The Flood Hydro raph Conclusion

6. THE SEDIMENT YIELD A?JD Ciiý1NNEL MORPHOLOGYOF THE CANON'S BROOK 289

Review The Channel Morphology of Canon's Brook Rates of Erosion in Canon's Brook: The Reservoir Study Conclusion

7. CONCLUSIONS AND IMPLICATIONS 345

i 5

Figures I

1.1 Urban expansion in the eastern Thames Conservancy area in the period 1929-1972.

2.1 The ratio of urban to rural water yield compared to annual rainfall in Morrison Creek, California, for 1950-1960. (after James, 1965)

2.2 (a) Precipitation and runoff in the urbanising Sharon Creek and a rural Los Trancos Creek Tributary, Palo Alto, California for 1959-65. (after Crippen and Waananen, 1969)

(b) Flow regimes for Sharon Creek and a Los Trancos Creek Tributary, Palo Alto, California, showing the Sharon Creek regime before, and after urbanisation. (after Crippen and Waananen, 1969)

2.3 Changes in water yield following urbanisation.

2.4 Changes in water yield following urbanisation as they are affected by rainfall.

2.5 Graph showing variation of flood-frequency ratio with percentage impervious cover. (after Martens, 1968)

2.6 Gauged urban and simulated rural flow for Sharon Creek at Menlo Park for selected periods. (after Crawford and Linsley, 1966)

2.7 The relationship between the ratio of urban to rural flood flows and simulated rural flows for Sharon Creek at Menlo Park. (after Crawford and Linsley, 1966)

2.8 (a) Effect or urbanisation on mean annual flood for a1 square-mile drainage area. (after Leopold, 1968)

(b) Flood frequency curves for a1 square-mile drainage basin in various states of urbanisation. (after Leopold, 1968)

(c) Increase in number of flows per year equal to or exceeding original channel capacity (1 square mile drainage area), as a ratio to number of overbank flows before urbanisation, for different degrees of urbanisation. (after Leopold, 1968)

2.9 A general relationship between the increase in flood flows and the percentage of the catchment paved for various recurrence intervals. 2.10 The increase in flood flows of various return periods to be expected from the paving of 20% of a basin. 3.1 Map of the Canon's Brook catchment and environs. ýý. 3.2 Geological map and cross section of the Canon's Brook. 6

3.3 The major surface water sewers of the Canon's Brook catchment, the areas drained and the date of completion of the schemes.

3.4 The gauging station on the Canon's Brook.

3.5 Rating curves for the gauging station and the low flow station.

3.6 Diagrams comparing the accuracy of the gauging and low flow stations.

3.7 The duration of records for the rain gauges.

3.8 Double mass plot of annual rainfall at the Eastwick Lodge/ Terlings/Sports Stadium site and Rothamsted.

3.9 The duration of sunshine records for the stations in the Harlow area.

3.10 The intended stages of development of Barlow. (after-Gibberd, 1947)

3.11 The sampling design for the land use analysis.

3.12 The urban areas of the Canon's Brook according to the O. S. one inch maps of 1940 and 1964.

3.13 The land use of the Canon's Brook catchment, 1950-1968.

4.1 Double mass analyses, 1950-1968.

(a) Canon's Brook runoff & Canon's Brook rainfall.

(b) River Ash runoff & River Rib runoff.

(d) Canon's Brook suamgr (June-Sept) runoff & River Ash summer runoff.

(e) Canon's Brook winter (Oct-Mar) runoff & River Ash winter runoff.

(f) Canon's Brook spring (Apr-June) runoff & River Ash spring runoff. 4.2 Trend in the summer (Apr-Sept) runoff from the Canon's gook analysed by moving averages. 4.3 Runoff from the Canon's Brook predicted by the multiple fý4.8. regression equation, 4.4 Runoff from the Canon's Brook predicted by the multiple regression equation, 4.5.

4.5 Flow diagram of the computer simulation model. 4.6 The relationship between potential and actual soil moisture deficits. 4.7 Simulation model error analysis.

(a) Variation of the variance of the calibrat ion period with degrees changing of error in 'the rainfall input. (b) Variation in the variance of thecalibra ion period with changing degrees of error in the evapotra nspiration input. 7

4.8 The impact of a correction factor for the catch of standard rain gauges on the variance of the model during the calibration period.

4.9 Run 1 of the simulation model.

4.10 The final run of the RURAL model.

4.11 The functioning of the simulation model during the study period expressed as runoff and soil moisture conditions for each land use and groundwater storage.

4.12 Soil moisture conditions for Cardington for 1964-66 calculated by the Ministry of"Agriculture Fisheries and Food. (after Ministry of Agriculture, Fisheries and Food, 1967)

4.13 Double mass analysis of the annual runoff from the River Ash and the RURALmodel.

4.14 its The increase in annual flow as a result of urbanisation and relationship with rainfall.

4.15 Graphical plot of the results of the URBANmodel simulation and gauged flow of the Canon's Brook 1950-1968.

4.16 Frequency distributions of the residuals for the calibration and prediction periods of the URBANmodel.

4.17 A graphical plot of runoff from a completely grass covered Canon's Brook against the runoff from the same catchment in various stages of urbanisation.

4.18 Accumulated total number of days with Canon's Brook mean daily f lows of :-

(a) 2 cusecs.

(b) 3 cusecs.

(d) 5 cusecs.

(e) 10 cusecs.

(f) 15 cusecs and over. 4.19 The changes in the modal, median and quartile flows of the Brook, 1950-68.

4.20 The changes in the median and quartile flows of the Brook, 1950-68, standardised for annual rainfall variations. 4.21 The estimated and projected median flow of the Canon's Brook and their relationship to rainfall and the percentage of the catchment paved. 8

...

4.22 Flow duration curves for the Canon's'Brook during a year of average rainfall with three degrees of urban development estimated from a regression analysis of the quartile flows in Table 4.16.

4.23 Simulated rural flow duration curves and gauged flow duration curves for the calibration period and alternate water years from 1953-54 to 1967-68.

4.24 Accumulated total difference between the gauged flow durations and the simulated rural flow durations plotted against the percentage of the catchment paved.

5.1 The effect of urbanisation on the flood hydrograph. (after Leopold, 1968)

5.2 The location of places and instruments used in the flood study.

5.3 The strategy for the time series analysis of the maximum monthly flood data for 1950-68 for the Canon's Brook.

5.4 Methods of time series analysis. Source: Imperial College London, Department of Civil Engineering Post-Experience Course.

5.5 The generation of time series. Source: Imperial College London, Department of Civil Engineering Post-Experience Course. N 5.6 Maximum monthly floods and moving average for Canon's Brook, 1950-68.

5.7 Frequency of flood peaks for Canon's Brook, 1950-68.

5.8 The hydrograph parameters measured.

5.9 Changes in hydrograph parameters with urbanisation.

5.10 Unit hydrographs for three stages of development.

6.1 A cycle of erosion and sedimentation in a Peidmont river undergoing successive modification by man. (after Wolman, 1967) 6.2 The major components and relationships in the catchment system and the effects of urbanisation. 6.3 Climatic variation of yield of sediment as determined from sediment stations and reservoir surveys. (after Langbein and Schumm, 1958)

6.4 Facsimile of a drawing from the Harlow Development Corporation's survey of the channel of the Canon's Brook in 1956.

6.5 Location map.

6.6 The cross-sectional morphology of the channel of the Canon's Brook in 1956 and 1970.

4 9

6.7 The excavated channel of Canon's Brook at Fourth Avenue in 1956 and 1970.

6.8 Plate of St. Andrews church and the site of the Netteswell flood regulating reservoir.

6.9 Facsimile of the engineering drawing for the reservoir at Netteswell.

6.10 The rate of completion of dwellings in the catchment of the Netteswell reservoir.

6.11 Sediment accumulation in the Netteswell reservoir from 1954 to 1970. 4 10

'Tables r

1.1 The urban component in national populations.

1.2 Hydrological effects during a selected sequence of changes in land and water use associated with urbanisation. (after Savini and Kammerer, 1961)

2.1 Summary of urban area increases in precipitation and related conditions. (after Changnon, 1969)

2.2 Rainfall and River Flow in the River Lee catchment above Fielders Weir, 1865-1965. (after Medrington, 1966)

2.3 Summary of some effects of present and future urban development on the Waller Creek, Dallas, Texas at the 23rd Street and 38th Street gauging stations. (after Espey, Morgan and Masch, 1966)

2.4 Summary of the effect of urbanisation on floods of various magnitudes.

3.1 Catchments in England and Wales with hydrological data covering a period of urban expansion. N.

3.2 The land-use classification used in the air'photo surveys.

3.3 Results of the analysis of the survey of the 1965 air photograph mosaic, for the determination of the minimum acceptable sample size.

3.4 Preliminary results of the air photograph surveys.

3.5 Cross tabulation table of surveys 1965 and 1965G.

3.6 Results of the land use surveys.

3.7 The results of a pilot survey of imperviousness in Potter Street using a line sampling method. 3.8 Results of the survey of the paved areas of the neighbourhood units in the Canon's Brook catchment. 3.9 Total number of dwelling completions and associated acreage of paved surfaces calculated from the Architect's Plans. 3.10 Summary table of the indices of urbanisation.

3.11 Final land use analysis. 4.1 Analysis of variance table for the double mass analysis of the Canon's Brook runoff against rainfall and the runoff from the R. Ash. 4.2 The significant results of an analysis of the time trends in the variables listed in Tabl 4.3.

4.3 The fo variables tested time trends in the period 1950-68. ii

4.4 (a) The variables in the multiple regression study.

(b) The equations calculated in the regression analysis of water yield.

4.5 The land use characteristics and descriptive parameters of the RURAL and URBAN simulation models.

4.6 A tabular comparison of the Penman estimates of the potential evapotranspiration from the Canon's Brook and gauged figures for nearby places.

4.7 A qualitative comparison of errors involved in hydrological modelling with analogous errors in standard statistical analysis. (after Dawdy, 1969)

4.8 The frequency distribution of the residuals for the calibration period of the RURAL model.

4.9 The confidence limits associated with the predictions of the RURAL model.

4.10 The impact of urbanisation on monthly, seasonal and annual water yields expressed as the difference between the gauged runoff and that predicted by the RURALmodel.

4.11 Regression analysis of the annual impact of urbanisation on water yield and related variables.

4.12 The water yield of the catchment for May-September 1968 assessed by the URBANmodel, the charts for the flume and the punch tape recorders.

4.13 Runoff from the Canon's Brook under varying land use conditions.

4.14 Regression analysis of the runoff from the Canon's Brook under varying land use conditions.

4.15 The yearly values for the modal, median and quartile flows of the Canon's Brook.

4.16 (a) Regression analysis of the quartile and median flows of the Brook standardised for annual rainfall. (b) The predictive form of equations calculated in (a). 4.17 2 Chi analysis of the simulated flow frequency and actual flow frequency during the calibration period. 4.18 Summary table for the impact of urbanisation on the water yields of the Canon's Brook calculated by several techniques. 5.1 Descriptive statistics and frequency distribution for the 18 year 12

historic flood record of Canon's Brook and eighty nine synthetic blocks of data. (Raw data in natural number form).

5.2 Descriptive statistics and frequency distributions for the 18 year historic flood record of Canon's Brook and eighty nine synthetic blocks of data. (Raw data transformed logarithmically).

5.3 Matched pairs of floods for Canon's Brook in its rural and developed state.

5.4 Unit hydrographs and mean unit hydrographs for three stages of development.

5.5 Variables in the regression analysis.

5.6 Regression equations for hydrograph parameters and associated statistics.

5.7 The variation in the exponent of X50 the percentage of the basin paved, with various dependent variables and sub-samples of floods.

6.1 Published rates of erosion for British river basins.

6.2 The measured characteristics of each channel cross-section surveyed.

6.3 Descriptive statistics for channel morphology variables for 1956 and 1970.

6.4 Results of "t" test for matched pairs of channel sections.

/ 1; j

PREFACE

It is still too early to attempt scientific method in discussing (the reaction of man on nature), nor is our present store of necessary facts by any means complete enough to warrant me in promising any approach to fulness of statement respecting them. George Perkins Marsh

It is fitting that this thesis should have been completed on the fifth

anniversary of the floods in south-east England which occurred on 14th and

15th September 1968. These floods, which many claimed to have been caused

by runoff from urban paved surfaces, coupled with my frequent observation

of the Wealdstone Beck in north-west London provided a stimulus and a

context for my research. A dearth of published material and the ýý

substantiation of my tenet, that there is ample pre-recorded hydrological

data to satisfy any postgraduate student, showed that an original contri-

bution to knowledge was both necessary and feasible.

The contribution of this thesis, which is an in depth study of a single small catchment in Essex, is the quantitative description of the effects of urbanisation on water yield, flow regimen, flood frequency, flood magnitude and channel morphology and the derivation of relationships between these changes and indices of urbanisation. The discovery that very large floods and channel cross-sectional area seem to be unaffected by development urban is of particular significance for it is these parameters that most concern the planner and river engineer.

Special thanks due are to the Chief Engineers and staff of the Harlow Development Corporation and the Lee Conservancy Catchment Board for all the basic hydrological data. At the start of my work I was much 14

helped by Mr. Prestwich and the late Mr. McMahon of the Lee Conservancy and the Development Corporation respectively. The award of a University of London studentship enabled me to work full time on this study for almost three'years. Professor E. H. Brown, who supervised the work, kept me firmly on course whilst giving me a great deal of freedom. Postgraduate colleagues in Room 9 provided stimulation and light relief. Sincere thanks are due also to Bill Campbell for friendship, encouragement and teaching in statistics, FORTRAN and data processing. My wife's steadfastness and encouragement were invaluable as was the urgency generated by my father's incessant enquiries about my progress. Finally, thanks are due to Professor W. R. Mead and University College London who allowed me time to finish the work whilst in their employ. ý, N

i 15th September 1973

London i5

CHAPTER 1

'INTRODUCTION

Man ... has done much to revolutionize the solid surface of the globe, and to change the distribution and proportions, if not the essential character, of ... even the waters. George Perkins Marsh

Recent years have seen the development of a widespread appreciation of the complexity and delicate balance of "The Environment" and more particularly of man's relation to it. A large number of books have been written on the theme some with evangelical zeal (e. g. Ehrlich et al-19709

Dasmann 1972, Ward and Dubos 1972), in others are described computer simulation exercises which predict the date of doomsday (e. g. Meadows et

"Man's al. 1972) and collated scientific analyses attempt to describe

Impact on Environment" (Detwyler 1971). Americans have celebrated "Earth

Day" (Lowenthal 1970) and the United Nations has organised an international conference on the human environment (Dept. of the Environment 1972).

This research examines one very small aspect of the relationship between man and his environment; namely, the effect of urbanisation on the hydrology of river basins.

I Urbanisation represents one of man's most fundamental and widespread modifications of the natural environment. Urban areas attract an increasing proportion of the world's population and in some countries over three quarters of the people already live in towns and cities, Table 1.1" Erlich (1973) "Between writes 1950 and 1960, the populations of cities in 16

Table 1.1 Table removed due to third party copyright

Source: U. N. Demographic Yearbook for 1971.804pp.

The definition "urban" is of not the same for all countries. The national definitions are given on pages 154-158 of the yearbook. 1'1

the under developed countries increased by 55 percent. In Latin America,

flood impoverished into has (seen) ... the of peasants urban areas ... the development of characteristic shanty towns. The trend in Africa has been

is 8 similar, `... Accra, the capital of Ghana, growing at almost percent ; per year; urbanisation in the United States has been much less rapid than in the underdeveloped countries, but suburbanisation has been extremely r.. rapid since World War I, I" (Ehrlich et al 1973). In Britain, as in the

USA, here has been considerable post war_suburbanisation a! well as the planned development of new towns. The effect has been to reduce slightly the percentage of the population in England and Wales living in urban areas in 1971 (78.3%) compared to 1962 (79.6%). However, the 78% of the population of England and Wales who live in urban areas are concentrated on only 10.8% of the land area (Best and Coppock 1962). It is this concentration as much as sheer numbers which has lead to environmental change.

The significance of this "urban explosion" for hydrology does not appear to have concerned planners until the 1960s for, as a later chapter shows, few studies were undertaken until the middle 60s. More recently, a number of authors have pointed to the importance of urban hydrology.

(1967) Chow said "for wise management and intelligent use of water resources, there is a need to ascertain the magnitude and consequences of man's influence for develop- on the physical environment, example ... the ment of cities". The Council of the International Hydrological Decade (1972) has included "the study of the effects of man's activities on hydrological factors land " ... urbanisation and other use changes ... Long within their Term Programme in Hydrology. A number of fields of fall study within the compass of urban hydrology. The interdigitation of 1 18

human activities in cities with catchment hydrology involves the design

and construction of surface water sewer systems, the attenuation of floods

resulting from runoff from paved surfaces; the protection from periodic

floodng of urban areas situated on flood plains, the dispIsal of domestic

and industrial wastes from the city and lastly, the provision of a reliable

and wholesome supply of water for the urban dweller and the city's industries.

The design of surface water drains or combined sewers is properly

the field of the civil engineer, and the hydrologist's contribution is normally limited to studies of specific processes in'the urban context.

Methods of sewer design have advanced greatly from the early days of the rational method (Lloyd Davis 1905) using a very simple model: - Q- CIA where, Q is the peak discharge, I the intensity of rainfall, A the catchment area and Ca factor representing imperviousness. A major advance in British design practice was the publication of the Road Research

Laboratory Hydrograph Method (Watkins 1962) whilst in the United States, the Stanford Watershed Model has been used, (Hydrocomp 1969). Present design methods are unsatisfactory in some situations; Steele (1973) has suggested several areas where research is necessary and the Institute of

Hydrology (1973) has "an awareness of the need to improve the prediction of runoff into storm sewers by studying the process of runoff from a complex of impervious surfaces".

Related to sewer design but studies, more hydrological in approach, is the investigation the of effects of the development of urban land use on runoff, water floods. yield and Such development can involve considerable areas; Andrews (1962) has estimated that between 1939 and 1960, 19

15,000,: acres of the Thames Basin were paved, and Figure 1.1 illustrates

the m4ssive extent of urban expansion in the eastern end ofj the Thames

Conservancy area. Nixon 11972) has commented on the flooding problems

that occur "because of the changing pattern of runoff due to urbanisation" and

the Ministry Housing Local Government in 1969 . of and acknowledged the

importance of surface water runoff from urban developments for they stated

that "representations continue to be received that development carried out

with planning permission has resulted in the flooding of farmland and dwelling houses". Local authorities were informed that unless "they are

satisfied ... that the volume of discharge would not be significant they

should seek the advice of the river authority". The government sponsored Institute of Hydrology recognised the significance of urban development in 1968 when they said "Milton Keynes will provide an excellent example of

the drastic land use change..., there is a lack of the quantitative knowledge needed to design flood prevention schemes of maximum efficiency

at minimum cost". In their 1969 report are set out plans for a classic paired catchment experiment at Milton Keynes, one basin remaining rural whilst the other is urbanised. The gauging structures were completed in 1972 and data collection initiated, but the 1973 report states that "the been original concept ... has overtaken by an awareness of the need to improve the prediction of runoff into urban storm sewers".

The development of urban areas on flood plains is a further aspect of hydrology, urban the study of which involves not only hydrological expertise but planning and legal skills. Bowen (1972) found that the for reason the large increase in mean tidal range in the Thames Estuary was "largely man made and result(ed) primarily from the continual process of embanking and bank raising" to facilitate flood plain development. The PAGE/PAGES EXCLUDED UNDER INSTRUCTION FROM UNIVERSITY Figure 1.1 Urban expansion in the eastern Thames Conservancy area in the (Sosece; period 1929-1972. 0,5. '/,. 't h Ai ps) 21

great significance of flood plain storage of water has been further

emphasised by the Thames Conservancy (1960) who stated that "in the case

Thames flood 1947 if flood had been filled of the of ..., the whole plain

ifi have increased flood levels by 4 feet, ..., this would the peak some and would have been disastrous". The Ministry of Housing and Local

Government (1962) urged planning and river authorities to liaise over development in flood risk areas for this is subject to normal planning control and development "which is permitted without regard to land drainage

health, life ". considerations may endanger ... and property ...

A further aspect of urban hydrology concerns water quality rather than water quantity. The chemical and sedimentological characteristics of water draining from roofs and particularly roads is likely to be markedly different from the natural inflow to rivers. The effects of oil, rubber and road salt on the biology and chemistry of streams is of obvious importance (Fruh 1969), as is the modification of river temperature patterns by the changed inputs of water from urban surfaces (Pluhowski

1969). Of far greater significance are the discharges of sewage, treated effluent and trade wastes to u ban watercourses (Jeger Cotrmnittee, 1970).

Related to these "human" forms, of pollution is the degradation of streams by sediment from soil disturbed by building activity or bank caving associated with increased flood flows after development has been completed (Wolman, 1967)i

The final area of interest for the urban hydrologist is the provision domestic industrial of and water for the city dwellers. The number of is problems here enormous ranging from the construction and operation of multi-purpose reservoirs to regulating groundwater exploitation; from studies of the value of water metering to investigations of public perception of water quality; from the resolution of conflicts between 22

domestic and industrial interests to research into the long term effects of groundwater recharge. Savini and Kammerer (1961) have summarised these hydrological effects of urban water supply along with many of the other hydrological aspects of urban growth. Their table is given as Table 1.2 below.

The investigation. of all of these aspects of urban hydrology is beyond the scope of a single investigator, working within the constraints of a postgraduate research programme, and consequently, this thesis is based on an examination of the effects of urban development on the river hydrology and geomorphology of one small catchment in Essex. Included in the analysis are studies of catchment water yield, river regimen, flood

N frequency, hydroggph shape, flood magnitudes, and channel morphology.

Urbanisation involves three major changes in the hydrological characteristics of a catchment. First, much of the vegetation is removed and large areas of soil are bared and disturbed, at least whilst construction is in progress. Second, large areas of the basin are effectively rendered impervious by the construction of roofs, roads, car parks and playgrounds. Third, the drainage network is greatly extended and increased in density and the resulting "channels", i. e. the surface water sewers, are much smoother and more efficient than their natural equivalents.

From these three features of urbanisation, a series of intuitive hypotheses derived are which are examined in detail in the rest of the thesis. The inhibit paving of the soil should percolation to aquifers and so groundwater levels be ought to lowered. A result of the lowering of the watertable be in should, a decline the magnitude of low and dry weather flows. There is likely be to very little evaporation or evapotranspiration from the 23

Tab l. 2

Hydrologic effects during a selectgd sequence of changes in land and water use associated with urbanisation. (after Savini and Kammerer, 1961) Image removed due to third party copyright 24 0

impermeable surfaces except during and just after rainfall and so total

amounts of water lost to the atmosphere from the basin should be reduced.

Consequently, the total outflow of water from the basin as streamflow should

be increased and have a "flashier" regime. The paved surfaces also have

a relatively smooth surface so presenting little resistance to surface

water flow and the interception and depression storage of urban areas

may well be less than that of rural lands. Thus rainwater is prevented

from soaking into the soil, it is less likely to be stored on the surface

and is able, therefore, to move rapidly towards the drainage channels

or sewer inlets. The increased density and efficiency of the drainage

net reduces the distance that the water has to flow overland and transmits

it basin The very rapidly to the outlet or main river. combined effects . of these changes are the production of runoff from even the smallest rain-

storm and the exacerbation of food conditions in the river; i. e. flood

volumes and peaks will tend to increase and lag times and times of rise

will tend to be decreased. However, these exacerbated flood conditions may

only apply to relatively small floods; for during a severe rainstorm when

the soil is saturated, the natural channel network is greatly extended and

the river is flowing at a high level, the hydrological differences between

a rural and urban situation may be slight. The clearing of vegetation and

soil disturbance should lead to increased rates of Erosion from slopes and

consequent aggradation in channels. Upon completion of the building

activity, erosion should be minimal on slopes and the increased frequency

and magntiude, of floods should initiate a period of channel scouring and bank erosion.

In Chapter 2 currently available literature on urbanisation and river is regimes reviewed. The papers, largely drawn from American research, 25

are examined individually and an attempt is made to synthesize the results

from each of the papers into general relationships between, for instance,

the increase in overall catchment water yield and proportion of the

catchment paved. In Chapter 3 sources of data that might have been used

in the study are evaluated and reasons why the already recorded data for

the Canon's Brook at Harlow New Town was selected, are given. Chapter 3

also includes an evaluqtion of the quality of the Harlow hydrological data

and a quantitative assessment of the development of the urban area within

the Canon's Brook catchment; this latter data being derived from air

photographs, maps and the Development Corporation's records. Chapter 4

examines the monthly water yield and flow regimen of the Canon's Brook for

the study period, October 1950 to September 1968. The results of initial

analyses by double mass plots and regression are confirmed and elaborated by a daily water balance simulation using a digital computer model.

Differences between the simulated rural water yield and flow regimen and the actual recorded figures for the urbanising catcluient demonstrate the effects of urbanisation. Chapter 5 looks at the frequency and magnitude of floods in the Brook. Descriptive statistics and unit hydrograph studies show that floods are exacerbated by urbanisation but regression analysis lends credence to the idea that large floods are unaffected by land use. The penultimate chapter discusses erosion and channel morphology. The considerable literature on urban growth and sediment movement is reviewed along with the sources of data about the morphology of the Canon's Brook and the reservoir bed levels in the early 1950s. The results of field resurveys of channel sections and the reservoir Floor are compared data with'the secondary for the pre-urban phase and an assessment is made of channel changes and reservoir sedimentation rates. Chapter 7 forms the conclusion in is which emphasis placed not only on the relationships between the Canon's Brook results and the synthesis of results from other but studies also on future research strategies. .6 26

This thesis and associated papers presents somi of the earliest results for studies of surface water runoff from urbanising catchments in S. E. England. 1968 saw the initiation, as descrfbed earlier, of the

Institute of Hydrology's Milton Keynes Project. Outside the South East of England, the Gloucestershire Joint Surface Water Study, a cooperative venture between the Gloucester R. D. C., Gloucester County Council,

Cheltenham R. D. C., Severn River Authority and Bristol University, began an investigation of the left bank tributaries of the Lower Severn in the

Gloucester and Cheltenham area in 1969. Their hydrometric network was completed in the early 1970s (Waller and"Shaw 1970) and initial results JThorpe have also been obtained from a hardware simulation model 1973).

This, long term project aims at the eventual construction of a mathematical model of the streams and their floodplains. The only other major research project of this-kind is that based on a small catchment on the outskirts of Exeter which was instrumented just before housing construction began.

So far the publications have concentrated on building activity and sediment yield (Walling and Gregory 1970) but further studies of the hydrology of this catchment are underway. The Departments of Civil Engineering at City

University and Birmingham University both have graduate students working on urban hydrology. The former is working on Crawter's Brook at Crawley using an analogue computer and the latter is studying the R. Teme by digital simulation methods. This thesis whilst following a substantial number of American studies, casts them in a new light. More importantly, it contains quantitative results for a small catchment in the heavily urbanised South East of England, it demonstrates the value of digital simulation as a research technique and shows pre-recorded data can be employed by research workers to gain useful results. i 27

Bibliography_

Andrews, F. M. 1962 Some aspects of the hydrology of the Thames Basin 'Proc. -InSt. 'Ciy. 'Ertgrs:, 21, pp. 55-90. I 4f Best, R. H. and 1962 The Changing Land Use Britain. Faber, London. Coppock, J. T. 253pp.

Bowen, A. J. 1972 The tidal regime of the R. Thames's; long-term trends and their possible courses. -Phil. Trans. Roy. 'Soc. London; 'Series A, 272, pp. 187-200.

Chow, V. T. 1967 New trends in hydrology., Natureand Resources, III (2), pp. 4-9.

Co-ordinating Council 1972 Report of the Seventh Session. Nature and of the International Resources, VIII (2), pp. 13-19. Hydrological Decade

Dasmann, R. F. 1972 Planet in Peril? Penguin Books and UNESCO. 135pp.

Dept. of the 1972 The Human Environment: The British View. Environment H. M. S. O. 42pp.

Detwyler, T. R. 1971 McGraw Hill. 731pp. .- Man's Impact on Environment.

Ehrlich, P. R. and 1970 Population, Resources, Environment. W. H. Ehrlich, A. H. Freeman, San Francisco. 509pp.

Ehrlich, P. R., 1973 Human Ecology. W.H. Freeman, San Francisco. Ehrlich, A. H., and 304pp. Holdren, J. P.

Fruh, E. G. 1969 Urban effects on quality of streamflow. In: Effects of Watershed Changes on Streamflow, edited ~_ by: Moore, N. L. and Morgan, L. W. Texas University Press. p. 255-282.

Hydrocomp Inc. 1969 Simulation of continuous discharge and stage hydrographs in the north branch of the Chicago River. Mimeo. 56pp.

i Institute of 1968 Research 1968.48pp. Hydrology

Institute of 1969 Research 1969.36pp. Hydrology

Institute of 1973 Research 1972-73.66pp. Hydrology

Jeger Committee 1970 Taken for Granted. H. M. S. O. 65pp. (Working Party on Sewage Disposal) 28

Lloyd-Davis, D. E. 1905 The elimination of storm wa er from sewage systems. 'Min. Proc. 'Inst. C'v. En rs., 164(2), 1 pp. 41-67. Lowenthal, D. 1970 Earth Day. Area, 4, pp. 1-10.

Meadows, D. H. et al. 1972 The Limits to Growth: A Report for the Club of Rome's Project on the Predicament of Mankind. Earth Island, 204pp.

Ministry of Housing 1962 Liaison between planning authorities and river and Local boards. -Circular 52/62 4pp. Government. +

Ministry of Housing 1969 Surface Water runoff from development. and Local Circular 94/69 2pp. Government.

Nixon, M. 1972 Problems of Water Resource Management in the Trent River Basin. In: Advanced Techniques in -River Basin Management: the Trent'Research "Programme. Proceedings of Birmingham Symposium of Inst. of Water Engrs. pp. 1-12.

Pluhowski, E. J. 1969 Urbanisation and its effect on the temperature of streams on Long Island, New York. U S. Geological Survey Prof. Paper. 627-D.

Savini, J. and 1961 Urban Growth and the Water Regimen. U. S. Kaiamerer, J. C. Geological Survey Water Supply Paper, 91-A, 43pp.

Steel, P. H. 1973 Present position and areas in which further research is required. Paper 4 in CIRIA Research Colloquium on Rainfall, Runof and Surface Water Drainage of Urban Catchments. Bristol 1973.

Thames Conservancy 1960 Evidence to Inspector during the Public Inquiry into the development of Hurst Park Racecourse. Mimeo. Surrey County Council File No. 1088/ 40621/30.

Thorpe, G. R. 1973 Forecasting runoff from storms moving over partly urbanised catchments. Paper 8 in CIRIA Colloquium on Rainfall, Runo and surface Water 'Drainage of'Urban Catchments. Bristol 1973. Waller, R. S. and 1970 Drainage and Flooding in the Gloucester Region. Shaw, T. L. Civil Eng. and Public Works Review, April 1970, pp. 368-369.

Walling, D. E. and 1970 The measurement the building Gregory, K. J. of effects of construction on drainage basin dynamics. Journal 'of *Hydrology, 11, pp. 129-144. 29

Ward, B., and 1972 Only One Earth. Peng in Books. 304pp. Dubos, R.

Watkins, L. H. 1962 The Design of Urban Sewer Systems. Road Research Techrýical'Papýer'55. H. M. S. 96pp.=

Wolman, M. G. 1967 A cycle of erosion andtsedimentation in urban river channels. Ceog. Ann 49A, pp. 385-395.

f' t 30 1 ..

'' CHAPTER 2

THE LITERATURE ON' URBANISATION AND HYDROLOGY

Natural science has become so vastly extended, its recorded facts and its unanswered questions so' immensely multiplied, that every strictly scientific man must be a specialist and confine his researches of a whole life within a comparatively narrow circle. George Perkins Marsh

The published work on the hydrology of urban areas contains two remarkable paradoxes. First, there is an enormous volume of work, under-' -1 taken largely by civil engineers, on the drainage of paved surfaces and surface water sewer design (e. g. Watkins 1962, CIRIA 1973, Viessman 1966), but only a handful of papers discuss the effect of urbanisation on the hydrology and river flow of a once rural basin. Second, whilst the studies of urban drainage design date back to the early years of this century (e. g. Lloyd-Davis 1905, Horner and Flynt 1934), the investigation of the hydrological effects of urbanisation on river flow only seem to have begun in the early 1960s and to have gained momentum in the middle 60s.

Moreover, there is a great regional imbalance in these works. At present almost all of the documentary reports have emananted from the U. S. A., with

Japan contributing one; in the United Kingdom a series of research projects have been established (N. E. R. C. 1970) and Australian needs for research have been reviewed (Aitken 1972). This peculiar situation is probably the result of the long standing need to minimise the costs of civil engineering works and the much more recent appreciation of man's impact on the environment and particularly the supposed exacerbation of flooding problems by urban development. The national origins of studies probably results, to some 31

f ' extent. from the American experience of uncontrolled and rapid expansion

of urban areas often into unsuitable areas (Reagen et al. 1971). The lack

y of European work on the hydrological effects of urbanisation may be the

resulli of the limited availability of material in language other than

English. However, it seems more likely that there has been very little

work in this field in Vurope, in view of the absence of European work in

major UNESCO reviews (e. g. UNESCO Working Group on the Influence of Man on

the Hydrological Cycle 1972, The UNESCO/FAO Working Group on the International

Hydrological Decade 1973), and the welter of work on urban climates in

Europe which does become available to an English speaking audience, WHO (1970).

This chapter critically reviews published work, looking at the effects

of urbanisation on individual hydrological phenomena as follows:

(a) Precipitation

(b) Snowfall and snowmelt.

(d) Runoff and Catchment Water Yield.

(e) Flood Frequency and Magnitude.

A discussion of the published work on erosion and sedimentation is deferred 6, to chapter where building activity and urban growth are considered as further variables influencing the geomorphological system. This chapter aims to provide both a framework for the study of the Canon's Brook and a synthesis of results against which the effects of Harlow New Town may be judged. 32

Precipitation.

Within the field of urban climatology, precipitation has received

only limited attention. It seems probable that this is not due to a lack

of interest amongst climatologists but more to the fact that "the

influences of the city on precipitation are most complex and are not

easily unraveled" (Landsberg 1961). Nonetheless, four main factors are

acknowledged in the literature as possible contributors to the modification

of precipitation over cities. First, increased turbulence is known to

result from the relatively great frictional drag exerted by the aerodynamically

rough buildings present in urban areas. The consequent increased upward

motion of the air in some localities may produce cooling, condensation and

precipitation or it may simply trigger off convectional movements.

Second, the intense heating of urban surfaces particularly in the summer

months may foster localised heating of the air and convectional movements.

Third, the release of air pollution over cities carries with it large

numbers of condensation nucleii and freezing nucleii which by their

profusion over cities may either encourage precipitation forming processes

or increase the efficiency of such processes. Finally, it has been

suggested that the release of water vapour into the atmosphere over cities may increase the probability of rainfall, but the amount of water released

into the atmosphere by man is minimal compared to the massive transfers of moisture that are undertaken by natural processes, and so this last factor

seems to be the most speculative.

There is conflicting evidence on the view that air pollution generated by urban activities increases the number of condensation nuclei in the

atmosphere and thereby tends to increase both the possibility and total VV

amount of subsequent rainfall. Telford (1960), in a study of freezing

nuclei produced by industrial sources in Australiap'found that steelworks'

smoke was a prolific source of nuclei, increasing counts by a factor of

50 compared with those of nearby clean air. Similar results have been

reported by Landsberg (1938) and Georgii (1959). There is, however, some

evidence to suggest that an excess of condensation nuclei may reduce

precipitation by causing the formation of a great number of small water

droplets which so reduce the humidity of surrounding air that further

condensation and raindrop growth is inhibited; Aynsley (1969) reported

such a situation where reductions in precipitation of up to 25% proved to

be the result of smoke from sugar cane fires in Queensland. Circumstantial

evidence to support the view that air pollution influences rainfall was

provided in the 1920s by Ashworth (1929) who, in an analysis -of the rainfall

of Rochdale, found that the mean Sunday-rainfall was 0.37 inches less than

the mean for weekdays. This he ascribed to the closure of factories and

consequent reduction in pollution on Sundays. In an analysis of the mean annual rainfall for successive decadal periods from 1898 to 1927 he found

that the Rochdale rainfall increased 13% to 48.65, while the surrounding rural areas showed no increase. This he ascribed to the increase in industry and pollution in the town during this period. More recently, Dettwiller (1970) has used exactly the same method to show that mean weekday precipitation for the period 1960-1967 is 45% higher than weekend

in Paris in precipitation and that four other cities in northern France the weekday increase from varies 14 to 327. He concluded that air pollution from active factories is the cause of these enhanced precipitation amounts. Barrett (1964) working on precipitation trends in south east Lancashire from 1860 to 1960 showed that mean annual rainfall in the centre of the conurbation 157. 'higher during was the period 1925-59 than 1890-24, whilst 0 34

the surrounding areas showed little change. This increase came in the winter months, and Barrett was not able to prove that any one process was

responsible for the change, but did suggest that urban growth might be

important. Other British work in this field suggests that urbanisation has little, if any, influence upon long term precipitation amounts.

Veryard (1958) failed to find any significant differences in total precipitation that might reasonably be caused by large English conurb-

ations, whilst Chandler (1965) described the main controls upon average annual rainfall in London as the synoptic situation and topography. An

examination of urban and rural precipitation amounts in and around London

from 1881 proved to be inconclusive (Chandler 1965).

American and European work appears to confirm that urbanisation has

a marked effect upon rainfall. Landsberg (1956) in an analysis of the

rainfall of Tulsa, Arizona, which grew from a trading post in 1890 to a city of 183,000 in 1950, found that urban rainfall tended to increase over

the period until in the 1940sit was 6.87. greater than the average in the

surrounding rural area. Comparison of the down-town and airport data, the latter 6 miles out of the city, for a fourteen year period showed that in summer urban amounts were 4.7% greater than rural, and winter amounts 11.5% greater. Changnon (1961a) in an analysis of urban, rural, and lake rainfalls around Chicago, showed that urban annual totals are 7% greater than rural and 31% greater thanljlake totals; the urban/lake differential in being winter largely the result of urban effects, and in summer almost wholly the result of lake'effects, but this situation is particularly

complex, both relief and land-water relations contribute and there is a minimal number of rainfall stations available. ) study of rainfall in the 35

university-residential city of Champaign Urbana by Changnon (1961b) was free

from topographic, instrumentational, and pollution complications and as such

is most instructive. A 12% excess of annual precipitation in the urban

area over the surrounding stations was found for a thirteen year period.

The ex-urban airport station recorded 8% less annual precipitation than its

urban counterparts. The increases in precipitation were located towards the

eastern margin of the urban area suggesting that the rainstorms which move

predominantly from west to east are disturbed by their passage over the

urban area. Seasonally the main increases in precipitation were in winter,

spring and autumn showed some increases whilst that in summer was minimal.

On a daily basis, precipitation maximisation occured during the late- after-

noon and evening, suggesting t at urban heating effects were influential

in the increases in precipitat on. In a further study, Changnon (1968) has

investigated the precipitation of the La Porte area to the east of the

Chicago-Gary industrial complex. He found that mean annual precipitation, number of days with heavy rain and days with thunder were increased by between 30 and 40% at La Porte compared with regional averages. Days with hail showed a staggering 246% increase. His La Porte results, together with a general summary table from Changnon (1969) are shown in table 2.1.

Changnon presented points in support of these increases which "certainly outweigh those for a fictional increase". He ascribed the changes to the influence of the industrial complex to the test which tends to increase both condensation nuclei and surface temperatures. He presented some remarkable time series diagrams to. show the relation between steel output, smoke pollution and rainfall at La Porte.

There have been several other reports of urban nduced increases in storm type precipitation. Schumauss (1927), whilst' tressing the liZ 36

Table 2.1

Summary of urban area increases in precipitation and related conditions. (after Changnon, 1969)

Urban-rural dUR..rance (Incr..., ) eaprened a%a µc cunt Id #"#. I vable

I I Chicago La Porte St. Lout CA;. TYI. a Wadi.. Ir. C. New 1'rwlt

Frccilditation ! Annual 3 31 7 s Warmcrhalf-year 4 30 " 4 S ; 6 12 Colder half-) car 6 33 " a 11 9 All rain days Annual 6 0 " 7 " " " Warmer half-year 8 0 " 3 " . " Colder half-year 4 0 " 10 " " Moderate-heavy rain days " " Annual 10 34 " " s " " " Warmer half. ) car 13 34 9 Colder half-year 0 S " 0 " " Thunderstorm dais Annual 6 38 11 7 " " " Summer 7 63 20 17 " . " `" Results unavailable or data insuflicicnt to make a conmpariwn.

I 37

increase in light rainfall in Munich compared to its environs, stated that

the urban area was subject to corresponding increase in heavy showers and

thunderstorms. Similar results have been reported from Budapest by Berkes

(1947), and Kratzer (1937) showed that Nurenberg had 14% more days with

thunder than its surroundings. Parry (1956) described a very severe

inches rainstorm, 1.36 in 2 hours, which was restricted in areal extent-

to the urban area of Reading. The general situation was one which often gives rise to thundery outbreaks, but Parry suggests that the 3°F heat

island may have been responsible for initiating the "urban rainstorm".

Experience in Detroit (Brater 1968) suggests that urban effects have little influence upon annual total rainfall, but that 24 hour stunner rainstorms with a return period of once in 10 years may be increased in intensity byý N as much as 10% by the existance of a large metropolitan region. Studies of thunder rainfall in London (Atkinson 1968,1969) have shown that a maximum of thunder rainfall exists over London during only the summer months. Moreover, the urban effect only seems to be operative during specific synoptic conditions, warm frontal situations being the most conducive. A further in depth study of a single day's thunderstorms over

London (Atkinson, 1970) showed that "the storms were triggered by the high urban temperatures" and that other factors played a negligible role.

A succinct summary of the influence of towns on precipitation has been given by Landsberg (1961), who says that gross amounts are increased by 5 102, to while days with 0.2 inches or more are increased by 10%. In the light of more recent research it seems that one must add confidence limits of + 10% to figures each of these and also include some comments about the distribution seasonal of these changes for this will be an important factor begins when one to consider the functioning of the hydrological cycle as a The by whole. work Chandler, Veryard, and Brater suggests that urban development has little effect upon total precipitation amounts, whilst 38

Ansley's report indicated that increases in the number of atmospheric condensation nuclei by air pollution may, tend to reduce rainfall in certain cases. Landsberg's average figures of 5 to 10% increase in gross precipitation amounts covers the majority of published reports but the La

Porte investigation presents a remarkable 31% increase in annual precipitation. The work in S. E. Lancashire, Tulsa, Champaign-Urbana, and Chicago - Lake Michigan suggest that the urban influence on total precipitation is greatest in winter. The La Porte and London thunder rainfall works suggest maximum influence upon precipitation amounts in summer, while the studies of thunder, hail, and heavy rain almost all point towards a peak of urban influence during the warm seasons of the year. Parry (1956) lends weight to this argument when he quotes from

British Rainfall the dates of "rare" rainfalls which were localised over London for the period 1870 to 1950; three in June, two in July, and one in each of April and May.

There can be little doubt that urban areas can and do modify precipitation amounts and storm patterns. The reasons remain conjectural with evidence being available to support several of the physical explanations advanced. Nor is it clear at which season the urban effect is at its greatest, but the weight of British evidence would suggest that it is summer rainfalls which are enhanced most by the presence of an urban area. Finally, almost all of the studies reviewed here have been concerned with conurbations or very large cities and so it seems reasonable to suppose that modest towns situated in the British countryside will have a minor or negligible effect on local precipitation. 39

Snowfall and Snowmelt

There is an extremely scanty literature upon the influence of urban development on snowfall and snowmelt, this situation stems largely from difficulties of observation and comparison of stations with different exposures and altitudes as well as differing local environment. Kassner

(1917) reported that Berlin had only 72% of the occurances of snow that fell in surrounding rural areas, and that 14% of the cases of urban snowfall were in association with rain. Kienle (1952) has reported exactly the opposite effect of urbanisation. In Mannheim during January 1949, snow fell through still air from a thick stratus cloud onto the urban area alone, as a result of supercooling of a local pollution induced fog. Manley "it is (1958) in a study of snowfall in metropolitan England stated that in possible that the warmth of a built up area in winter plays some part diminishing the frequency with which falling snowflakes will be observed in inner London. (but) This be " Lamb (1964) stated effect ... must small. that south London has appreciably less snow than north since "the snow bearing winds are warmed up a degree or two during their passage across the (conurbation)". Chandler (1965) took an opposite view and said that

"the built is differentiate frequency up area unlikely ... to the of in snowfall ... the region" since snow usually falls on windy days when the heat island is poorly developed. He did argue, however, that "fallen snow

it will melt more quickly in central parks than in suburban gardens, where will often disappear several days before that covering the farmlands around London".

In an analysis of the long term trends of snowfall for seven Canadian (Potter cities 1961), it has been shown that Edmonton and Winnepeg have increased slightly their snowfall over the period 1860-1960 largely due 40

to improved observations; Victoria, B. C., Sidney, N S., and Quebec have no overall trend in their snowfall which seems to by controlled by prevailing meteorological conditions. Montreal and Toronto, though, display significant downward trends in their snowfa 1, Montreal at a rate of 14 inches per century and Toronto at a rate of between 6.3 and 22 inches per century depending upon the period of analysis taken. These two cities are in an area where there has been a general increase in annual temperature, and consequently Potter estimates that the decrease in snowfall at Toronto due to the urban effect is 2", in a mean annual snowfall of

61.2". All the evidence, therefore, suggests that Manley was correct in sayiýg the effect of urbanisation on snowfall must be smal1l.

Evaporation and Evapotranspiration.

The removar of vegetation and the paving of soil surfaces has a marked effect upon water loss to the atmosphere by evaporation and evapotranspiration. Evapotranspiration is a continuous process from a naturally vegetated soil controlled by the energy available (normally solar radiation), the ventilation of the leaves and soil surface by wind, the vapour pressure gradient from the leaf to the atmosphere and the availability of moisture in the root zone and on the surface as interception and depression storage. Evapotranspiration is greatest during suuner days, minimal at night and very small in the winter months. Penman (1950) showed that monthly open water evaporation at Mildenhall varies from -0.1 (slight inches condensation) in January to 4.3 inches in June. Pegg and (1972) in Ward a study of a boulder clay catchment in Eastern England found that evaporative losses in July 1966 varied between 2.5 inches and 4 inches depending upon the method of assessment used whilst comparable figures for 41 I-

December 1966 were 0.0 and 0.6 inches. The drying of the soil and the

consequent soil moisture deficit found in many soils during the late summer

certainly depletes the rate of evapotranspiration, but it is doubtful if

it ever falls to zero. In an urban paved situation, there is no long

term storage of water on or near the surface and so evaporation cannot occur

continuously. Consequently, water is only lost by evaporation during and

immediately after rain has fallen. The rate of evaporation from wet urban

surfaces is probably greater than from natural lands for the asphalt,

slates, tiles and aged dirty concrete of towns are very dark and will

absorb most incident solar radiation. However, rainfall is a fairly

infrequent event occurring on 175 days per year in S. E. England (Lamb 1964)

and about 11 days per month in the period May - August at Kew (Chandler

1965). Simple observation shows that paved surfaces are generally dry, I

therefore, evaporative losses from urbanised areas are probably lower than

from vegetated surfaces, and this difference is likely to be most marked

in the summer, when evapotranspiration is at its peak and of little

quantitative significance in winter, when evapotranspiration is minimal.

No studies of evaporation from urban surfaces were reported during the

WHO-WMOsymposium on Urban Climates (W. M. O. 1970) but inferences may be

drawn from other studies of humidity, cloudiness, and total river flow.

The latter parameter is considered in a later section. Peterson (1971)

"the states that main reason to expect differences in the humidity of urban

is and rural areas that the evaporation rate in a city is lower than in the

country... Even though little research on humidity has been done, the

consensus of urban climatologists is that the average relative humidity in is towns several percent lower than that of nearby rural areas whereas the average humidity is absolute only slightly lower in built up regions". Landsberg (1956), in a summary review of published work, found that annual 42

mean relative humidities are 6% lower in cities than rural areas, but that

in summer the reduction is 8% and in winter only 2%. Extensive analyses

of the climate of London (Chandler 1962 & 1965) have shown that relative

humidity in the city centre may be 20% below that in the surrounding rural

areas and the nature of the humidity profile is related to the intensity of

the urban heat island and the density of building. The mean annual vapour

pressure in London was' shown to be slightly lower (0.2 mb) than at a nearby

rural location. Further work on night-time absolute and relative humidities

in Leicester (Chandler 1967) confirmed that relative humidities in cities

are lowered by the effects of the urban heat island, but showed, more

significantly, that in many parts of the city absolute humidities were

generally greater in Leicester than in the surrounding country. Chandler

stated that "clearly reduced eddy diffusion of water vapour in air trapped

between buildings important ... was a more control upon the distribution

of water vapour than reduced evaporation in the city". Critics, however, have

pointed to the fact that the surveys were done during a rainy period and

that retention of rainwater in the pores of concrete and brickwork as well

in as depression storage could easily account for the observed increase in

vapour pressure.

The lowering of the rate of evaporation from cities may also be due in

part to the reduced amount of solar radiation reaching the surface in urban areas and the reduced speed of the wind in towns. These factors will influence not only the rate of evaporation from paved surfaces but also the from rate of evapotranspiration plants and exposed soil within the developed Landsberg (1956) found area. an increase in cloudiness of between 5 and 10% and a decrease in radiation of 15 to 20% for a range of cities, wind speed declined by 20 30% to and the tuber of calms increased by 5 to 20%. The 43

reduction in radiation, and to some extent the increase in cloudiness,

is the result of particulate matter floating in the air over cities. These particles are most effective as attenuators of radiation when the suns angle is low, for at these times the rays have to pass through a greater thickness of particle-ladened air than when the sun is more vertical. Consequently,

the greatest reductions in radiation are found in high-latitude cities and during the winter. Chandler (1965) reported that the central part of

London received 270 hours of bright sunlight less than surrounding' areas, whilst Monteith (1966) has demonstrated that the smoke control acts of the middle 1950s have increased radiation in the Kingsway, London by 1% during the period 1957 to 1963. Studies by De Boer (1966) in Rotterdam showed a reduction of radiation in the city centre of 3 to 6% when compared to suburban areas and 13 to 17% when compared to rural areas. Mateer (1961) found that in Toronto the average annual energy receipt was 2.8% greater on

Sundays than during the rest of the week. The urban wind field differs from its rural counterpart in several respects. First, the increased roughness of the city causes increased frictional drag and so tends to reduce wind speeds overall. Second, the urban heat island effects are most intense in the centre of cities so there are often local urban winds blowing normal horizontal to the thermal gradients. Finally, the roughness and heat island aspects of urban areas often combine to increase turbulence and gusting of Chandler's (1965) winds. investigation of London's wind field illustrated all of the expected effects. He found that when regional wind speeds are low, usually at night, then the recorded wind speed at Heathrow is smaller than Kingsway; that at however, with strong regional winds, the reverse is The true. overall reduction in wind speed was only 5% and in London the in number of calms the city centre was smaller than in outlying rural areas. 44

Runoff and Catchment'Watar Yield

The paving of a catchment and the drainage of he impervious surfaces

by surface water sewers has a profound effect on th' runoff process and

consequently upon the annual and monthly water yiel of a catchment.

Infiltration to groundwater is effectively halted by urban surfaces and

so watertables and base flows should fall regardless of the abstraction of

water from wells. Evaporative and transpiration losses of water to the I atmosphere will be reduced since there is no aeration zone of water storage, it and seems likely that interception and depression storage are smaller

in weil graded and drained areas than in comparable rural settings. The net

effec7! s of these changes are that total flow should be inc-eased by

urbanisation, but that low flows should be both lower and more frequent

than formerly. In general the regimes of the river should become more "flashy" with frequent and rapid changes from a low flow situation to moderate or high flow and back again.

The East Meadow Brook in Nassau County, Long Island, New York has twice attracted the attention of the U. S. G. S. because of its urbanisation and hydrological records. Sawyer (1963) indicated the rate of urbanisation by for means of population totals 1950 and 1955 which showed a 130% increase for East Meadow Brook and only a 30% increase for the neighbouring "rural"

Mill Neck Creek. By comparing the direct runoff and base-flow components for the two streams,. with precipitation at a nearby station, he was able to show that urbanisation had increased the proportion of direct runoff to baseflow in the East Meadow Brook. For the two periods 1938-51 and 1952-60, increased rainfall by 9.4% and runoff in Mill Neck Creek by around 7.0%. However, in the urbanising East Meadow Brook direct discharge rose by 123.1% and baseflow by 5.3%. rose nearly He argued that 2% of the baseflow had "been lost because in of the change land surface" and put the 45

volumetric figure at 63,000 galls/day. In a later analysis of the same pair of basins Seaburn (1969a) characterised the extension of the urban area by reference to the population of the major settlements in the basin and more importantly to the area of the catchment served by storm sewers.

Those paved surfaces and storm water sewers, which drained into basins, (Seaburn 1969b)ß from which water percolated directly into the shallow aquifer Plots were not considered to be a source of direct runoff to the river. 1962 of cumulative annual precipitation and annual direct runoff for 1937 to showed marginal increases in rainfall but dramatic increases in runoff after 1944,1952 and 1960, all of which are periods of intense building from activity. In tabular form, he showed that an increase in sewered area

570 to 3,600 acreas in the 31 sq. mile catchment increased direct runoff,, from 920 to 3,400 acre feet; a 27% increase.

In California, there have been three major studies of urbanisation and water yield. Harris and Rantz (1964) used double mass analysis on data for rainfall, runoff from an upstream rural area and from the whole of

Permanente Creek, including the lower urbanised portion. They found that the ratio of flows from the partly urbanised catchment to the upper rural part of the catchment was 1.18 in 1945 and 1.70 in 1958. During this same period the extent of impervious surface in the lower part of the basin had increased from 4 to 19 per cent. James (1965) used the Stanford Watershed

Model, a digital simulation model, and a long record of rainfall, to generate a continuous hydrograph for 1905-1963 for the Morrison Creek,

Sacramento where there was onl a short streamflow record. For the years

1950-1960 he simulated both a4 holly rural and a wholly urbanised situation.

He found that the urban water yield was 2.29 its rural value and the urban base flow was only 0.7 its rural value. The monthly ratios of urban to 46

rural runoff varied, being lowest (0.71) in the baseflow dominated suamer months and highest (4.11) in January when surface runoff was taking place.

The annual ratio of urban to rural runoff varied inversely with rainfall

amount (Fig. 2.1) since a saturated soil reacts very much like an impervious

surface. Crippen and Waananen (1969)-have examined seven year records

from a Los Trancos Creek tributary which was rural and Sharon Creek which began as a rural catchment, was urbanised during the fourth year and then

300 remained unchanged until the end of the record. These approximately

acre catchments, are near Palo Alto south of San Francisco and are in the

lee of the coast ranges. "During the seven water years 1959-65, Sharon

Creek flowed for 20 days, 21 days, 1 day, 288 days, 355 days, 366 days and into 365 days", which suggests that urbanisation changed an ephemeral stream

a perennial one. However, Crippen and Waananen show that 1963 and 1965 were particularly wet years and after urbanisation there was a considerable

importation of water for irrigation. Nevertheless, Fig. 2.2(a) shows that

"for a given amount of precipitation, more water now flows from Sharon

Creek than would have been discharged under predevelopment conditions .... in is the change streamfiow regimen .... strongly associated with the

development within the basin". Fig. 2.2(b) shows the flow duration curves

for the two basins for 1959-61 when they were both undeveloped and for

1963-65 when Sharon Creek had een developed. It is clear from the diagram

that when imported water is i cluded, the Sharon Creek's curve departs even

further from that of the rural Los Trancos Creek tributary. Crippen and

Waananen conclude that "the days flow (for greater number of of at .... example 0.04 cusecs. ) in Sharon Creek is principally caused by the more

rapid response of the streamflow, after development in the basin, to

precipitation during light storms. " A similar considerable increase in the flow Assunpink of the Creek, New Jersey has been reported by Miller (1966) 47

I Jf ý\ r

L "- 3"5- 0 4-

- . fl

1.5 ro ro 1.0 c c c c Q Q 0.5 )ý 0 0 .ý ro 0 5 10 15 20 25 30 cr_ Annual Rainfall (inches)

Figure 2.1 The ratio of urban to rural water yield compared to annual --- rainfall in Morrison Creek, California, for 1950-1960. (after James, 1965)

j ) v 48

25 PRECIPITATION so Sharon Creek r! Los Trancos Creek s-'' 8 tributary 20 40 E ; 't: ý; '' RUNOFF ä:.; lýiý"::Y ® ; ß: ý'<ý" Sharon Creek ýýý :. ". "; i". ',s', },; '}. il'r.;ý, ® Los Trancos Creek tributary 30" w is -30 0 ý;0

'' ,,; "'fl. W 1, . n ý: 20 ".. ýi ý7 '"'ý: ý. 'r v. ac '' \' v. 10 . ".: ýwCß. ". 1.. ý'7 'i"'riC, 6gth a natural i: ý. "ýÖ p `1 rýý ri'_' 'ý: w'"'" S/, f, ;{ r' . r. i A`tý"ý. ". ''1 `L3 1, ýr "; º"i "". :; , ö 10 ý 0-4 10 t. Wp '. "r: fir,::. r"i. '. ý'i j".,..,.. t" r r .,, r"+ :: t7 ,. r"Jýýý .r 5

1959 1960 1961 1962 1963 1964 1965 YEARC ENDING SETTEMBER30

Figure 2.2 (a) Precipitation Ind runoff in the urbanising Sharon Creek and a rural Log Trancos Creek Tributary, Palo Alto, California for11959-65. (after Crippen and Waananen, 1969)

II -- - LOS TRANCOSCREEK TRIBUTARY 04 U SHARON W Mý CREEK

W a

ýs' u 119G ! + 63 iý 0.6 z 0.4 1959- 61ý rtý water i,! I` included 02

0.1 I! Iý 0.06 I 0.04 NC A+A A w. ------v. ýý ý"ýv v. cv u. bu 1.9 2.0 4.0 6.0 1 IS 20 30 PERCENTOP DAYSTHAT INDICATEDDISCHARGE W EXCEEDED

'Figure 2.2 (b) Flow regimes for Sharon Creek and a ýos Trancos Creek Tributary, Palo Alto, California, showing the Sharon Creek regime before and after urbanisation. (after Crippen and Waananen, 1969) 49

but industrial effulent appeared to be the cause and a nearby stream had comparable increases as a result of return flow from irrigation.

Waananen (1969) has reviewed other American work. Ramey showed that during a 30 year period, the discharge of the Chicago area had increased

21 times. Stall and Smith's comparison of the 38% impervious Boneyard

Creek with the agricul; ural area of the Kaskaskia River near Urbana,

Illinois is reputed to show that during "wet years the annual discharge

(in inches/unit for basins, .... area) averaged nearly the same the two but in dry years the urban Boneyard Creek watershed had a better sustained flow approximately twice that in the rural basin. " Despite the large number of urban runoff research projects presently underway in Britain and outlined in chapter 1, there is a paucity of published results.

Medrington (1966) presented the annual flow figures for Fieldes Weir on the River Lee during an informal discussion on the hydrology of a highly developed river basin (Table 2.2). He stated that "the table indicates a general picture of decreasing flow from a given rainfall and this may be partly due to the effects of development'of various sorts within the catchment area". He does not elaborate on this statement at all, and so it must be presumed that the observed reduction inflow is the result of lowered water tables in the chalk, irrigation abstraction or bypassing of the weir by foul water sewe s which contain water derived from the river or ground water. Kent River Authority (1967), in a study of into percolation the chalk of north Kent used "the Penman method" to calculate potential evapotranspiration and to estimate actual evapotrans- from piration both long and short rooted vegetation. About 5% of their study area was paved and they sought a method which would acco=odate this into area their studies. They stated "as there is little published 50

Table. i2.2 Rainfall and riverflow in the River Lee above Fielders Weir, 1865-1965 (after Medrington, 1966). lI

" Flow 20 Rainfall year period (inches) (cusecs)

1865 - 85 26.86 206.1

1885 - 05 23.73 144.4

1905 - 25 26.25 182.7

1925 - 45 25.19 144.1

1945 - 65 25.17 122.4

Total 127.20 599.7

Average 25.44 159.9

(1PýQý1Mý. I-_ ýMº i1 `Lý, Me.. Of ºýs 04 dd :.. l hMý NIýt^ T b Cý % weAýý iiqýt M Lr) 1J LI tks. ...

6 51

information on evaporation and percolation in paved areas the assumption

that all winter rainfall runs off and percolates and all summer rainfall

is evaporated was taken as being the best available approximation". This assumption is at odds with both intuitive reasoning and published American work, 'but it can be said to provide a relatively conservative estimate of percolation and therefore potential resources.

Overall then, urbanisation does increase water yields and Figure 2.3 presents a compendium of statistics from the papers reviewed. The relationship between paved area and increased water yield is clearly very weak because of the host of other variables which are not incorporated into

the analysis. When an intuitively satisfying line is fitted by eye and the residuals taken and plotted against annual rainfall, Fig. 2.4; it is clear that the increase in water yield following urbanisation declines as rainfall amounts increase. Consequently, urbanisation in a moist climate may produce minimal changes in yield, whilst the same type of development in a drier clime would increase the total streamflow very significantly.

There has been very little work on flow regimens after urbanisation and so it is difficult to gainsay the intuitive reasoning of Leopold (1968) that "imperviousness results in decreased ground water recharge and decreased low flows".

Flood Frequency and Magnitude

The social and economic losses resulting from the inundation of urban areas and riverine agricultural lands, together with the "obvious" exacerbation of the situation as a result of city development has directed in most research urban hydrolog to the investigation of floods. In spite the of present prediliction fo such studies, they only began in the early 52

t 3.5 106 °7'5 0 ö0 -3-0 4' 22 o s 22.5 7L o

jC Q"0 o (1964) Q 00 Harris 8. Rant7 1.5 + Seaburn (1969) + o James (1965) a Crippen&Waananen (1969) 10 + e Stall & Smith (cited byWaananen1969)

05 0 . 10 20 30 40 50.60 70 80 90 100

Percentage of catchment paved

Figure 2.3 Changes in water yield following urbanisation.

30 0675 + 2.5 0 Harris & Rantz(1964) 20 65-6 Seaburn(1969) 00+ 15 0 James(1965) 1.0 Crippen &Waananen _a (1969) 05 ® Stall& Smith 0® (cited Waananen1969) 000++ 0'5 0 0 00 1.0 00

1.5 00 0 20 2-5- 05 10 15 20 25 30 35 40 45 50 Annual Rainfall (Inches)

Figure'2.4 Changes in water yield following urbanisation as they are affected by rainfall. 53

1960s and most of the work dates from 1966. This r view is structured around groupings of papers which used similar analytical methods. The work of the U. S. Geological Survey is characterised by relatively large amounts of data and the use of regression and regional flood frequency in methods,. The unit hydrograph technique has been used in major studies

Texas and Michigan and by several other individual researchers. Simulation, by digital, analogue or hardware methodsi forms the final group of studies wed revi here.

The. U. S. Geological Survey Investigations

The magnitude and frequency of floods in suburban areas of Washington,

D. C. was investigated both intuitively and empirically by Carter (1961).

He assumed that the rainfall-runoff coefficient for flood peaks and flood `. volumes grew from an observed 0.3 in rural catchments to 0.75 on impervious surfaces. Moreover, he assumed that "the effect of-the changes in impervious areas is independent of the size of the flood". On the basis of these-assumptions, the factor, K, by which all floods are increased by the paving of I per cent was found from:

K-0.30 + 0.0045 I 0.03

Since a 10% impervious catchment could have aK factor of 1.15, Carter calculated that "the effect of imperviousness is small relative to the effects of suburban development on flood peaks". This conclusion may be valid but it is based upon weak, simplistic and wholly indefenceable assumptions. For instance, the 0.30 coefficient for rural conditions is an observed mean, but the range is probably 0.0 to 1.00 with runoff coefficients being near 0.90 to 1.00 for large significant floods.

Similarly, there is no justification for the assumption that the effect of

is independent urbanisation of the size of the flood, and the paper by 54 M

Martens(1968), reviewed later demonstrates that this assumption is invalid.

The second part of Carter's analysis concerned he lag time, T, between

the centroids of rainfall excess and flood runoff. line was plotted r, through a scattergram of 13 points with axes T and L

where L was the length of the basin from the gauging station to the

remotest part of the watershed and

s was the weighted slope of an order of 3 or greater of all

stream channels in the basin.

This line represents rural conditions and two further lines were drawn

paralle3i'to the first, to represent partially and completely sewered

conditions. However, the 5 and 6 points representing these two conditions.

do not justify such parallel lines, in fact, the two sets of points

suggest two lines, one steeper than the rural one the othe much less steep. I From these very tenuous lines and associated equations, and a multiple regression equation describing mean annual flood divided by K in terms of basin area and lg time, T. Carter derived a ratio of suburban (12% impervious) mean annual floods to undeveloped catchment mean annual floods. This factor, 1.8, was stated "to be the maximum effect of complete suburban development on flood peaks of any recurrence interval for drainage basins larger 4 than sq. miles in the Washington area". This conclusion may be cofrelý to" because, as subsequent sections show, the effect or urbanisation is large floods smaller on than small floods, but the equation for the K factor the rs and spurious lines relating T to L/, lead one to suggest that Carter's main contribution has been in the area of methodology rather than substantive empirical research. 55

Anderson (1967) has made use of Carter's methodology for an analysis of approximately eighty gauging stations within fifty miles of Alexandria and Fairfax County, North Virginia. He derived the following'equation analytically:

230 K AO'82 T-0.48

Q where, is the mean annual flood,

A is the area of the basin

K is the factor by which all floods are increased by the

paving of 1% of the catchment

T is the lag time.

The greater amount of data avoided the need to fit parallel lines by eye to the T versus L/[ relationship, but the work was based on Carter's ý"ý doubtful assumptions. Anderson concluded that the lag time for a completely sewered basin is one-eighth of-the rural value and one-fifth of the rural value when only tributaries are sewered. "On small steep basi/ins, drainage improvements alone may triple average flood sizes and complete development of stream channels and the basin surface may increase

floods average by a factor of nearly 8". Average floods are not really so important large infrequent as events and these received little attention.

A study of two basins near Detroit, Michigan by Wiitala (1961) used Carter's approach and relationships very directly. The urban Red Run basin 36.5 of sq. miles was completely sewered whilst the Plum Brook was 22.9 in sq. miles area and relatively free from urban and suburban develop- Wiitala ment. used Carter's exponent in the relationships between T and L/fTto calibrate the relationship between T and L/[ for the Plum Brook then he Plum and used this Brook relationship to calculate the lag time of the Red Run it in its when was rural state. He found that urbanisation of the Red Run had reduced its lag time from twelve to three hours. Use of 56

Carter's multiple regression equation relating mean annual flood to K, A and T and the empirically derived values for T, allowed Wiitala to calculate the ratio of the mean annual floods in the 25 per cent impervious Red Run and undeveloped Plum Brook. The ratio of 2.3 was a little larger than

Carter's from Washington and Wiitala states, "it is believed that the same ratio would apply to larger floods as well".

The most recent development of Carter's method is reported by Putnam

(1971) who found that in North Carolina storm sewering and impervious land cover decrease the overland flow time and infiltration rate, resulting in an increase of the total rainfall reaching the stream as runoff. He deprecated the use of a family of curves for different degrees of urban development, to relate basin lag time to L//s-. As an alternative he

showed that one curve to estimate basin lag time can be defined by

including the ratio of impervious surface to total drainage area as a parameter in the lag time relationship. He argued that the use of one curve greatly reduces the subjectivity of interpolating between several curves each of which is based on a specific amount of development.

One of the major assumptiöns of all of this work, stemming from

Carter's original paper, is that "the effect of the changes in impervious is independent area of the size of the flood", (Carter 1961, p. 10). This idea was seriously challenged by 14artens'(1968) work on metropolitan

Charlotte, N. Carolina. He plotted a series of lines, each for one

. particular return period, on a graph relating ratio of the high return flood period to the mean annual flood against impervious area in per cent, (Fig. 2.5) and found that, 'whilst the mean annual flood is increased 2.5

.4 C

5 `5oir ý. ý. . -rr---rr-r--r- - r.. r.. ýrý. ýr.. ýrrrrý.

s + a awe 0 30 0J u 25

20 {

z z 1S f

W 23 0 I.- 0

2 { .. . X33

1 u 20.40 60 80 100 IMPERVIOUS AREA, IN PERCENT

Figure 2.5 Graph showing variation of flood-frequency ratio with percent of impervious cover. (after Martens, 1968) 58 times, the fifty year flood is largely unaffected by even 100 per cent paving of a basin. However, the derivation of the graph gives grounds for suspecting the precise quantitative conclusion, if not the qualitative one. The-lines were drawn by connecting points located on the extreme left and right of the graph. For 0% impervious, i. e. rural conditions, the ratio of floods to the mean annual flood was derived from an empirical composite flood frequency curve prepared from regional data. For 100% impervious, the ratios were established by selecting appropriate ratios from an empirical composite rainfall-frequency curve (plot of the ratio of high return period rainfall intensities to mean annual peak rainfall intensity against recurrence interval) and multiplying these ratios by

2.5, the effect of K in the equation,

K-0.30 + 0.45 I 0.30 which was taken from Carter's paper. The qualititative result that the `-, effect of urbanisation is diminished as flood magnitudes increase is probably correct, but the selection of the 50 year flood is little more than an arithmetic coincidence. The derivation of the graph relies on all rural floods having 30 per cent runoff and all urban areas having

75 per cent runoff. The diagram excludes all consideration of scale for only very small catchments could be 100 per cent impervious and most basins 10 of, say, sq. miles are never likely to be more than 40 per cent paved. Finally, if one extends Martens' analysis to the one hundred year flood, it appears that complete urbanisation reduces this flood by about 8 per Whilst cent. the surcharging of manholes, the throttling of flow in sewers, and ponding of water in roads and behind kerbs may in fact mitigate floods; very severe this extension of Marten's method exposes the lack of to reference physical processes in the work. Some empirical support for Martens' findings been has advanced by Skelton (1972) who found that in a 60% 59

urbanised basin in St. Louis "the 25 year flood was 2.4 times greater than is the 2 year flood, whereas the same flood from a rural basin about 3.4 times the 2 year flood".

The paving of around 25 per cent of the East Meadow Brook, Nassau

County, Long Island, New York and the consequent sewering of 18 per cent of the catchment area between 1937 and 1962 (Seaburn 1969a) increased the peak of the unit hydrograph 2.5 times from 313 to 776 cusecs. The width of the unit hydrograph at 50 and 75 per cent of the peak flow declined to

0.38 and 0.28 of the rural values. Seaburn demonstrated the change in the

rainfall-runoff relationship between the periods 1937-43 and 1964-66 by a

pair of regressors lines, each of which was' plotted through a scattergram

of points for one of these periods. The lines for 1964-66 was above that

for 1937-43 and it had a steeper slope, showing that a greater percentage

of rainfall ran off under urban conditions and that the greatest increase

in runoff after urbanisation was after heavy rainfall. However, Seaburn

argued that "theoretical considerations and observations suggest that the

lines for two trend ... should ultimately converge", showing that very

severe rain storms there is relatively little difference between urban and

rural catchments.

A similar investigation to the East Meadow Brook study undertaken near

Palo Alto, California by Crippen and Waananen (1969) showed that paving and

draining of 30.8% of the Sharon Creek basin raised the unit hydrograph peak

from 180 to 250 cusecs and reduced the lag time between centcoids of rainfall

excess and runoff from 69 to 54 minutes. Comparison of the rainfall-runoff

relationship in the developed Sharon Creek and undeveloped Los Trancos

tributary revealed that *in the latter, storm runoff ranged from 4.9 to 17.0% 60

of storm rainfall whilst in the Sharon Creek corresponding figures were

37.8 and 45.5. Similar results have been reported by Soule (1971) who found that land clearing prior to development, had caused a significant change in runoff characteristics of a 33 acre basin near Washington, D. C.

The peak discharge increased from a maximum of 4 cusecs for the period

December 1966 to September 1969 to a peak of 39 cusecs for the 1970 water year. He also found a 'noteworthy" decrease in lag time for the drainage.

The final U. S. Geological Survey paper to be reviewed here, is one of the few in which an attempt has been made to analyse flood frequency using real data. Wilson (1966) used 43 station years of data for four gauges in and around Jackson, Mississippi and found that the "urban" fifty"' year flood was only about twice the magnitude of the mean annual flood, whilst in larger rural areas the ratio was three times. He accredited this difference to the "man-made storm sewers, gutters and ditches which function well during low order floods but are overtaxed during extreme floods". Moreover, when plotting. the ratio of mean annual floods in the urban catchments of comparable sizes, which were derived from a regional analysis, against percentage f the basin with storm sewers and improved he found channels; that the ratio increased from nearly 2 for 20% of the catchment served with sewers to over 3.5 when 45% of the catchment sewered. He concluded the urban-rural annual mean flood ratio might be as high as 4.4

Unit Hydrograph Studies.

The hydrograph unit method was used for prediction in an incisive by r manner Espey, Morgan and(iasch (1966) for the Waller Creek, Austin, Texas. They derived multiple regression equations, for. both rural and 61

base, urban conditions, in order to predict the peak, time of risa, time and width of the hydrograph at 50 and 75% of peak flow for thirty minute from unit hydrographs. Eleven rural and 24 urban catchments central and 1 -0 north central U. S. A. were used in the analysis. The independent variables /were: used I /L, catchment length along main channel,

s, weighted mean Catchment slope,

A, basin area,

I, impervious area, and a factor

1, which was given three arbitrary values to represent various

degrees of channel improvement.

Only the time of rise was expressed in terms of these variables alone, other dependent variables were related to one or more independent variables and one or more hydrograph parameters derived from other

equations. For instance, the peak flow equations included as an independent variable - time of rise, and those for hydrograph width included as an

independent variable - peak flow. The statistical analysis undertaken was thorough and precise and ended with a predictive test of the equations

using new data for the Beargrass Creek in Kentucky, which was 70%

impervious. The test proved successful in that the errors were within 6%

of the true value for all unit hydrograph characteristics except time of

rise (-31%) and time base (-19%). The equations were then used to

estimate the thirty minute unit hydrograph for two stations on the Waller

Creek in Austin, Texas for three degreees urbanisation. The results in roi Table 2.3 reveal that a 50% paving of the Waller Creek would reduce time of

rise by over 50% at both stations and increase peak flow by over 60%.

The authors do not appear to have considered the effect of channel changes

I 2 future developmentG Tabl1e 2.3 Summary of some effects of presentp and urban Morgan on the Waller Creek, Dallas, Texas. (after ISpey, and Masch, 1966)

23rd Street Gauging Station

ttra Percent Percent Stage Peak Difference of Difference, o f Discharge based on Also Based on (cla) Development (minutes) Rural Values Rural Values

Rural 0 'I "02 105 0 1,460

Present 31 2 I- 272 31 - 462 2,200. +

Future 621 I" 302 50 - 522 2,360 +

38th Street Gauging Station

Portent Percent TL e P: Stage Differene" sk Diftarone" of Die h of Based art* Based on D Rise on (eta)8 evelopment (minute. ) Rural Values Rural Values

Rural I" 0Z 103 0 880 0

Present I- 212 33 - 47Z 930 +62

Future I- 502 47 - 342 1,460 f66 Z

Equation 37

31 26 T4-40.3'A'21 i . s . Watershed

Pr, dittod tar. Time Value " (minutes) I

23rd Street so 2 20

33th street 502 14

Present Valueet 23rd Street 27 2 44

38th Street 21 : 31 63

because "the k (at 0.8) during value of ... was assumed constant the

development". In a subsequent paper, Espey et al (1969), the i factor was lowered denoting further channel improvement and culverting, the flood

peaks were higher and sooner than before. In an extended analysis

including further data for six more rural and nine more urban basins from

Houston, the t factor into 12 was changed two elements, -11 and 0

The former represents4channel improvements and ranges from 0.6 to 1.0,

the latter figure representing a rural situation. 1.2 represents channel vegetation and varies from 0.0 for no vegetation to 0.3 for dense well developed vegetation. They improve the predictions in the expected fashion but they can only be regarded as fudge factors even though they have some ordinal-type existence in reality. In the investigation of the rainfall-runoff relationship, Espey et al (1969) derived a regression equation from 24 storms on the Waller Creek which related percentage runoff to impervious cover, amount and duration of rainfall and an A. P. I. index.

They found that a 20% impervious cover increased runoff by 180% and a 30% cover by 210%. These are dramatic increases but they apply to a relatively arid area where small absolute increases produce massive percentage increases in runoff. Espey, Winslow and Morgan (1969) concluded both studies with the findings that peak flows may be tripled, time of rise reduced by 33% and that the precise increase in peak runoff is influenced by the degree of channel improvement, impervious cover, channel vegetation, and the type of secondary drainage system.

The unit hydrograph method has been employed by Brater (1968) and Brater Sangal and (1969) in an investigation of the "urban runoff process" in and around Detroit using thirteen gauging stations on six river systems. 64

They placed great emphasis on a hydrograph separation technique and claimed

that the use of a groundwater depletion curve and a rising straight line'

from the beginning of the hydrograph rise "eliminated personal judgement in

separating surface runoff from ground water discharge". They may have

minimised personal judgement but in no way can the technique be considered

objective by their own admission, the baseflow curve is probably not a

straight line. Their initial findings refer to basin retention and changes

in the unit hydrograph. When they plotted surface runoff in inches against

storm rainfall two facts emerged. First there is never 100% runoff from

any of the catchments and normally an envelope curve, showing basin

retention as 0.2 inches, enclosed all the points. Second, the surface

runoff volume of many storms can be wholly explained by 100% runoff from N 11 the paved surfaces minus 0.05 inches which is the assumed retention on

paved surfaces. The plotting of the 13 values for unit hydrograph peak

and time of rise against drainage area for the six river basins shows that

as one moves from rural basins to the Red Run with 7,500 persons per

sq. mile and 100% surface water sewering of the basin, the peak rises by

up to four times and the time of rise contracts by a factor of about five

for basins of comparable size.

An unpublished report by Curtis, Lee and Thomas cited by the American

Society of Civil Engineers Task Force on Urban Hydrology (1969) contains detailed, aw thoughtful and constructive approach to the problem. lrl.

- This is all the

more remarkable because of the relatively early date of the work (1964).

The Task Force report as follows: u;)

"A flood routing model of the 69.5 sq. mile Wolf Creek drainage

basin in southwestern Ohio using one hour duration unit graphs

and Muskingum channel routing techniques was developed and

verified for floods observed during the period 1939-61. Unit

graphs were then modified according to the techniques discussed

in the Van Sickle discussion of the Eagleson paper in the

'Journäl'of'the'Hydratilics'Division, American Society of Civil

Engineers, November, 1972, to simulate completely urban

conditions. Resul; s of studies with the revised unit graphs

with no changes in channel routing coefficients show that little

increase is to be expected for the relatively long duration, six

hour to nineteen hour, storms which now cause the annual floods.

Study of one hour duration storms which can be expected to be

critical under urbanised conditions show that an increase of 50%

is to be expected for a ten year frequency flood. A trend

toward convergence of the urbanised peaks is evident for r frequencies over one hundred years. "

A further contribution to our knowledge of urban hydrology has been made by Kinosita and Sonda (1967) who worked on the Syakuzii River on the outskirts of Tokyo. Using a macroscopic analysis with both a continuity equation relating outflow to changes in storage, and the Sugawara hardware tank model; they found that a flood in 1966 after urbanisation was three times as large as a comparable one in 1958 when the catchment was rural. A "microscopic analysis" using sub-catchments revealed that, had flood plain inundation not taken place during the 1958 flood, the peak' would have been doubled in size.

Simulation Studies.

It is in the field of simulation studies by digital, analogue and hardware models that the most progressive and incisive analyses have been 66

made. These normally aim to construct a model and fit it to a limited

record covering either or both rural and urban situations. When calibrated

and tested, the model may be used in an experimental fashion to answer

research questions.

Crawford and Linsley (1966) seem to have pioneered the development of digital simulation methods with the Stanford Watershed Mod 1 IV. A short

section of their report is devoted to the "Hydrologic Effects of

Urbanisation". They discuss the hydrological changes wrought by urbanisation and present the results of a simulation exercise on the Sharon Creek the stream studied by Crippen and Waananen (1969). Fig. 2.6 (a) and (b) display Crawford and Linsley's simulation of the catchment as if it had been rural compared to the gauged urban situation. They say "small storms throughout the year show the greatest increase, and at the start of the year runoff was measured when none would have occurred if the watershed was undisturbed. The largest storms change less dramatically". This final point is demonstrated very effectively in Fig. 2.7 which was prepared from

Crawford and Linsley's original data. James (1965) refined the Stanford

Watershed Model still further and used it to develop a long term continuous hydrograph (1905-1963) for Morrison Creek, Sacramento, California. By varying constants describing the physical conditions in the catchment according to the amount of urban development and channel improvement within the tributary area, a number of continuous hydrographs were developed. He drew important three conclusions with regard to urbanisation and floods. First, the ratio of flood peaks for urban to those for rural conditions decreased from 2.33 for the 2.33 year flood to 1.57 for the 200 year flood. Ratios for largest the flood in an individual year ranged between 5.76 and 1.31. Second, increased urbanisation autumn flood peaks 90 fold and spring 67

U SHARON CREEK AT MENLO PARK MEAN DAILY FLOW WATER YEAR 1963

etaleu ' o. a . ««.. lireunerý..u. u. yýrarl 1 --- huýut uu Ue u rr.

L al{OAU ý[[olDfoN[[OID(O Iu4&AtrYWA. rdl ." Wwr. 31 "H. 1. as . o. Ij. 36 ýýeaaHß. 1. reto u..U..

L.I

1 I IttSlu urtrul Itttrlu 11rwn" 11111411 Mtar APIN nt

}

November 10,1114 SHARON CREEK 30 Menlo Park d a 23 Recorded "' Simulation (Prior 10 -. to A . urbanization)

OM i 12

Figure 2.6 Gauged urban and simulated rural flow fo Sharon Creek at Menlo Park for selected periods. (after Crawfo d and Linsley, 1966) 68

22 20 18 tutu Q. Q. \` 16 - "0 14- 0 = to- 12 c 1- 10 \ý . L ý cc 8- 0 6 co 4 2 012345 Rural flood peak in cusecs (simulated)

Figure 2.7 The relationship between the ratio of urban to rural flood flows and simulated rural fows for Sharon creek at Menlo Park. (after Crawford and Linsley, 1966)

1 1 69

flood. peaks by up to 37 times. Except for extremely dry years, winter

flood peak ratios varied between 1.2 and 2.0. One result of urbanisation

was thus found to be a lengthening of the flood season by increasing the

danger from autumn and spring floods. Finally, the hydrographs of

individual floods rose and fell much quicker under urban conditions.

James concludes "each hydrologic effect of urbanisation stems from the

reduced role of soil moisture storage as urbanisation restricts contact

of rainfall with the soil. This reduction magnifies off season and

lesser floods where soil mositure is a major factor but has a much lesser

in larger floods. found effect midwinter and ... Urbanisation was thus ...

to be most influential in increasing peaks of lesser floods".

.ý

\` Simulation by means of an analog computer was used by Riley and

Narayana (1969) to derive computer models for the urban catchment of the

Waller Creek, Texas (also used by Espey et al. 1966,1969) and for its

"equivalent rural watershed". This latter theoretical watershed had uniform rural land use and for a given input produces outputs identical to those from the urban catchment. The reason given for this unique and almost unimaginable "equivalent rural watershed" is that the variation in land use and paved/non-paved areas in urban catchments is so great that modelling is very difficult. This hardly seems to justify this radical departure from reality and no other convincing reason is advanced for the use of the concept. Two measured urban parameters are employed to characterise the spread of paved surfaces. These are percentage impervious cover and characteristic impervious lenght (Lf). This latter quantitO' is derived from: Lf ai i Eai L w is where a; the area of each paved surface, 70

1; is the length of channel from the centre of each paved

area to the river gauge and in in Lw is the maximum length of travel for water a channel

the basin.

When the properties of the equivalent rural watershed were plotted against length these two urban parameters, the authors found that the maximum of rural travel was shortened by more than 60%; the slope of the equivalent

from 26 447. the time watershed was steepened, with an increase to paved,

from 58 to 51 and the of rise of the unit hydrograph was shortened minutes, However, despite peak discharge was increased from 1540 to 1700 cusecs. and the sections of paper extolling the success of model verification between of the sensitivity exercises, all the relationships properties impervious by the factor equivalent rural watershed and the characteristic at appear to be exponential with all of the lines becoming near vertical an Lf value of 0.453.

(1973) built As a precursor to digital simulation work, Thorpe and 8.5 3.3 experimented with a sand filled rectangular catchment, x metres and 60 cms. deep, served by 10 sets of 22 atomised spray nozzles which simulated moving storms over the catchment. Thorpe covered either the upper or lower parts of the catchment with polythene to simulate urban conditions and in the initial experiments used a single straight channel lead with straight side slopes. He found that downstream storm movements to higher peak discharges than upstream movements with the same velocity; with upstream movement slower storms create longer peaks for a given surface condition. When considering part urbanised catchments, peak runoff from upstream movements is magnified more by impermeable surface in the upper catchment. Downstream movements cause higher peaks with a covered lower catchment. Thorpe expressed some of his conclusions in the form of dimensionless ratios. Further studies with the model are in progress under 71

the aegis of the Gloucestershire Joint Surface Water Study.

The remaining literature on the changes in the flood hydrograph

induced by urbanisation can best be described as being assertions based

on deductive thought, statements of faith or sensible guesses. The need

to derive a flood flow formula for Conne scut led Bigwood and Thomas Masor;'' 6 (1955) to conclude, on the basis of two gauging stationsjdraing jroM

suburban areas, that, "for streams draining urban residential watersheds a

multiplier of 3.0" needs to be applied to the answer given by the normal

predictive equation incorporating only catchment area and slope.

Mills (1968) has argued that "substantial property damage can occur

if the effect of urbanisation on runoff is not taken into account in the ,n design of storm channel improvements and bridges". He cited April

storm of about 4" in 5 hours over Dallas which produced 92% runoff from

a wholly urbanised catchment and 85% runoff from a nearby partly urban

catchment. This evidence may prove his point, but it could be argued

that, given the errors is gauging storm rainfall, there was no

significant difference between the two situations.

An infiltration study was ysed by Felton and Lull (1963) to show that in suburban growth the WissahiI an Valley, Pensylvania had made the intermittent stream but had increased total flow by concentrating it into high flood short peaks. They found that not only were paved surfaces important in these changes, but also that the infiltration capacity of lawns (0.1 urban and parks inch/min. ) was much lower than that for the fields (0.28 and woods they replaced and-40.58 inch/min respectively).

-4 a 72

An introductory paper from the Gloucester Joint Surface Water Study

(Shaw & Waller 1973, Thorpe 1973), sets out their research progress for the

Gloucester-Cheltenham-Tewkesbury area. "Results from the first stage of the studies indicate that substantial urbanisation of an area can increase peak runoff, following storms which recur annually, by as much as 10 times the flow for a similar rural area". 4

The influence of urbanisation has been recognised if not measured in two other parts of England. Medrington (1966) says of the River Mimram, a'chalk stream in Hertfordshire and its tributary that, "variations of flow are not great and are gradual, whilst, at Alcazar (gauging station on the Pymmes Brook in N. E. London) a change from low flow to peak flow can occur in about half an hour". Of the Upper Calder Valley in Yorkshire,

Muller (1966) writes "man himself has a significant influence" on floods, but "the effect which urban development has had on flood flows has, in this case, not been determined owing to the absence of data".

An overall view of urbanisation and floods

Despite the relatively large number of individual studies of the problem there are few reviews or syntheses. Some studies, such as those of Espey, Morgan and Winslow, Anderson, and Crippen and Waananen, refer to the work of Carter whilst Seaburn has examined the work of Espey,

Morgan and Masch. The American Society of Civil Engineers Task Force on

Urban Hydrology Report (1969) discussed intuitively the hydrological processes modified by urbanisation but did no more than list articles and make bibliographical notes. 73

Leopold (1968) undertook a major taks of synthesis and simplification ". A in his "Hydrology for Urban Land Planning -A Guidebook ... major part (1967), Martens (1966) of which is the translation of the studies of Anderson

Wilson (1966), Carter (1961), Wiitala (1961) and Espey, Morgan and Masch during the (1966) into "terms that the planner can use to test alternatives in planning process". The three main graphs from Leopold's work are shown by Figure 2.8, but it should be stated that many of the figures used

Leopold are projections by other authors well beyond the observed empirical and evidence. Figure 2.8 (a) relates the percentage of the catchment paved

sewered to the relative increase after urbanisation of the rural mean is in it depicts annual flood from a1 sq. mile basin. This valuable that 6 dispels urban induced increases in floods of between 1.5 and times and so figure the idea of a single ratio which is "the effect of urbanisation". The 1 is less useful when one has to use it for a real catchment of more than

for flood fifty interval. These latter sq. mile and a of, sa ,a year return

points are all the more important when one considers Lepold's claim that

he is producing a guidebook for urban land planning; for few urban areas

can be defended againsttlessýthan the 20 year, flood.

Figure 2.8 (b):, again a bold attempt at useful synthesis, falls down

on several counts. First it does not include floods which cause

significant damage. Second, with the exception of the urbanised line and

the three points on the 2.33 year flood line, it is all speculation. Finally,

the lines converge in the'direction of decreasing flood magnitude. Not only

is this illogical, but Leopold ignores one of the major findings of two of

the papers that he cites, namely James (1966) and Martens (1968), who found

that the effect of urbanisation declined with increasing flood magnitude.

46 74

l00 I3 io

Figure 2.8(a) Effect of urban- fo isation on mean annual flood for 4, a1 square-mile ö drainage area. (after Leopold, 10 1968)

40 'O to $0 109 /IICIM! AGI 01 AIEA IMºEIYIOU

raw- ......

" AYERAOENUMILI Of ROWS IN A 10"YIAI U1100

jug PI(CIMS11 of Ste& 50. erea- {mptrýýoui PIrt, ataýý j0o (I ISO ýÖ

0 V t Figure 2.8(b) Flood frequency y0 1 yo 200 curves for a1 square- MAO drainage basin AD mile in various states of ISO (after V urbanisation. Leopold 1968)

I00 o .% n'Os oi

0L 11[i 0.2 ' 0.3 1.0 2 2.2 S 'o IEf01RINCE INtIRYAI, IN HANS

,f1

Figure 2.8(c) Increase in number tooý of flows per year 0 equal to or exceeding original z channel capacity ucm ý W (1 square-mile C drainage area), as W 0 a ratio to number 0 º. of overbank flows 2 ec before urbanisation for different degrees 1 of urbanisation. (after Leopold, 1968) Percer

Peºcuntog

NC iumrlLTE 75

The final diagram reproduced from Leopold's paper, Figure 2.8 (c), is a

valuable distillation of published data and would seem to afford a

useful guide for planning of rural areas downstream of urban areas without

associated flood balancing structures. But, it is b sed upon speculative

extrapolations at the highly urbanised end of the axis and only depicts

the number of oveank discharges, not the severity of such floods. 4 I One of the most valuable aspects of Leopold's 1968 paper is the

synthesis of results from a large number of studies and reports to

produce a practically useful series of diagrams. Prompted by this, the

salieit data from all relevant articles reviewed in this t esis has been

collated in Table 2.4 in an attempt to express the ratio of flood peak

after urbanisation to that before urbanisation as a function of the

percentage of the catchment paved and the return period of the rural

flood. As far as possible only empirically derived figures have been

included and where it was necessary to make estimates, comments have been

appended. When plotted, Figure 2.9, the data derived from Table 2.4 appears

logical and gives credance to the suggestion, made earlier, that the

effect of urbanisation on floods declines as flood magnitudes increase because of the reduced importance of interception, depression storage infiltration and in a rural catchment undergoing a very severe rainstorm. have Lines been fitted by eye to the data in Figure 2.9, and in general is there good agreement between lines and points. Inconsistencies in the figure inevitable were and arose because the catchments vary in size, found in they are widely differing climatic environments from the semi- arid parts of the U. S. A. to Japan and western Britain. The type and 76

Table 4

The increase in flood discharges after the urbanisation of a rural catchment.

Reference Values of the Flood Recurrence Percentage ratio, Interval (Years) of the flood discharge Basin Paved after urbanisation to flood discharge before urbanisation

Bigwood and Basiit 1 3 2.33 (20)8 Thomas (1955) Basin 2 3 2.33 (20)

Carter (1961) 1.8 2.33 12

Wiitala (1961) 3.0 2.33 25

James (1965) 1.4 2.33 101 5.00 101 1.3 1 1.2 10.00 10 1.2 25.00 101 - 1.1 100.00 101 1.1 200 0 101 .0 Crawford and 20 0.12 6.7 Linsley (1966) 13 0.52 6.7 1.6 3.02 6.7

Espey, Morgan 38th Street 3.2 2.33 21 and Masch (1966) 5.9 2.33 50

23rd Street 4.4 2.33 27 6.0 2.33 50

Wilson 1.9 9 2.33 3 2.2 2.33 11 2 8 18 . 2.33 3.6 2.33 273

Anderson (1967)4 2.86 2.33 20 2.35 25.00 20 2.24 50.00 20 2.20 100.00 20

3.85 2.33 50 2.61 25.00 50 2.36 50.00 50 2.20 100.00 50

Kinosita and 25 (100)6 44.3 Sonda (1969) 77

Curtis, Lee 1.5 10 (15) and 7 Thomas. (Reported by 1 100(+) (15) ASCE Task Force 1969)

U. S. Geol. Survey Study of 1.6 2.3 15 Little Sugar Creek, 1.3 10 15 N. Carolina. (Reported by 1.2 20 15 ASCE Task Force 1969)

Shaw and 10 1(+) (20) Waller (1973)

Hammer (1973) 2.5 1.50 25 2.2 2.33 25 2.0 5.00 25 1.9 10.00 25 1.8 20.00 25 1.7 50.00 25

4.3 1.50 50 3.5 2.33 50 3.0 5.00 50 2.8 10.00 50 2.6 20.00 50 2.5 50.00 50

Putnam (Cited by 3.3 1.5 25 Hammer 1973) 2.9 2.33 25 2.6 5.00 25 2.4 10.00 25 2.2 20.00 25 2.0 50.00 25

4.2 1.5 50 3.7 2.33 50 3.2 5.00 50 2.9 10.00 50 2.6 20.00 50 2.3 50.00 50

J 1

/ 78 1_I Footnotes for Table 2.1r

1. Watershed condition was 22% urban including 10% of the basin

actually paved and 17% of the tributaries improved.

2. Estimated from gauging station records published by Crippen and

Waananen (1969).

3. Only the percentage of the basin with storm sewers and improved

channels was given. Following the work of James (1965), 0.45 of

this area was assumed to be paved.

4. All the ratios refer to a basin with natural channels, sewered

tributaries and a L/ ^/s ratio of 1.0, where, L is the length of

the basin in miles along the main channel and s is the slope in

feet per mile between points 10% and 85% of the distance along

the main channel in the basin.

5. Data is taken from the mimeographed paper circulated at the

symposium.

6. The rainfall was the heaviest on record in Tokyo.

(+) 7. indicates that the figure is a conservative underestimate.

8. Brackets indicate figures assumed from qualitative descriptions

contained in the papers. 79

4.141 too as soso Is '9 ""t" 03 Y YY iLý 0 Sp values or tn" ratio:. ý' 4+ be . 3, oK 0uehaPp. afar Urb e.ot an n' Peak Ol. MoP9.6~9 UMOnlsoteon Z ,pº. T

4 /f 43 30 ý.. Yip Ö : °s. " ++o. is .t to w (9 20- '*3s J-4 Ile age a W'1 lot 15 "., Ii.: aw "1.1 N. ý N. t . "t 1.+. CV4 "u ý'' .+"1 ... p 0$ 1"0 t0 100 200 FLOOD RECURRENCE INTERVAL (YEARS)

Figure 2.9 A general relationship between the increase in flood flows and the percentage of the catchment paved for various recurrence intervals.

zz 00 ii NN z <

Wa ý Wp Fy 4

U2U NN as bo tar

FLOOD RECURRENCEINTERVAL (YEARS)

Figure 2.10 The increase in flood flows of various return periods to be expected from the paving of 20% of a basin. 80

degree of urbanisation will vary between the catchments na way that caxnot be fully expressed by "percentage paved" and the degree of improvement of the drainage network is likely to produce significant variations in the American sites. However, the figure does show how various degrees of paving will increase floods of particular magnitudes and more importantly, Figure 2.10, which was derived from Figure 2.9, shows how the effect'of, say 20% paving of a catchment, declines as flood magnitudes increase.

Conclusion

Urbanisation seems to affect almost all aspects of the hydrological cycle, from rainfall and atmospheric humidity to ground water and flood flows. There is clearly no single figure which will describe the effect of urbanisation on a particular hydrological process, but at the same time investigations in this field do generate soluble problems and useful answers. Succeeding chapters examine, in the context of the

Canon's Brook, many of the hypothises and relationships suggested by this review and it is hoped that a fusion of published literature and the results of empirical research will produce valuable and applicable answers. 0 81

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Changnon, S. A. 1961(a) Precipitation contrasts between the Chicago urban area and an offshore station in southern Lake Michigan. ' Bull. Am. Met. Soc. 42, pp. 1-10.

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-CHAPTER 3

DATA SOURCESp'DATA COLLECTION AND THE CONCEPTUAL FRAMEWORK

We are very imperfectly acquainted with the present .. o precipitation and evaporation of any extensive region, even in countries most densely peopled and best supplied instruments The with and observers. ... exact measurement of the geographical and climatic changes hitherto in effected by man is impracticable, and we possess, relation to them, the means of only qualititative, not quantitative analysis. George Perkins Harsh

It has been shown in the previous chapter that for the measurement of individual processes and the examination of overall changes in catchment-. hydrology, there a*e a multiplicity of techniques available. Further, although previous studies of the influence of urbanisation on hydrology for have recognised the complexity of the changes and the consequent need comprehensive and accurate data, few have used it. This chapter looks briefly at the relevance and utility for this study of the various research methodologies set out in the previous chapter; the choice of the Canon's

Brook, Harlow, Essex as the research catchment is explained and the data available for that basin are described and evaluated. The latter part of the chapter discusses the urbanisation of the catchment and the methods forms used to assess it. The concep ual framework of the investigation a conclusion.

Hydrological Research and the Graduate Student

A graduate student, when considering the suitability of particular

approaches to the study of the hydrologic impact of urbanisation, must

1 F "

/ 88

consider two major constraints, time and resources. Doctpral research formulatäon, data reqt/ires ideally that research training, problem

of collection, analysis, thesis presentation and possibly the publication for individual results be undertaken within three years. Resources the The postgraduate student tend to have finite and low limits. only help labour available, apart from one's own, is*the intermittent of relatively large groups of undergraduates. Money, although easily

for instrument- accessible in small amounts, is not readily available the in ation of even a single catchment. Co-operation with others a increases university research team or with outside bodies certainly

available resources but does not remove the time constraint.

The collection of basic hydrological data in the field was rejected

for three reasons. First, at anything other than the scale of the

individual house, urbanisation is a relatively slow process. It must be is doubted if during the time available for data collection, which

eighteen months at the maximum, any catchment would have undergone

significant urban growth. Related to this point is the fact that the

installation and operation of instruments is time consuming and any time

spent on these procedures necessarily reduces the data collection period.

Second, the construction of all but the most primitive river gauging

structures and the purchase of other instruments is not generally feasible

within the present structure of financial provisions for individual

research students. Finally, while examining the possibility of co-operat-

ion with other bodies and the feasibility of field collection of data,

it was found that there was a large amount of pre-recorded data available

be for which would suitable a study of the overall hydrological effects of urbanisation. 89

The Choice 'of a' Study of 'the 'Canon's 'Brook 'and Catchment 'A survey of possible sources of pre-recorded data or the study was initiated at the inception of the problem. A letter was sent asking if the addressee had or knew of the existence of any data from stream and rain gauges which covered a period of urbanisation in a catchment.

Information on the size of the catchment, the nature of storm and foul water drainage, the type and quality of other climatological and hydrological data available regionally, the nature of human interference with the hydrology of the area such as watershed changes, water supply abstraction, and river control works, and the details of research already undertaken with the available data was also sought. Every River

Authority and New Town Development Corporation in England and Wales together with certain local authorities who were considered to be potentially helpful were circulated. The Institute of Hydrology, the

Climate and Environment Section of the Road Research Laboratory, the

Meteorological Office, the Ministry of Agriculture, Fisheries and Food, and the Building Research Station were also contacted since the proposed study coincided with their interests.

In order to ensure that the pre-recorded data discovered from the circulation of the letter was of an adequate standard for the proposed research a series of quasi-objective criteria were set up against which each set of data could be judged. The criteria were as follows: 1. unbroken records of streamflow and rainfall during a period

when significant urban growth took place.

2. hydrological for either records the urbanising basin which covered a pre-urban period, or contemporary data for a nearby non-urban catchment which was otherwise hydrologically similar gýý 90

to the urbanising basin.

3. town growth was to have produced no changes in the watershed.

4. the flow through the stream gauge was to be free from foul water

or sewage effluent.

5. if abstraction of water took place from the stream or ground

water upstream of the gauge then this was to be recorded

accurately.

6. no leakage of water from the catchment by way of foul or

combined sewers.

7. the ground watertable should follow the topographic divide.

It was found from the 100% response to the circular that 33

catchments had streamflow and autographic raingauge records for a period

of urbanisation. Table 3.1 below gives a complete listing of the

catchments and 'shows their characteristics. Comparison of the information

in the table with the criteria set out above reveals that many of the basins

are unsuitable for study. For example, the R. Maun at Mansfield has a

very high proportion of its dry weather flow made up of sewage eflýlent

of unknown volume. The Cut, Bracknell, has had its watershed radically

altered by the civil engineers during the expansion of the town. The

quality of the hydrological data for many of the catchments is low, for

txample at Stevenage the gauge has a weekly drum but the chart is only

changed when a large flood occurs and these may be months apart. Finally,

some gauging structures are of relatively poor design and large

inaccuracies appear at certain flow conditions as on the Pymme's Brook,

Edmonton.

6 91

Taº10 M C. teluwntt to Logland and Vii.. with Aydr. 1o ie date t.. trln{ a period of rrbal tytn. lii t+arke Me. Rivet Cousin$ Authority Typ. of Cause* basin Arta Ceetoly Vatrrchr6 $wrf. ta Me. of No. of Duratteo Auto. Central (fq. eller) Mater of Drain. {t lain- Catch- Drter4 SAW864 usage

I bal. 1. TheCut Th... a ConservancyD.C. Yale6 11.6 Morlfted trperate 3 0 1131" LargeNumber ei atlq lashrn 1 sea. 2. Canon's )rook Lo. Conservancy Flute 1 1.23 Clay ! table separate 3.1 2 1150 " 3. Motrelll River Themes Conservancy Flute 26.2 Clay Stable Separate 11 0 1136 - 6. it. too " Hampshire R.A. Crump 1 36.3 Chalk 6 stable separate 1 1 1961 " A small Nilhis teartlt" t. rtlartru Y{. " tbo arbosiieiiN 3 0 1966 S. Mallington Stream SummonR . A. Crump 1 1.27 stable Separate " 6. docking keck lucre: I. A. Crump 1 - 1.50 Chalk 6 Stable separate 3 3 1966 - Clay 7. isms River Cara 1. A. Crump 1 11.2 Clay 1 3 0 1965 " S. t. rtvoo4 brook tea.. I. A. Crump 1 2.40 1 1 1966 " f. Beverly Brook C. L. C. flu" stable separate 11 0 41931 " 10. IL. Irrat C. L. C. Veit 1 " 9.0 Clay I Separate 16 0 i 161 0 11. It. grant C. L. C. Natural 3 t 3.0 Cie? Separate Settles

12. 1. Brest C. L. C. plums, 1 . 40.0 Clay t separate 16 0 13. 1. stoat O. L. C. Natural 3 460.0 Clel f. separate 16 0 Bottes 16. It. We Shames Conservancy Veit 1 32.0 t Separate 4 at 1161 " 1161 15. 1. Mole That.. Conservancy plume 1 11.0 i Separate 4 cl " Conservancy Natural f f 1952 C. wtck Airport se. ette- 16. It. Mole Thames 3 12.0 separate at - /y. tectles vtea 411 01 Of rnt" surfaces bare

17. 1. Mel" Crowley D. D. C. B. C. Wait 2 5.30 t separate 4 at 1132 1t. Crawley Ditch Crawley D. D. C.. Culvert 2 t Separate 6 t2 1932 " It. Crwter'" brook Crowley Y. D. C. flume 1 1 Separate 4 "2 1132 " Chart lrr., ularb 20. Trib. of R. Lee ttevenaq D. C. Flume 3 410.00 1 Separate 1 0 1533 " ek. atd 21. ryme'o break Lee Coarervaaq B. C. Veit 3 16.00 Clay MorlfIN Coaºisol 410 0 22. Salaam brook Lee Cona. rveacy B. C. Wit 3 7.1 Clay Modified tartly 610 0 Coeºlae4 from 23. Hermitage Straw Hampshire R. A. Flown 2 variable 1 0 1135-60, Water ch. natlleI Is 19ii " out boats .. rt. $ site flow p. fi. " 24. R. Adue $uaea River A. Crump 1 19.3 Modified combined 'i 2 0 1967 " I Coeºtn. 1 3 1157 - 23. 1. Tama Treat k. A. ::. tur. l 3 311.0 6 la7. ft46 fetttM beob eillwrt voter 1111 Iu bows 26. 1. Tame Treat t. A. «uurel 2 U4.0 I rt++f"*A 3 " . fettlos 1169 27. 1. Do" Mersey A Weever Crump 1 20.0 t - 1. A. 21. A. Dellis Mersey A Weever Crump. 1 22.1 1 3 1166 " R. A. ý! 29. $lnderlaad %rook Mersey 4 Weever Crump 1 17.3 t 3 1466 " R. A.

30. Kicker k. ok Mersey & Wearer Crump 1 23.4 t 3 1167 " R. A. Coventry's 31. A. $ew Severe A. A. 11uey 1 t Separate 3 1111 " Centeln. "ttlw. t "M Imported water Downstream 32. Maw Treat R.A. Mae 1 11.1 Modified Co. 16.4 2 it3$ " of "w. 64 1. wtt. ll 33. ttsur $reok tteea 1. A. staff 3 8.0 t Separat. 2 1 U62 " veekly: sSaff Gaeta

" Onute "cenracy rated o" a tbre" pint excl. With I tt restating the 1"nt saut... 92

It can be seen from Table 3.1 that there are only really two

catchments which meet the minimum data requirements, Canon's Brook,

Harlow, Essex and the River Mole upstream of Horley Mill where there are

some six gauging stations. The River Mole was rejected because of the

number and size of balancing reservoirs constructed to regulate river

flow, the geological variability of the area, and the somewhat

imprecise nature of the records from three of the gauges. The Canon's 4 Brook fulfilled all the criteria and had a number of other advantages

for the study.

The location of the catchment of the Canon's Brook is shown on

Figure 3.1. It is 5,270 acres in extent and rises from 120 feet OD at

the gauge to 360 feet in the south. It has been mapped by the Development

Corporation and field checking revealed that the maps supplied by the

Chief Engineer were substantially correct. The broadly rectangular basin

is drained by two streams, the Parndon and Todd brooks which are confluent

south of Hare Street from whence they flow northward to the Stort

Navigation Canal as the Canon's Brook.

The whole of the catchment is underlain by London Clay overlain by 4 glacial deposits. Clayton (1957) using borehole data has described the

sequence of deposits and prepared a map showing exposures of both drift

and: solid geology. Figure 3.2 reproduces part of Clayton's map and incorporates information from a Geological Survey Map of 1889 for those

parts of the catchment outside Clayton's map. The London Clay, Malden Till, Springfield Till and Hanningfield Till can be considered impervious. The lenses Chelmsford of gravels are aquifers and field checking reveals 93

I\L

V0 Stevenage

Luton Bishops Stortford and Stansted

Rothamsted Eastwick Expt. station + Old Harlow Green Hoddesdon ... "tMulberry "4* , +, on ý5 00 k o Bayford AZ Rye W ö St. Albans ill

Hemel Hempstead {:fý a Cheshunt

ý. ý ý': v, i _ _"- -. _i .ý. ` _-_ - }4"'ß' :ti$i : !. "}A __ _ -- Canon's Brook r":r: ::' r f,:..;.::":: ">:: ">:":: fi:.;; Catchment ....

Rivers ------Major ý urban areas ý :a"'"".. ý GRE ATER::. '<"-':?' '": ý>;?: 'ý ý =cam;. ° ... : ";"`": ", ": ______- Meteorological station 3 ::LONDON: o, _- _ __ mile

Figure 3.1 Map of the Canon's Brook catchment and environs.

/ý 94 Tc + HörICW Station

-{- The High"

a toaa. roller Street'

'' '`. RAY "" ." Q". ''. ' ^e°ý

ýýj, ". :: 'ý'; " ' ". ý, tire ý. ý Stewardw ., y'

''Y, Alluvium III I' I Maldon Till y" Ll. l ® x Springfield Till Hanniong6eld Till .ý II Chelmsford Grovels London Cloy Gauging Station / *Canon's Brook Catchment 040 boundary

mass J

OR 44e116 04 435057

Todd Brook Epping Long Green

R. Stort N*w Town centre 280 tt. O. D. "S London Clay -- -R : , -- ý"s" ""' 3 ý. r.. ryG "ý- 6 t2 ý London Cloy 92 ft O. D .

I. M. mili. yli. ld TIN ' t. 01d Cover Grovel 3. M. le. " Ti,i4, CU11r.$I. rl Grovels ' S. s. rie. l;.. Till G. Riviti, Grovels 0 miles 2

Figure 3.2 Geological map and cross section of the Canon's Brook. -q- -- 95 J \ý r

small seepages at the outcrop of these gravels. It seems realistic to

assume for the purposes of this study that the only source of ground is little water storage is the Chelmsford Gravels and that there or no

leakage from the catchment via groundwater. Small isolated beds of peat be little are found in the main river valleys but these appear to of

hydrological significance.

The watershed was not altered during urbanisation; the engineers lines being most careful to build their surface water drains along the foul drainage of of natural water movement. The surface water and water The foul the town were designed and constructed on separate systems. in Stort Valley. water drains to the Rye Heads Sewage Treatment Plant the into The surface water sewers discharge directly the semi-natural open 3.3 channel of the Canon's Brook and Todd Brook. Figure shows the v drain position of the major surface water sewers and the areas each

together with the year in which construction of the sewers was completed.

Unfortunately, there is no other detailed map available of the

configuration of surface water sewers in the catchment. The only other

significant modification which has been made to the hydrology of the area

has been the construction of a balancing reservoir at Netteswell. This

had a volume of 238,800 cu. feet, if its water level stood at 196 feet

OD at its completion in November 1954, by September 1970 it had a volume 6). of only 173,940 cu. feet because of sedimentary infilling. (See Chapter

The reservoir has a weir with two rectangular notches which restrict

outflow from the lake and attenuate flood peaks down stream. An analysis

of the effect of this reservoir on peak flows measured at the gauging

station is given in Chapter 5. 9.6

ý' ý_' "f"f. `ý__ ý-"i 'ý !ý 'rte=ý? __ , j. I"ýý -. ý

3 %J/, ' tom= -,

rr'" 1.11M nýrwý "

`,, Y r, "" , ha, n 1.. ," ýM. w IVA"M ýM"N %. two O S. "W"W N" C-Vo

3.3 YýrIYýiýFigure The major surface water sewers of the Canon's Brook catchment, the areas drained and the date of completion of the schemes. 97

There has only been one previous study using data from the gauging

Watkins (1956) describes "an investigation station. ... carried out

(between September 1951 and April 1953) into the relation between rainfall

and runoff" in which he evaluated the Area/Time Diagram and Lloyd-Davies

methods of calculating flood hydrographs and examined the effects of storage

and soil moisture conditions on the runoff curves. He concluded that

"The diagram the area/time method ... gave good agreement with recorded

runoff curves, provided that the calculated curves were corrected for

storage and were also corrected by means of the total impermeability

factor to make the calculated total runoff the same as the recorded total

runoff". Also that the impermeability factor showed considerable seasonal

fluctuation following roughly the trend of the soil moisture deficit,

calculated from meteorological data. While the area/time diagram method

is a suitable design technique for engineers, where impermeability

factors can be assumed, it is of little predictive value to the

hydrologist when he has to know the result before he can apply the method.

Finally, two purely practical points favoured the choice of the

Canon's Brook as the research catchment. First, it is readily accessible from London, consequently fieldwork and personal contact with the

Development Corporation and the Lee Conservancy Catchment Board situated /Second, nearby at Cheshunt, was easy. the size of the catchment is such field that surveys on foot are feasible and field measurement of channel characteristics could be undertaken.

Instrumentation The of-Canon's-Brook and ' other' Data' Sources

The gauging Canon's station on the Brook is situated at TL 432104

r i 9S

ý! '4; ý; ý: ý..

!"" ; 't .r` + ..

Figure 3.4 The gauging station on the Canon's Brook. 99

about'40 yards upstream of the Elizabeth Way culvert. The structure,

a rectangular flume, is shown in Figure 3.4. It was constiructed by the

Harlow Development Corporation in 1950 and became operational on 1st

October that year. It has worked continuously since that date and has

provided a record of streamflow for the period up to September 1968 with

breaks totalling 99 days (1.5% of the time). During most of these breaks,

an integrator was worIing which provided exact information on total water

flow even though the chart recorder was non-operational. The instrument

is of the Lea type of a rather primitive 1948 design. The drum which

carries the chart is horizontal and revolves once every seven days. The

trace is made by a7 inch long pen which is suspended from the gearing

mechanism. The chart is 19 inches long and 5 inches wide. The lower 21

inches represents the flow range 0-40 cusecs (16 cusecs per inch)

while the upper 21 inches represents 40-400 cusecs (144 cusecs per inch).

The chart revolves at a rate of 0.11 inches per hour.

The Lee Conservancy Catchment Board's doubts about the accuracy of this gauging station prior to its purchase from the Development

Corporation in 1966, prompted the Conservancy to construct a further

gauging station immediately downstream of the original one. This new

gauging station, hereafter called the low flow station, was completed in

late 1966; it provides too short a record to be of any value in this study

other than to serve as a check of the accuracy of the original station,

hereafter referred to as the gauging station. The low flow station is a Weir Crump designed to read on a9 inch chart the flow range 0-9 cusecs. Reference to Figure 3.5 shows that this flow range necessitates a head 0-9 inches of which is therefore recorded on a 1: 1 basis by the chart Twin side weirs ensure that at no flow of the Brook will there be any drowning the of gauging station by water backing up behind the low flow loo

16 15

14 Low flow Station (1966 - continuing. ) 13 High flow Station (1950 - continuing ) The full curve finishes at point 12 a : Head . 6'0' 11 Flow . 405.0 Cusecs 10 rrrr' 09 f rýrr'r 08 ýºýý 07 . ö 06 0.0 i . 00 05 00 'o wo 04 ow " ýýý 03" 0o00ý 02 / 0.1 ý/ 00 10 15 20 25 30 Rate of flow in Cusecs.

Figure 3.5 Rating curves for the gauging station and the low flow station.

I 101

station. The accuracy of the gauging station may be questioned on six

counts. First, ' as with any instrument there is the almost certainty of

some human errors. The reading of the chart by the research worker is

liable to inaccuracy especially when the line is rather thick as described

below. On several occasions during the period 1950 to 1965 when the gauge

was operated by the Harlow Development Corporation the zero of the gauge

was clearly out of adjustment. In some cases this can be detected from

notes scribbled on the chart, in others the line falls below 0.0, while

in a few the break of slope of the line where the scale of the gearing mechanism changes from 16 to 144 cusecs per inch does not occur in the proper place at 40 cusecs. When zero errors were noted and corrected no

attempt was made by the Harlow Development Corporation to correct older

affected charts and so some cases readings for a week or two may be a few cusecs in error. Other human errors which have occurred include the pen running out of ink for several days, the chart being inserted upside down and the weekly chart being allowed to record a fortnight's flow by revolving twice. Second, the gauging station tends tobe insensitive to small changes in flow for two reasons. On the one hand the throat of

flume is the 8 feet wide which means that small changes in water depth (i. head) in e. the flume result in relatively large changes in flow.

The rating curves for the gauging station and the low flow station shown in Figure 3.5 illustrate this point. In the flume of the gauging station in a change water depth from 0.0 to 0.51 feet covers the flow range 0- 10 is cusecs which represented on the chart by a pen movement of 0.6 inches. On the hand other the gauge's insensitivity may result from the 7 inch has in its pen which play mechanism and also has a propensity to in catch the paper and consequently does not move in response to. small changes in flow. Third, the narrowness of the chart adds to both 102

the inaccuracy and the insensitivity of the gauge. The dual scale facilitates the recording of very high flows of interest to the HHDC engineers but the allowance of 16 cusecs per inch in the low flow ranges is unsatisfactory when one considers that between 1953 and 1964,15 cusecs or less occurred 93% of the time (Lee Conservancy Catchment Board

Engineer's Report 1964). This poor performance of the gauging station at low flows is discussed in a later section when a critical comparison is made with the low flow station. Fourth, changes in the pressure of the pen on the chart and consequent changes in the rate of flow of ink over the nib has produced a line of variable thickness. The line is normally 1/32 inch wide (0.5 cusecs - low flow scale/ 4.4 cusecs - high flow scale) which is precise enough for most purposes. The line in some cases is 1/16 inch wide and in rare cases the line thickens to 1/8 inch (2 cusecs/18 cusecs) in which cases there is clearly very considerable likelihood of imprecise readings. The fifth source of error in the gauging station record results from the inlet to the stilling well being i some 14 feet upstream of the throat of the flume. This makes the station particularly susceptible to weed growth, sedimentary deposition and children's pranks, for all of which tend to raise the water level in the approach to the throat of the flume and consequently cause the gauge to err. During the period 1950-1965 when the gauging station was operated by the Harlow Development Corporation this type of error was more frequent in than the subsequent time when the Lee Conservancy have maintained the structure regularly. Finally, although the flume was designed to measure flows, flood on 1st July 1958 water overtopped the gauge during a flood estimated by the Lee Conservancy Catchment Board to be of 500 cusecs.

The performances of the g ging station, thought to be inaccurate 103

at low flows, and the low flow station were compare for 1967, this being

the first complete year of concurrent operation. The flow recorded by

each chart at midnight every day during the year was noted and Figure 3.6

prepared. There is considerable inaccuracy in the estimation of low

flows by the gauging station if one assumes that the low flow station

is exact. In all cases, except one, the gauging station overestimated by flow an average of about two cusecs. "In percentage terms the error

varied from +50% at 3 cusecs to +20% at 10 cusecs. Consisýt ent over-

estimation of flow by 2 cusecs for one month produces an error of +0.27

inches in the estimate of monthly water yield for the basin. This is

significant when-one considers that during the first three years of

operation of the gauge the mean monthly water yield was 0.66 inches and

the standard deviation was 0.65 inches. When shown Figure 3.6 the Lee

Conservancy's hydrologist recalled that during a regular maintenance in check 1968 the stilling well of the gauging station had a considerable amount of sediment removed from it. This accumulation may go some way to

explaining the inconsistencies discussed. It seems likely that similar errors are common to the record as a whole and consequently confidence limits ± inches of 0.3 on monthly water yields appear to be appropriate.

At the commencement of operation of the gauging station there were three autographic raingauges and associated daily read British Standard in pot gauges operation in or near the catchment. Their locations (TL 441117,

TL 455064, TL 477116) are shown on Fig. 3.1. All of the gauges were of the Dines tilting syphon type and all were fitted with a 12 inch daily On 7th October 1962 chart. the Eastwick Lodge gauge together with the rest of the climatological station was moved to the grounds of the Harlow 104

i6 10 ......

I, .. ß.f. ».. 1 ... ok"wMr+ -1 Low 11tied MOYe 1 " M"

I 1. " s" " ' . u.

I it 1... "". """ I+wc1N º«"tanNw N ill ý 9wo.. rý tunctýnwy "ecwNMy. Dot1 t"? or 'a" -Lmw lilt" IV 0"

3 1

1"1"7""DN 60 70 00 90 WO . 60 -00 do . 30 "00 "10 0 10 90 30 40 SO Flow /e. p. 0 W Npo 11wr NNeM M CWi1 ºwt1". H Nru1*u .IN. /ar $* sl. l.. w M . rw V. º... low . N. M.

..... « Nigh IOW station ý--ý L*w flow station

1) Data used for true diagram cons'stod of o sdmPr of noo. ngS lasen syatamabtaly each mdn. 9M 2) CM 1OWrfd analysis of v4se (19 was Y. ows that tnI froawncy or flows of 9 Cwsscs or tas on Conon's Groot is signl-contty affected 10 1% bevel) by lM cno. co of 9aug, np Station vied. (XI-42 68. dA a 17 )

w V

v .Z

D 70 ]o 40 7o l'k7 70 00 90 100 Percentage of tons flow oowoUN of ". cIca"

Figure 3.6 Diagrams comparing the accuracy of the stations. gauging and low flow 105

Development Corporation's Offices at Terlings (TL 448116). It was moved

(5.5.70) Sports agaiýi on 7th December 1966 to its present site at the 1 Stadium (TL 446108). The Mulberry Green and Rye Hill gauges ceased operation in April 1956. Despite the apparent continuous operation of for these three instruments, no complete record of hourly rainfall the area exists even today. A search of the files and stores of the in Development Corporation revealed records for the gauges as shown

Figure 3.7. The Eastwick Lodge/Terlings/Sports Stadium gauge has almost complete records, except for certain winter months when the recorder was

inoperative as a result of frost and years 1952 and 1953. The other two

fortunately do have gauges have much more fragmented sets of data, but they for Eastwick records of the period 1952-53 which are missing the gauge'.

The accuracy of these three gauges in assessing the rainfall over the

catchment is in doubt for four reasons. First, the gauges, being of the

standard type with their rims above the ground surface, are liable to

wind eddy, splash and other effects described by Rodda (1969). The

conceptual model of raingauge performance given by Rodda (1969) fits the

situation in the Canon's Brook and suggests potentially large errors

in the point rainfall estimates. Comparison of the catch of standard

gauges at Wallingford with ground level gauges set in pits with plastic

louvre surrounds suggests that the conventional methods consistently

under-record rainfall and in certain months may err by over 10% (Inst. of

Hydrology 1968). While the rainfall records for the Brook are almost

certainly in error, the lack of any definitive measure of this inaccuracy

precludes the modification of the data. Second, the estimation of a real

rainfall over the catchment necessitates the extension of point rainfall

measures. Figure 3.7 shows that only the Eastwick Lodge/Terlings/Sports -Ir 106

Eastwic k /Tarlings/ Sports Stodium

Rye HIM

Mulberry Green

,; - Autographic records of rainfall

Pot gouge records of rainfall (read daily at 0900 )

Figure_ The duration 6f records for the rain gauges.

i 107

Stadium gauge's readings may be taken as representative of the three gauges and therefore may be used as an estimate of catchment rainfall.. In an analysis of flood producing rainfall, which is more variable than daily or monthly totals, it seems appropriate in the interests of simplicity to take a mean of the falls recorded at all gauges which were in operation at the time. The third possible source of error in the rainfall figures results from the resireing of the Eastwick Lodge/Terlings/Sports Stadium raingauge. The Climatological Station, including the raingauge is part of the Meteorological Office's network and consequently comes under their supervision. The following extract from letter AF/M301/64/Met 0 3a describes the quality of the sites as judged by the Director General:

1It must be stressed that at each move all the meteorological

instruments appear to have been moved without overlapping observ-

ations so that no opportunity occurred of comparing simultaneous

readings from old and new sites. Neither of the first two sites

have been quite ideal as is shown by the following extracts from

our inspectors' reports: -

Eastwick Lodge Farm. Inspector's Report, October 1956. "It

is agreed that the site is. not ideal. The ground is rather irregular but has a general upward slope towards the north

of about 50 feet in a quarter of a mile. To the south it drops away about 12 feet to the road A414 some 30 yards

away. Beyond the road there are fairly flat fields with only southwards, a gentle slope the lower half of this area being

flooding from subject to the River Stort,. The enclosure itself is small, measuring about 7 feet by 27 feet. It is

surrounded by fence 2'6" a wooden high su ounted by a 108

protecting wire about 1' higher. The fence is composed of

wooden supports connected by three wooden beams about 4 inches

deep. The rainfall recorder is about 4 feet from the north end

of the enclosure, the raingauge in the middle, and the screen

about 2'6" from the southern end. It will be seen therefore,

that the fence itself obstructs to some extent both the rain-

gauge and the rain recorder. " 4

Eastwick Lodge Farm. Inspector's Report, September 1961. This

states that the general features of the station have not

changed since the previous (1956) inspection and hints that in

due course it might be possible to re-site the station at

Terlings.

Terlings Site. Inspector's Report, August 1966. "Since the

last inspection in 1961 the site of the station has' been

changed from Eastwick Lodge to a nearby site at Terlings

(approx 1 mile away). The new site although satisfactory is by

no means excellent but is apparently the best available.

Originally it was intended to move into the local park but it

was decided that'this would subject the instruments to

possible vandalism ...... due to the somewhat sheltered state the of enclosure wind and visibility observations were made at

points from on route the observer's place of work. " Thus it is not surprising that the raingauges at this site are described as "slightly over-sheltered".

Routine inspection of the is station not again due until mid-1971 109

so we are unable to comment on the present Sports Stadium site other

than to the effect that rainfall measurements at this site appear

to be in good agreement with those at other stations in the area".

The homogeneity of the rainfall record from the resited gauge was checked by using a double mass plot, Searcy and Hardison (1960), of culmulative totals of4annual rainfall at Eastwick Lodge/Terlings/Sports

Stadium and Rothamsted which is the nearest station having the necessary rainfall data. Figure 3.8 shows that the record from the Eastwick Lodge/

Terlinga/Sports Stadium gauge is homogeneous and that the gauge's resiting had no apparent effect on the monthly catch of rainfall. Finally, there are no comprehensive records of the contribution of snow to the precipitation of the catchment. Lamb (1964) shows that on average Harlow has snow lying on between 10 and 30 mornings per year, while the observer's notebook for the Eastwick Lodge/Terlings/Sports Stadium climate station shows snow to have been lying for 77 days during the period Oct 1950 - September 1958.

Since snow seems to be a relatively insignificant hydrological phenomenon in the area and because there are no comprehensive data on its occurance or

it depth, will be necessary for the execution of the study to neglect snow as far as is possible.

Data from a number of other hydrological and climatological stations has been in used the analytical work. The two gauged rivers nearest

Harlow are the Rivers Ash and Roding (Fig. 3.1). The former is gauged by broad a crested weir at Mardoc' Mill, the latter by a standing wave flume Redbridge, at and both have had gauges in operation since October 1950. Neither of these be catchments can considered to be true control catchments iio

u 4n rL G) vº u o ci w oE v'° o r_S L to

00 *" a o V)

_ rn o Er01- U3

Cumulativa total rainfall at Rothamst¢d - inches

Figure 3.8 Double mass plot of annual rainfall at the Eastwick Lodge/ Terlings/Sports Stadium site and Rothamsted. 111

for the Ash has a considerable proportion of its catchment area on the chalk while the Roding, although similar to the Canon's Brook geologically, is 117 sq miles in area and has undergone considerable urban development in its southern portion near to the gauge.

The calculation of rates of evapotranspiration which is undertaken in later chapters demands data for the monthly mean actual vapour pressure, saturated vapour pressure, temperature, sunshine and wind speed at 2 metres. The first three variables were measured at the Eastwick Lodge/

Terlings/Sports Stadium station at 9-00 a. m. each day during the period

1950-68. The values of mean monthly sunshine were derived from a mean of the values recorded by the stations operating at that time in the region,

Figure 3.1. Figure 3.9 shows that Rothamsted took sunshine records throughout the study period while other stations operated for only part of the time. The only stations in the vicinity which recorded wind speed at 2 metres were Kew and Rothamsted, It was assumed for the purposes of this study that the data on wind speed at 2 metres for Rothamsted applied directly to the Canon's Brook.

The general introduction in Chapter 1 and the literature reviewed in Chapter 6 shows that the effects of urbanisation and building activity on channel morphology and sediment yield are no less important than the hydrological results. As with the streamflow data, it was found that time and resources precluded the collection of field data on the operation of current geomorphic processes. However, investigation of the archives the Development of Corporation revealed a plan dated, 12-4-56, of the the Canon's Brook channel of at a scale of 1/500,32 cross sections at a scale of 1/240 similarly dated, and a 1/500 plan of the Netteswell 112

Rothomst. d

Chsshunt

Baytordsbury

Hodd. sdon

Harlow

Figure 3.9 The duration of sunshine records for the stations in the Harlow area.

f 113

Regulating Reservoir dated 11-8-52 and showing the floor of the

basin with one foot contours. Connunication with the Engineer's

Departments of both the Development Corporation and the Lee

Conservancy revealed that none of these features had been

substantially modified by conscious human action in the period

between the preparation of the plans and summer 1970. Field survey,

detailed later in Chapter 6, of the channel plan, the cross sections

and the reservoir capacity was undertaken during the summer of

1970 to ascertain changes from the initial state.

The Estimation of the Degree of Urbanisation of the Catchment.

The assessment of the land use changes which occurred in

the Canon's Brook catchment during the construction of Harlow

New Town is fundamental to an understanding of possible hydrological

changes. In addition, in order that relationships between the

degree of urbanisation and hydrological processes should have predictive value some measurement of the type of urbanation must be

Dade. As Leopold (1968a) has said, "our ability to talk about the degrees of hydrologic effect becomes more or less useless unless these effects are correlated with some quantitative measure of land use intensity.

The job ... is ... for a social scientist with a background in hydrologic principles to deal with the problem of intensity of use and to develop field methods which are practical for describing the intensity of use".

This section considers methods used in the past to determine urban growth

it hydrology as affects and describes the various attempts which have been made to measure the degree of urbanisation of the Canon's Brook. Stages in the intended development of the urban area are shown in Figure

3.10 which is taken fron the Piaster Plan (Gibberd 1947). Each method 111

Image removed due to third party copyright

Figure 3.10 The intended stages of development of Harlow. (after Gibberd, 1947). 115

is evaluated in terms of its merits and disadvantages and a surrogate 0 measure is given in conclusion for use during the rest of the work.

Examination of the indices of urbanisation used in previously

published work reveals a dichotomy between those which use

hydrological variables and others which employ surrogates or

indicators of true hydrological conditions. The most commonly used

hydrological variables are the area of impervious surfaces and the extent

and nature of the sNrm water sewerage system. Carter's (1961)

empirical equation for flood flows had as its only "urban variable",

the percentage of the catchment made impervious. Espey, Morgan and Masch

(1966) incorporated within a version of Carter's equation a channel rough-

ness factor (1), which was determined by informed guesswork, but did represent

the three main categories of channel type. Watkins (1962), in an

evaluation of methods of calculating sewer discharge, suggested that

only those impervious surfaces which drain directly to the sewer system

should be considered, because water draining from other impermeable areas

has to cross pervious surfaces and so is effectively lost to the quick

draining sewer system. He appears to have measured these hydrologically

significant impermeable areas from large scale plans and maps. Most

of the U. S. Geological Surve studies, for example Harris and Rantz (1964), (1969), Seaburn Crippen and laanen (1969), Wiitala (1961) and Anderson (1967), employed "impervious area" or "percentage of the catchment paved"

as variables in their work and in each case aerial photographs were used

to assess this area. The only published British work in the field, Walling (1970), and Gregory employed detailed field mapping methods. It appears that the construction of each house was carefully monitored

and the exact date of cor}nection of its surface water sewers was noted.

i 116

Leopold (1968b) has reported an indirect method for estimating the paved area by employing data on the percentage of an average

"lot" rendered impervious by residential development. Ile quoted the work of Antoine who suggested that a 6,000 sq. ft. site is likely to be 80% impervious, whilst a 15,000 sq. ft. lot is only likely to have 25% of its area paved. Felton and Lull (1963) have quoted comparable figures for the suburbs of Washington but Stanowski (1972) has extended these studies and developed a method for determining the paved area for various urban and suburban land-use categories using readily available population figures as the only independent variable. His formulation was based on correlations between population density and the proportions of land area in each of six urban and suburban land-use categories. The proportions of different land- use categories were, in turn, weighted by the average percentage of impervious area found in each land-use category. Illustrations were presented for New Jersey and Stankowski argued that despite the inherent averaging processes used, the method was inexpensive and rapid. His approach is laudable but his results are not appropriate to the British situation because of the wide differences between land-use practices. A comparable British method of equal efficiency in the U. K. and North America is that employed by the Meteorological office in the estimation of land use for their forecasting of soil moisture deficits.

For urban catchments they assume that the paved area is 25% of the area shaded grey on the O. S. Seventh Series One Inch maps (Meteorological

Office undated).

The extent and distribution of surface water drains is a second 117

major "urban variable", but it seems to be one of particular relevance to the American situation where not all new developments are provided with underground storm water drains and street-side swales and ditches are often the only form of surface water drainage. In many cases the actual length of sewers or mean length of overland flow to sewer inlets was used, e. g. Watkins (1956), Espey et al (1966), James (1965); in U. S. Geological Survey publications the use of the percentage area served by storm sewerage is favoured without regard to their extent or disposition. This latter variable is clearly easy to measure, and is shown by Seaburn (1969) to be closely related to changes in streamflow. The use of this variable is at odds with

Watkins' (1962) view that only hydraulically connected impervious surfaces should be considered. Riley and Narayana (1969) have attempted to incorporate both paved area and a measure of drainage efficiency into their mean characteristic impervious length factor,

Lm, where,

a. 1. m ra. i and 11 is the length of trave to the basin outlet from the centre of

the ith impervious area

ai is the area of the ith impervious area.

The surrogate measures of urban development are more closely related to the design and construction of foul water systems (Grava (1969)., Savini Kammerer (1961)). and A number of the variables have been in hydrological used studies, usually because of their ease of estimation. Population, with its direct relationship to buildings impervious and thereby area in residential areas, has been used by Seaburn (1969). Harris and Rantx (1964), counted the total number dwellings in of completed the catchment, whilst Earl Jones (1970a and 118 b) suggested the use of the Federal Housing Administration's "Land Use

Intensity Scale". This is based upon the ratio o the floor area in buildings to the total area of ground and as such it is analogous to the British "plot ratio" which is used to evaluate the suitability of proposed developments. The Land Use Intensity Scale is of acknowledged value n determining the degree of urbanisation but it is of little value in hydrological studies since it is related more to the number of storeys in buildings than to their areal extent.

In the Canon's Brook study, the total area of imperious surfaces and the area of impervious surfaces drained by surface water sewers has been assessed by means of air photographs and large scale plans.

The sewer network of the catchment has been charted and measured with the aid of Development Corporation Plans, whilst surrogate measures of urbanisation have been derived from population statistics, records of the completion of dwellings in the basin and one inch topographic maps using the Meteorological Office method.

An exhaustive inquiry into possible sources of air photographs of the basin revealed five sets. The Ministry of Housing and Local

Government were able to supply a set of 1: 10,000 vertical air photos in the form of stereo pairs. These were flown by the RAF (Sortie

106/UK/1565) in June 1946. Essex County Council provided a similar set of photographs taken by Huntings (Sorties 5280) in May 1960, and were able to release a 1: 10,000 mosaic made by Huntings from stereo pairs taken in August 1965. Harlow Development Corporation have not undertaken any systematic coverage of their area by vertical air

They have, instead, photographs. concentrated their efforts upon the large production of a number of low and high oblique photographs of varying 119

dates of sites of special interest. Finally, the Department of

Geography, University College London, made available some

1: 50,000 stereo pairs taken for the G. L. C. Standing Conference on

Planning in South East England in 1968.

Two of these five sets of photographs were found to be of little value to the study. The obliques did not cover housing areas away from points of interest and it was extremely difficult to prepare maps from the prints. The 1: 50,000 1968 photographs were of such. a small scale that it was not easy to distinguish individual houses or footpaths even when considerable magnification was used. Further, when dot sampling was attempted, the points tended to cover several land types in spite of their minimal diameter.

The three remaining sets of photographs made it feasible to estimate the percentage of the catchment covered with impervious surfaces and to calculate the rate of change of this and other land uses. The calculation of the area of impervious surfaces which drain directly to the streams was possible when maps such as Figure 3.3 were used in conjunction with the photographs. Reports from the Chief Engineers of the Development Corporation, the Harlow Urban District Council and the

Epping and Ongar Rural District Council, the three authorities responsible for drainage in the Canon's Brook catchment, revealed that before the initiation of the new town there had been no major surface water sewers for the rural communities in the basin and that paved areas had drained

less directly more or onto permeable surfaces. This situation has not

designated changed outside the area up to the present (12.11.1970) day.

Consideration of the literature describing spatial sampling, incisively 120

reviewed by Berry and Baker (1968), leads to the se ection of a stratified unaligned random dot sampling method (Figure 3.11).

There are four reasons for selecting this particular technique. First, the method ensures that all types of land use are represented in the re'ults. Non-stratified sampling with dots might have resulted in concentrations in one part of the map while line sampling might have been biased by the lineation of houses and roads. Second, the unaligned ý Qp N dots; ensured a fairly/ distribution over the squares. Thrd, the constructionIhI of the sample frame was quick and easy with only 74 random co-ordinates being generated and subsequently located on the grid. Dot or line sampling would have needed far more points. Finally, the method was objective and accurate within the limits of operator variance. The dots were tiny in comparison with small-scale features in the photographs and therefore interpretation was not difficult. If line sampling had been attempted many of the distances to be measured on the line would have been beyond the accuracy of dividers and a millimetre ruler. Similarly, planimeter survey would have been grossly inaccurate in view of the large number of tiny areas (houses) to be measured. To execute the sampling, the watershed was traced from a six inch map onto Eflon, and this was superimposed over a sheet of centimetre squared graph paper to provide a grid. Squares of 100 sq. cms. were chosen because each covered about half a square mile of the catchment and they were convenient in use.

They located were by using random co-ordinates for the south west corner and those that had all or part of their area within the catchment were included in the sample. Fifty squares were located in this way and each was given a key number from 1 to 50. At the same time 50 computer sheets were given corresponding numerical codes. On a single 10 cm. 121

1ý

Ei 1:

" 00 de i -

00 rr 1% -ft Ei] - i Random unaligned dolt wlthln eachth randomly located square.

--- Cotthment Boundary

Figure 3.11 The sampling design for the land use analysis. 144

fashion. square of graph paper 23 dots were located in a random unaligned

The first random number determined the x co-ordinate of point one, the second random number determined the y co-ordinate of point one and also determined the x co-ordinate of point two. The third random number the y co-ordinate of point two and the x co-ordinate of point three and so on. The, 23 points were then traced onto the Eflon so that each of the randomly located squares had 23 points within it. Each of the squares had its points numbered 1 to 23 and corresponding coding was given to the 23 lines of the computer coding sheet. There were therefore 1150 points in the samile, but only 947 of these fell within the catchment and so this latter figure gives the true sample size of this survey.

Figure 3.11 shows the sampling design in its complete form. In order to optimise the use of time a second pilot survey was undertaken to determine the minimum number of points which must be surveyed to give a true measure of land use in the catchment. It was felt intuitively that 947 was too many but the exact number needed could only be discovered by statistical analysis. A full survey of all 947 points for the 1965 mosaic of the catchment was undertaken using the classification shown in Table 3.2. This classification was devised to fulfill the needs of subsequent analytical work and also to facilitate rapid and accurate air photo work. Since all the photographs were taken in summer, the categories reflect land use at that time of year. Cognizance fact of this has been taken in later chapters.

r 123

Table 3.2.

The Land Use Classification used 'ire 'the 'Air ' Phdtd Surveys

Land Use " Codel

Buildings 1

Roads & Footpaths 2

Other Impervious Surfaces 3

Gardens & Nurseries 4

Root Crops 5

Cereals 6

Woodland & Hedgerows 7

Grassland 8

Water 9

Bare Soil 0 4

1A '*' 1:, 2, subscript of with codes ""and 3 showed that the paved

area was not connected by surface water sewers to the streams. 124

For,, each of the points the code was noted in a single column of the coding sheets with subscripts in the next column, and then a d4 a card was punched with the code numbers, grid referrnce on the sampling grid, and the land use code, for the point on the 1965 mosaic survey. The results of this survey, neglecting subscripts, are shown below in Table 3.3 in the column headed 1965. Since it was felt intuitively that this 947 point 'survey represented the land use of the catchment correctly the results in column "1965" of Table 3.3 were taken as a base for comparison in the subsequent analysis of the sample sizes. From the grand total of 1150 data points, samples were taken systematically. For example in the cases of samples 1965

B, C, D and E every fifth card was included in that sample. The results of this sampling from the large 1150 sample can be seen in Table 3.3. '-.

Chi-square analysis reveals that when a survey is undertaken

464 481 with or points then the results are not significantly different from the expected value (as represented by 1965) but that when smaller samples are taken the results are liable to distortion to an unacceptable degree by random processes. It was concluded that it was not necessary to survey all 947 valid points in subsequent analyses but that half exactly this number, in other words squares 1 to 25 inclusive, would suffice.

The for stereo photographs 1946 and 1960 were then analysed using the same sampling design 1 and squares to 25 only. The sample points were located on the by photographs viewing a six inch map with the sample design it superimposed over and a single photo simultaneously using a free standing stereo viewer. It was generally easy to locate 125

Table 3.3. Results of the analysis of the "1965 Air Photo Survey"

to determine the minimum acceptable sample size

Code 1965 1965A 1965B 1965C 1965D 1965E 1965F

1 106 59 15 21 24 24 47

2 82 38 13 11 18 20 44

3 19 9 6 5 3 1 10

4 77 35 21 14 11 15 42

5 42 20 5 9 9 12 22

6 162 80 33 40 32 29 82

7 138 65 25 30 25 27 73

8 274 137 62 53 55 54 137

9 3 2 1 1 1 0 1

0 44 21 9 8 9 6 23

Total no. of valid 947 "464 190 192 187 188 481 points

l 2 Chi Sq. 1.6 7.7 4.7 2.1 7.8 1.7

Acceptable yes no no no no yes sample

1. The null hypothesis is "There is no significant difference between 1965 and the subsamples. "

2. The critical value of Chi Sq. at the 99% level with 9 degrees of freedom is 2.09. 126

the points on the photo and if there was any doubt about the land

use thin the point was viewed in more detail using a pair of

photographs. The only place where the location of the points was

difficult was in the densely built up areas of the new town. Here

it was decided that so long as three places around the po nt could

be superimposed from map to photograph then the sample point would

be considered accurately located. The variation of scale over the

air photographs made this task difficult in those cases where the edge

of the photograph was being employed. The preliminary results of

these three surveys, neglecting subscripts, are shown in Table 3.4

in the columns headed 1946,1960, and 1965. While most of the values

are logical, two points need to be examined. First, there appears

to have been little expansion of the impermtable area between 1960

and 1965. Second, the 1960 figure for woodland appears abnormally

low, while that for grassland appears peculiarly high. In order to

the the ., evaluate reliability of the survey technique and to check

doubtful figures mentioned above two resurveys were initiated.

The first resurvey involved a second analysis of squares 1 to

25 on the 1965 mosaic. The same computer coding sheets were used but

in order to ensure an impartial second survey the sheets were placed in a large envelope with holes cut in it so that the codes could be

entered and the grid references and code numbers of the points checked

without the operator seeing the result of his first survey. The second involved resurvey the use of a new sampling design with the 1960 air Here photos. a systematic grid of 16 points in a square 10cns x 10 cros was

drawn on a piece of Eflon and this was positioned centrally over each photo in turn. Viewing a pair of photographs enabled one to see a 127

Table 3.4 Land Use Study Results

Land use 1946 1955 1960 1960A 1960ß 1965 1965G 196511

Bare soil 1.3 --- 0.0, 2.7 0.9 4.6 4.4 4.6

Buildings 2.1 4.7 9.5 '. 11.4 10.5 11.2 11.5 11.3

Road 3.6' 5.4 7.6 4.0 6.9 8.6 7.6 8.3

Other imp. 4 Surfaces "0.6 0.4 2.3 1.8 2.2 2.0 1.3 1.7

Domestic Gardens and Nurseries 2.7 5.9 6.8 6.3 6.6 8.1 8.7 8.4

Root crops 16.5 ---- 4.2 2.7 4.0 4.4 2.1 3.7

Cereals 33.7 - 19.1 18.0 18.6 17.1 15.4 16.6

Wood and 1, Hedgerows 11.1 10.1 7.0 14.8 19.7 14.6 7.8 12.4

Grassland 27.5 ---- 41.0 38.4 40.0 28.9 41.1 33.1

Water 0.8 1.2 0.4 0.0 0.3 0.3 0.0 0.2

Sum of all Imp. Surfaces 6.3 12.5 19.4 17.2 20.0 21.8 20.4 21.3

Total No. of points surveyed 461 461 461 223 684 947 472 1412 128

three dimensional picture with a set of sixteen black dots on it. So long as the points fell within the Canon's Brook catchment then the land use was determined according to the classification shown in Table 3.2. The results of these resurveys are given in

Table 3.4 in columns headed 1960A and 1965G.

It can be seen from Table 3.4 that these resurveys, far from resolving the abnormalities of the earlier surveys have highlighted some of them. The spuriously high figure for impervious area in 1960 fell from 19.4 to 17.1 thanks largely to the percentage of the catchment covered by roads falling by almost 50%. The peculiarly low figure for woodland in 1960, has in 1960A been transformed into one which is exceptionally large. The differences between the 1965 and

1965G survey result largely from the greatly reduced area of woodland and the enlarged area of grassland. Since both 1965 and 1965G had been undertaken with the same sampling design on the same photograph the points should have fallen on exactly the same location each time.

It must be appreciated that this ideal could not be attained because the dots were only 0.3 ums in diameter and since the Eflon could not

Ir-refitted to this tolerance an approximation had to suffice. In

Table 3.5 there appears a cross tabulation of squares 1 to 25 of 1965 against 1965G. The number of points which fell in each land use category each time being shown. If the sampling design had been if perfectly relocated and there had been no errors of interpretation in either survey the one would expect all the figures away from the principle diagonal to be is zero. It considered that the small

V 129

Table 3.5 Cross Tabulation of Surveys 1965 and 1965C

Column 123456789 10

Row Codes 0 1 2 3 14 5 6 7 8 9 1965

0 12 1 2 2 0 1 1 0 1 0 1 1 25 5 4 11 0 0 4 4 0 .3 2 3 3 16 1 4 0 3 0 6 0 4 3 0 4 0 0 0 0 1 0 1 0 5 4 0 10* 6 1 14 0 0 2 8 0 6 5 0 0 0 0 0 9 0 1 0 0 7 6 0 0 1 0 1 3 64 3 1 0 8 7 2 1 3 0 2 0 3 21 4 i 9 8 5 3 10* 3 8* 4 8 25 27 i

10 9 0 0 0 0 0 0 0 0 0

1965G

* This indicates figures discussed in the text. 130 1'

figures which occur away from the principle diagonal are due to

imprecise relocation of the Sampling Grid and that the larger figures,

which are starred, are largely due to mis-interpret1tion. The figure

10 (Col 2 Row 5) results from mis-interpretation slight movement of the

sampling grid which transferred points from-a houses garden to its centre structure, and variations in the definition of the of a dot when

there was doubt whether the point fell in the gardei or on the house.

The figure 10 (Col 3 Row 9) probably results from mis-interpretation.

of dots which cover part of a very narrow road or footpath and the surrounding

verges. The figure 8 (Col 5 Row 9) probably results fron both mis- I interpretation and the only case where the land use classification does

not contain mutually exclusive categories, i. e. it is possible to have

grassland in domestic gardens and nurseries. The most disturbing figure

in 'able 3.5 is the 25 (Col 8 Row 9) which suggests some fconsiderable f confusion of woodland including hedgerows and grassland in the two surveys. This must be considered the result of mis-interpretation This error cannot be. readily explained for it is easy to distinguish the two land types of use on: -photographs of a scale of 1: 10 000 when 6x magnification is It used. may be argued that these two surveys represent the first and last be the to undertaken and that the intervening period produced some in subtle change the researcher's ideas. Alternatively, although the done independently resurvey was of the first survey, the knowledge of the first abonormalities of the survey biased the results of the resurvey by producing sub-conscious distortions in the operator's interpretation. In the light of the problems which have been discussed in the preceeding there is for paragraphs a need a radically different interpretation of the photographs which would benefit from the experience gained. Lack of time and resources precluded such further work and it was decided that the 131

results so far derived must suffice. In order to make the most of the data collected from the 1960 and 1965 photographs, the two surveys of each set of photographs were combined to give the total numbers of points surveyed as high a value as possible. The results of this analgamation are given in Table 3.4 in columns headed 1960B and 1965H.

One-. further source of land use data has been used in the preparation of Table 3.4. The 1955 data shown has been derived from the most recently published O. S. map of the area at a scale of six inches to the mile. The present Chief Regional Officer of the Ordnace Survey supervised the revision of the map and gives the assurance that those features shown by solid lines represent structures extant in Spetember 1955 and that those shown by dotted lines represent construction activity with at least the foundations finished. The sampling design was superimposed over the nap and the land use recorded on th coding sheets for squares one to twenty five only. The classification sed was as in Table 3.2, except that codes

0,5,6 and 8 were all recorded tinder 8 since they were indistinguishable on the map. The figures for roads and woodland given in Table 3.4 are not

entirely as they were recorded by the survey. The original figure of

10.42 for roads was artificially inflated by the fact that the map does depict not the limits of the asphalt but the fence lines. A study of the 1946 roads on the air photos revealed that the asphalt usually occupied distance 50% of the between[the fences and that in the vast majority of the lined cases roads were with hedgerows. On the basis of this further 50% has been deducted analysis, from the figure for roads and has been added to the figure for woodland and hedgerows. 132

The results so far presented in Tables 3.3 and 3.4 have been

in so far as th: ey neglect the subscripts which record preliminary i that the impervious surfaces are not connected to the streams by surface drain water sewers. Assuming that half of these unsewered surfaces onto

domestic gardens and nurseries and that the others drain onto grassland,

Table 3.6 presents the final results of the air photo analysis of the land use. The data in Table 3.6 does not represent full picture of four imprecise changing land use of the catchment, it merely gives rather in cross sections. The nature of the problems discussed the preceeding

isineeded. Estimates sections shows that a continuous record of change

of the continuous change of land use have been derivJd by linear

interpolation between the figures for the four surveys except where there

is evidence of a non-linear trend. A study of the Architect's

Quarterly report on the progress of building contracts showed that no dwellings

were completed in the catchment before October 1953, consequently there 1946 was no change in the impervious area or of domestic gardens between

and 193. A few lengths of road were completed before this date (Watkins

1956)/-but there appears to be no detailed, records and so hoLsing completions be must taken as an indicator of the connection of paved areas to the streams.

Appropriate modifications to the trends in other land uses during this so period were made as to ensure that the sum of the percentages of the

catchment under each land use came to 100%. - Linear extrapolations extend

the record from 1965 to 1968 in all cases except impervious surfaces and

woodland. Here, because of intuitive reasoning and general knowledge of

the situation, paved areas have expanded more rapidly than before 1965

and woodland less rapidly. These linear interpolations and extrapolations

do not represent the real trends in land use in the catchment, but in the 133

Table 3.6.

Results of the Land Use Surveys (Figures represent the percentage of the whole catchment in each land use)

(Land Use 1946 1955 1960 1965

Paved areas drained surface water sewers 04 14.5 19

Domestic gardens and nurseries, incl. some non-sewered paved areas 6 9 9 10

Root crops 17 * 4 4

Cereals 34 * 19 16

Woodland and hedgerows 11 10 10 12

Grassland, incl. some non-sewered paved areas 31 76* 43 34

Open water 1 1 0 0

Bare soil 0 * 0.5 5

*See text

i 134

absence of any substantive data are a good approximation.

A second attempt to measure the growth of the paved area in the catchment was made using 1: 2,500 plans prepared by the Architect's

Department. These plans were prepared at intervals for each of the neighbourhood units. The plans depict every building in the unit including those which pre-dated the new town, the true width of the roads and footpaths and other impervious surfaces such as car parks. The basis of the schema was that from the plans the total acreage of impervious surface in each neighbourhood unit could be estimated and a mean paved area per dwelling could be calculated by dividing the total area paved by the number of dwellings. The total area of impervious surface for the catchment, month by month, may then be calculated by summing the products of mean paved area per dwelling and the number of completed dwellings per neighbourhood unit.

There are three main points to consider here, the foundation for the conceptual

framework, the method of measur ng impervious area and the estmation of

the number of completed dwellings.

This study assumes that the rate of expansion of the impervious

surface is directly proportional to the rate of increase in the number of

is dwellings. This a necessary approximation to reality.. In most cases,

the Engineer's Department completed work on sewers and some roads for each

before housing area there ws any extensive construction work. Further, within any neighbourhood unit the shopping centre was assumed to grow linearly

during the main phase of dwelling construction. The major shopping centre, High, The was assumed to grow linearly throughout the period Oct 1953-Sept

1968. The New main roads of the Town area were( shown on the Architect's 135

Plans baut it was thought unrealistic to include them with any particular housing area. Instead the mean width of the main roads was calculated from a systematic sample of 68 road width measured from) 'the plans. The total area of imperviousness due to roads was the product of the total road length and the mean road width.

The, rate of growth in the absence of any firm data was assumed to be linear. 4

The area of impervious surface in each' housing area was estimated using a line sampling technique. A pilot study was undertaken on Potter Street,

Figure 3.3, to determine the minimum sample size required. A grid with squares 10cros x l0cros was superimposed over the 1: 2500 plan and the total length of each line falling over impervious surfaces was measured.

The length of each line between the margins of the area was also measured.

The percentage of Potter Street covered by impervious surface was then calculated as the length of line covering impervious surfaces divided by the total length of the lines all multiplied by 100. The same calculation was then undertaken using only East-West or North-South lines.

The results are shown in Table 3.7.

The pilot survey shows that it is not necessary to take measurements

from every line on the grid. The pilot survey also shows that there is

no significant difference between E-W and N-S lines. This may be real

it be or may a result. of Potter Street having few roads and rows

of houses N-S E-W. aligned or In the surveys of the other parts of the

catchment it was decided to survey every other line including ones in

both N-S E-W and alignment. The results of the survey of the other parts

of the new town are shown in Table 3.8. 136

Table 3.7.

The Results of the Pilot Survey of imperviousness in Potter Street using a line sampling method Total Total length over Percentage Impervious Length impervious surfaces impervious per house All lines 5013mms "891mms 17.77 0.034 acres

E-W lines 2504mms 455mms 18.17 0.035 acres

N-S lines 2509mms 436mms 17.37 0.034 acres 137

Table 3.8. "

Results of the survey of the paved areas of neighbourhoods in Canon's Brook Catchment. Harlow.

Total Area Surveyed Percentage Area of Total No Imperv/ Area Acres Impervious Impervious Dwellings Dwelling Acres Acres Stewards 128 23.65 30.3 1967 0.015

Passmores 221 22.83 50.5 1985 0.025

Tye Green 258 22.60 " 58.0 1586 0.0366 Great Parndon 264 18.80 49.7 988 0.050

Latton Bush 322 16.87 54.5 1403 0.0384

Brays Grove 189 26.40 48.5 1530 0.032

Potter Street 193 17.77 34.0 1108 0.034

Hare Street 173 20.00 34.6 1353 0.026

Little Parndon** 63 16.00 10.0 385 0.026

Netteswell** 195 13.40 26.2 1065 0.025

Mark Hall South** 82 22.30 18.4 779 0.024

Kingsmoor 276 23.45 64.9 1812 0.036

e* Only the parts of these areas which fall inside the Canon's Brook catchment were considered. 138

The variations in the mean acreage of impervious surface par

dwelling in the various units is considerable. It is due largely to variatipns in the type and density of properties in the various estates, but errors may derive from the sampling method and from data

inadequacies.

'The data used for the number of dwellings in each aria and the

total completed dwellings during the study period were taken fron the contacts Quarterly Reports on by the Architect's Department. These are available for every quarter from December 1949 to September 1961 and

then for every six months until }larch 1967, the period June 1967 to

September 1968 is covered by unsubstatiated estimates of building

completions. The reports give for each of the neighbourhood units the

total number of dwellings completed under Harlow Development Corporation

aegis or by private developers building on land sold by the Development

Corporation. They do nct include dwellings constructed by either the

Harlow Urban District Council or Essex County Council, but the numbers

involved in these latter categories are small. Table 3.9 gives the total number of dwelling completions and the associated aggregate acreage of paved surfaces calculated from the Architect's Plans. The final average of 585 is in accord with the 640 acres given by Watkins (1956) as the paved area on completion of the development.

The population of an area must be a measure of the degree of urbanisation of that area at least as far as western civilisation is concerned. To explore this relationship an-attempt has been made to measure the

Canon's population of the Brook catchment during the period 1950-1968. The 1951 census was taken at a time when none of the Canon's Brook catchment had been built over by the new town and therefore the districts for 139

Table 3.9 The Total number of dwellin s comp1etedlannuallyand the

associated acreage of paved surfaces calculated from the

Architect's Plans

Date (30 September) Completed Dwellings Paved Area in Acres

953 422 4.4

1954 1137 50.2

1955 3181 110.5 4 1956 4522

1957 5760 1159.8209.8

1958 6603 250.9

1959 7333 287.5

1960 8383 334.5

1961 9383 379.6

1962 10 136 416.6

1963 10 510 439.2

1964 11 127 469.8

1965 11 800 485.6

1966 12 441 508.7

1967 13 736 550.8

1968 14 663 584.7 140 which the information is presented bear no relation to either the catchment boundary or the present divisions of the New Town.

In the /961 census some population data for each of the neighbourhood units of the New Town in 1951 are recorded. Even from these it is not possible to estiuate precisely the population of the basin in

1951. / It has been assumed that the population in the rural part of the / atchment outside the designated area was equal to thý population living outside the catchment but within neighbourhood units which straddled the watershed. On this basis the population of the catchment in 1951 was 2,030. The figure of 35,083 quoted for the 1961 census again excludes the rural. part of the catchment. The neighbourhood units which transgress the divide have had their total population split into portions living inside and outside the catchment in proportion to the number of dwellings inside and outside the catchment. The

1965 figure of 40,274 is a Harlow Development Corporation estimate. based upon the total number of dwellings in the catchment multiplied by 3.5 which is the mean family size in Harlow determined by them in a special study. The 1968 figure of 51,320 is based upon the number of dwellings and a mean family size of 3.5.

Finally, the Meteorological Office technique of utilising the areas shown grey on 1" maps was applied to the O. S. 1" maps of the area. Figure 3.12 shows the extent of grey on the Seventh Series (Complete Revision 1961-62, Published 1964) 1" map of the Canon's

Brook and the area of building (black printing) on the Sixth Series (Published 1940) map of the area. Analysis, by counting the number of millemetre squares more than half filled with print when a grid is figure, superimposed over the shows that urbanisation covered 0.4% of the catchment in 1940 and 5.6% in 1961-2. These figures are not direct of course measures of urbanisation for the gray or black printing 141

Image removed due to third party copyright

The Built-up Area (Black Shading) from the O. S. 1 inch map (Sixth Series) published in 1940

Image removed due to third party copyright

The Built - up Area (Grey Shading) from the O. S. 1 inch map (Seventh Series) published in 1964

S. Figure 3.12 The urban areas of the Canon's Brook according to the O. one inch maps of 1940 and 1964.

_,x 142

is a symbolic representation of a built up area, but they do

represent an easy, rapid and fairly representative method of determining the degree of urbanisation of ä river basin.

A summary table of the various measures of urbanisation discussed in the preceeding sections is given in Table 3.10 the

lower part of the table shows the rates of growth of the urban area with 1968 as the base year and a value of 100. Three major points emerge from the table and the associated discussion.

First; the productivity of the five methods varied greatly. Method

1, population figures, cost nothing and took less than a day to

complete. The figures, however, are approximate and of little direct hydrological significance. Method 2, air photo analysis,

required the purchase of over £20 worth of photographs and took many weeks of painstaking work. The sources of the data are definitive and the results are influenced only to a small degree by operator

variance. Method 3, the Meteorological Office technique, is both quick and cheap but lacks objectivity for a small scale study of this type. It appears to be more appropriate to larger areas but the reduction factor on the basis of the results of this work might be raised to 304". Method 4, dwelling completions, was inexpensive and took only a few days Quarterly once the Reports had been located and secured. figures The however provide only an indirect estimate of hydrologically significant variables, for areas like the town centre, industrial estates included. and roads are not Me od 5, Architect's Plans of neighbourhood units, provide one of the best stimates of the growth of the paved The data definitive, area. sources are all types if impervious surface are included, the and technique provides information on the continuous spread of urban development Method unlike 2 which gives only a series of cross sections. It has the advantage of low cost, under E40 but takes some time to undertake. "1 143

Table 3.10 The Urbanization of. the Canon';, Brook sA Suruary Table

Method 1940 1946 1951 1955 1960 1961 1965 1968

1. Population resident in the basin --2,030 -- 35,083 40,274 51,320

2. Paved area, which is sewered, as - 0.0 - 4.0 14.5 - 19.0 22.0 a percentage of the catchment (Photos and 6" Map)

3. Percentage of the catchment paved, 0.4 ----3.5 -- by the Met. Office method

4. Total number of dwellings 000 3181 8383 9383 11800 14663 completed in the catchment, under HDC aegis

5. Paved area as a percentage of 0.4 0.0 0.0 2.4 6.6 7.2 9.3 11.1 the whole catchment (Architect's Plans)

Tha. Growth. Rates .. of Urbanization. 1968-100

Method

1. --4- 68 79 100 2. -0 18 66 - 86 100 3.

4. 000 22 37 64 81 100

5. 000 22 60 63 84 100

4 144

Second, the rates of expansion of the urban ar a, shown in the lower part of the Table, are approximately the same for all the methods. However, the intercorrelation of methods 1,4, and 5 by way of dwelling completions should not be forgotten. Third, there appears to be aIsignificant discrepancy between the extent of impervious surfaces as measured by methods 2 and 5. This could have resulted from one or all the following: pperator error, incorrect sampling, the Architect's

Plans/ being modified after completion of the maps, incorre}t housing numbers or acreages, or operator bias. It is proposed to take a mean

of-the two estimates of paved area and use this throughout the rest of the study. The changes in paved area, required by this averaging process, will be divided equally between do; estic gardens and grassland. ', ti Table 3.11 and Figure 3.13 give the finalised land use analysis.

The Methodological and Conceptual Basis

The preceeding discussion has shown the possible methods which exist for tackling the problem of the hydrologic impact of urbanisation and the nature of the available data. A study of urbanisation presently in progress was rejected on practical grounds. The choice of the historical approach using secondary data sources was vindicated by data. the mass of The choice of a. single catchment study of the Brook justified Canon's was for three reasons. First, the instrumentation

basin of the was good by the standards of the other catchments investigated

the basin fulfilled and all the. qualijZying requirement concerning foul and surface water drainage, watershed stability, substantial urban growth Second etc. a multi-catchment study would have been severely hampered by the complicating effects of varying geology, size, climate, shape, 145

Table 3.11- Final Land Use Analysis

(Figures represent the percentage of the whole catchment in each land use. )

------N------N-N - N-N ---- Ný-M- 1 LAND USE '1946 '1955 1960 1965

------N-- MNMN-- MýýN1ý-N- --- N --N ý-ý

Paved Areas drained by. surface water sewers. " 0.00 3.2 10.6 14.2

Domestic Gardens and Nurseries, including some non"sewered paved areas. 6.0 9.4 11.0 12.5

Root Crops. 17.0 * 4.0 4.0

Cereals. 34.0 * 19.0 16.0

Woodland and Hedgerows. 11.0 10.0 10.0 12.0

Grassland including some non- sewered paved areas. 31.0 76.4* 45.5 36.5

Open water. 1.0 1.0 0.0 0.0

Bare soil. 0.0 * 0.5 5.0 14 6 ,'

100- Bare soil and open water go- Grassland 801 a

70-

60 Woodland and hedgerows 4-9 50- Cereals I 40- 4) Root crops c3130- Domestic gardens 51-15 and nurseries 20 Paved area

1950 52 54 56 581960 62 64 66 68

Figure 3.13 The land use of the Canon's Brook catchment, 1950-1968.

ý. r 147

initial land use etc. It was thought that the effects of urbanisation would have been lost in the complexity of these other factors. Third,

the amount of data, streamflow charts, rainfall recorder sheets and

related information available for the Canon's Brook was considered

about the maximum which could be handled by one person undertaking

aLthree year project.

basic The conceptual basis of the study rests on two principles.

First, comparison of the hydrology of the catchment efore urbanisation with that during urbansiation and especially during he latter part

of the study period will reveal if urbanisation had any effect at all

on the movement of water in the catchment. This simple "before and

after" principles can be applied in many ways, but basically it aims

to compare an initial state with a subsequent one. The geomorphological

analysis of Chapter 6 aims to make a direct comparison between the bed post-urbanisation1 state of the river channel and the reservoir

and tIat which existed in the early 1950s. Some of the mole primitive

traditional analytical techniques used in Chapter 4 simply compare

before and after figures. The double mass analysis and multiple

regression analysis of Chapters 4 and 5 which aim to define relationships

appropriate for the catchment in its rural state and then to apply

them to the subsequent period when the land use was undergoing modification

are on a slightly higher conceptual plane. The most sophisticated pieces

of conceptual thinking involves the computer simulation work described

in Chapter 4, and the time series analysis in Chapter 5. In Chapter 4

it be will argued that the computer simulation model fitted to the

flow water yield and regime data for the calibration period Oct 1950

September in to 1953 acts a manner directly analogous to the catchment in its rural state. The predictions made by the model for the ensuing 148

period 1953-68 represent what would have been the h drology of the catchment had no urbanisation taken place. It is a gued that when the urban factors discussed in the preceeding secti n of this chapter are incorporated into this rural model, the model calibrated against the period 1950-60, and the model'tested against the period 1960-68 then the changes which were necessary in the rural model to make it simulat7 the urbanising situation faithfully are directly analogous to the changes in the processes which took place in reality. The time series methods of Chapter 5 attempt to build a stochastic statistical generotor of flood sequences in the belief that within the historical recoýd of floods for the Canon's Brook there are four components. , These are the trend resulting from the urban development the periodicity from the annual cycle of seasons, the persistence from storage in the basin and persistence of weather types, and finally the stochastic element results from the. chance factors governing storm types, their duration and intensity, the antecedent precipitation condition etc.

The intention here was a time series simulation to produce flood frequency curves for the rural period and each of the 15 degrees of development associated with the subsequent years. 149

Bibliography

Anderson, D. G. 1967 Effects of Urban Development of Floods Northern Virginia. USGS Open File Report. 39p.

Berry, B. J. L. and 1968 Geographical Sampling. Chapter III 3 Baker, A. M. In: Spatial Analysis: a reader in statistical geography. Eds. Berry, B. J. L. and Marble, D. F. Prentice Hall.

Carter, R. W. 1961 Magnitude and Frequency of Floods in Suburban Areas. USGS, Prof. Paper 424-B. pp. 9-il.

Clayton, K. M. 1957 Some Aspects of the Glacial Deposits of Essex. Proc. Geol. Assoc., 68, pp. 1-21.

Crippen, J. R. and 1969 Hydrologic Effects of Suburban Development Waananen, A. C. near Palo Alto, California, USGS Open File Report. 126p.

Espey, W. H., 1966 A study of some effects of urbanization on Morgan, C. W. and storm runoff from a small watershed. Masch, F. D. Texas Water Development Board Report 23.96p.

Felton, P. M. and 1963 Suburban hydrology can improve watershed Lull, H. N. conditions. Public Works, 94, pp. 93-4.

Gibberd, F. 1947 Harlow New Town: Master Plan. HMSO.

Grava, S. 1969 Urban Planning aspects of water pollution control. Columbia University Press. 223pp.

Harris, E. E. and 1964 Effect of Urban Growth on Streamflow Regime Rantz, S. B. of Permanente Creek, Santa Clara County, California. USGS Water Supply Paper 1591-B. 18p.

Institute of Hydrology 1968 Research 1968.43p.

James, L. D. 1965 Using a computer to estimate the effects of urban development on flood peaks. Water. Res. Res., 1,2 pp. 223-234.

Jones, D. E. 1970a Personal Communication.

1970b Land Use Intesity. US Federal Housing Administration Land Planning Bulletin No. 7

Lamb, H. H. 1964 The English Climate. English Universities Press. 212p.

Leopold, L. B. 1962a Personal Communication.

1969b Hydrology for Urban Land Planning -A Guide- book on the Hydrologic Effects of Urban Land Use. USGS Circular m. 18p. _, 150

Meteorological Office' The Calculation of Actual Evaporation and Soil Moisture Dýeficit over specified catchment areas. Document No. G. 14666/ LG/12/68/30.

Riley, J. P. and 1969 Modelling the Runoftf Characteristics of Narayana, V. V. D. an Urban Watershed by means of an Analog Computer. In: Effects of Watershed Changes on Streamfl'ow. Eds. Moore, W. L. and Morgan, C. W. Uriv. Texas Press. pp. 181-200.

Rodda, J. C. 1969 The Assessment of Precipitation. Chapter 3.1 (ii) in Water Earth and Man. Ed. Chorley, R. J. pp. 130-134. MaLk+'A.

Savini/ J. and 1961 Urban Growth and the Water Regimen. Kammerer, J. C. . USGS Water Supply Paper 1591-A. 42pp.

Seaburn, G. E. 1969 Effects of Urban Development on Direct Runoff to East Meadow Brook, Nassau County, "Long Island, New York. USGS Prof Paper 627-B. 14p. . Later Searcy, J. K. and 1960 Double Mass Curves. USGS Supply Hardison, C. H. . Paper 1541-B. 66p.

Stankowski, S. J. 1972 Population density as an indirect indicator of urban and suburban land surface modifications. USGS Prof. 'Paper, 800-B, pp. 219-224.

Walling, D. E. and 1970 The Measurement of the Effects of Gregory Building Construction on Drainage Basin Dynamics. Journ. of hydrology 11, pp. 129-144.

Watkins, L. H. 1956 Rainfall and Runoff: An Investigation at Harlow New Town. Proc. Inst. Nfun. Eng. 82,8. pp. 305-316.

Wakkiti s, L. N. 1962 The Design of Urban Sewer Systems. Road ''Research Technical Paper No. 55. HMSO. 96p.

Wiitala, S. W. 1961 Some Aspects of the Effect of Urban and Suburban Development upon Runoff. 'USGS Open File Report. 28pp. 151

CHAPTER 4

THE WATER YIELD AND FLOW REGIMEN OF THE CANON'S BR00K.

We cannot always distinguish between the results of man's action and the effects of purely geological and cosnical causes. George Perkins Marsh

The water yield and flow regimen of the Canon's Brook for the

period October 1950 to September 1968 are examined and changes

related to modifications of the catchment by urbanisation. The

water yield of the catchment is the total outflow of water from the

basin during unit time, and i cludes not only surface runoff in the

stream but also leakage of moisture by sub-surface flow into aquifers

whose groundwater divide is not coincident with the topographic

watershed. In Chapter 3 it was argued that the Canon's Brook is not

subject to groundwater leakage and that the total water yield of the

catchment passes the gauging station. The possible units of measurement

of water yield are many and varied; in this study inches per month and

inches per year are employed. This expresses the yield as the depth

of water over the whole catchment if the total yield per unit time were

spread evenly over the area of the basin. The floe regimen of a

catchment defines the relative importance of flows in the river and is

usually expressed as a flow duration curve. This graph is a plot of flows extant against the percentage of time that t ey are equalled or

exceeded. (Wisler and Brater 1959).

The impact of urbanisation on monthly river flow is of particular

1 152

importance to the understanding of water resources, i. e. "water for

the time being contained in any source of supply" (Water Resources

Act 1963 1.2,1) and their changes in time and space. The significance

of changes in supplies and demand is such that River Authorities are

charged to "carry out a survey of water resources of their area and

demand intervals of existing ... and shall carry out revisions ... at

of not more than seven years. " (Water Resources Act 1963, Section 14).

The monthly water yield of the catchment defines the maximum amount of

water which is available for supply schemes whilst the regimen defines

the flow conditions prevailing in a river from which abstraction might

be contemplated. From this data the reliable yield of a project may

be calculated (Wisler and Brater 1959).

Four distinct methods have been used here to investigate the monthly water yield of the Canon's Brook, but all rely upon the premise

that the rural hydrology of the catchment can be defined from the first

three years of hydrological records and that changes during the subsequent period were the result of land use changes, largely urbanisation, in

the basin or climatic trends. The latter is shown to be insignificant

in the area in the time period under consideration. The techniques and results are presented in order of increasing complexity, namely, double mass analysis, trend analysis, multiple regression and computer simulation methods. The latter technique was the main tool used to investigate the flow regimen and consequently a discussion of this research is included the at end of the section on simulation techniques. A comparison of the by results obtained the various techniques provides a useful and instructive conclusion. It also permits speculation concerning the likely effects of further urbanisation of the Canon's Brook catchment

the and magnitude of likely changes in water yield in other basins in Britain. 153

Double Mass Analysis.

The double mass analysis technique in this cons xt consists of

from Canon's Brook a plot of the accumulated total runoff the against hydrological the accumulated totals of other meteorological or

homogeneous and consistent variables. If the second variable is both between it and the water and there is a change in the proportionality then it may yield of the Canon's Brook during the period of analysis in of the be assumed that the change is a result of"a change the yield ted Brooke The graph of accumulated total monthly runoff plo against is accumulatedr total rainfall is shown in Figure 4.1(a). There a 1958 break in the slope of the line at 1957-8, suggesting that after

a greater proportion of rainfall went to riverflow than had been the

case previously. Analysis of variance (Table 4.1) indicates that

this break of slope is significant. An estimate of the hydrologic

impact of urbanisation can be gained from this analysis if one uses a

technique suggested by Bruce and Clark (1966). They correct for

changes in proportionality in a double mass analysis by the use of the

following expression:

P mm aP 4.1 a -- o ...... m 0 where Po is the observed proportionality before the break of slope

Pa is the adjusted proportionality after the break of slope is ma the slope of the line before the break of slope

no is the slope of the line after the break.

Table 4.18 the presents results of such an analysis, and shows that there is in a significant upward turn the levels of flow in the Brook at the

the 1950s. Searcy Hardison end of and (1960) have suggested that instead of simply using precipitation for comparison with runoff one should compute a hypothetical figure for runoff from a linear equation 154 Figur` 1 Double mass analyses, 1950-1968.

I s i

" e

CYTYI live %9191runoff be inch*&

(a) Canon's Brook-, unoff & Canon's Brook rainfall

CunrlUw fatal rwion of ing lee Mwr *, l. 4 "M. / K. dO MMM Md " runoff (b) River Ash runoff & River Rib

i 4 a.

i ty t

CYsvlsllyý 1. IN . YiM: I 111ýIIt11M IM Ca*. *. IIMº

t I 155

1 1o

/. c not " N. w" rN1 r~ý 0ý INI Mý 1/1" NN

O 1/. º ý MN

N Fg MNr. 6-1460IW1 " E Jaºº MN N M. rý JN. r 11 1 p to 1 30 CYm iIIve I0I. 1 rufoll In nth.. Ior CsM&s k. ok

(d) Canon's Brook summer (June-Sept) runoff & River Ash summer runoff

0 ºrr/ It/I M 11/I " IN/ HIS £ IQO /NI " IN/

I. 1" Y ý "1/N I/11

HI/ ' u'i1 I/N wel/M Wm f 111 seid Mood // rye.

amulotIve total runoff in Im for Coto s Brook.

(e) Canon's Brook winter (Oct-Mar) runoff & River Ash winter runoff

,t 16 0 ob 40

g NN

ö' 10 /wie

3 f a. l1 iui Ný. IIM liýý. IN U Sapd I, w. A..

to !o 30 40 Cumulative total runoff in inches for Cano, Brook

(f) Canon's Brook spring (Apr-June) runoff & River Ash spring runoff I 156 1

Table 4.1 An-analysis--of variance table for the double muse analysis of the-Canon's

Brook from the R. runoff against rainfall--and-the-runoff -Ash

Period of Double )! ass Number of Critical value is the grouping Analysis Analogues groups P ratio d. f. of F at 1 per significant? level Canon's Brook cent with df in Runoff and previous column

Annual Annual 2 15.7 1,214 c. 6.7. yes Rainfall

Annual Annual Runoff 3 4.7 2,213 c. 4.7. yes from R. Ash

Summer Summer Runoff 3 12.6 2.51 5.1 yes (July-Sept fron R. Ash

Spring Spring Runoff '3 "8.1 2,51 5.1 yes (Apr-June) from R. Ash

Winter Winter Runoff 3 3.6 2,105 4.9 yes (Oct-Mar) from R. Ash

V 157

and then compare this with observed runoff. The equation they advocate

is of the following form:

Q a(P-b) ...... 4.2 where Q is computed runoff

P is precipitation and

a and b are empirically determined constants.

Their reasoning that runoff computed from rainfall data is more analogous

to real runoff must be doubted since the only data input is rainfall and

the equation is a simple linear one.

To offset this critisisn of the lack of analogy between rainfall

and runoff in the Canon's Brook, the latter has been compared to the

flow of the River Ash above Mardock Mill, north west of Harlow

(Fig. 3.1), having a catchment area of 36.6 sq. miles and draining

southward to the Stort. Chalk underlies most of the basin which is

overlain by clay in a few places. The Ash catchment is also of lower

relief than the Canon's Brook basin. In the light of this it cannot

be considered a true control catchment (Wilm, 1949) and the results must

be treated with caution. There are, however, several advantages in

using the Ash in this study. It has undergone no major land use changes,

has no licenced water abstractors within its watershed and so has a

consistent homogeneous runoff record. A double mass analysis of the '(b) Rivers Ash and Rib, (Figs. 4. and 3.1), emphasises the point.

Double mass plots for the River Ash and Canon's Brook, for annual, (July-September summer incl. ), winter (October-March incl. ), and spring (April-June incl. ), periods are shown on Figure 4. l, c, d, e, and f. Corresponding analyses of variance which test the significance of the various breaks of slope are shown in Table 4.1. There has been, during P 158

the study period, a significant increase in the runoff of the Brook.,

a result largely of enhanced spring and sunner flows. The breaks in

" the double mass curves seem to occur at two main times, 1954-55 and

1960-61. The former is clearly related to the completion of dwellings

and paved areas from September 1953 onwards while the latter owes

something to both the continued spread of urban area (Table 3.11) and

the rather anaomalous sequence of wet and dry years which occurred

at the turn of the decade. It can be seen in Table 4.18 that a double

mass analysis of this type suggests that the construction of some

15,000 houses and the consequent paving of nearly 17% of the basin has

increased water yields by up to 100%.

Trend Analysis.

The data for the Harlow catchment can be treated independently

of other sources and changes through time may be analysed for trend

by either moving mean or linear regression.

A moving mean analysis is undertaken by calculating the mean of

overlapping sub-sets of data in such a way that high frequency fluctuations

in the data are filtered out (regory, 1968). A plot of the 51 month

moving mean for the 216 months of record showed a steady increase in flows,

but when onlyy(p data for summer Aril-S eptember incl. ) was used, there

appeared to be a more marked increase. Figure 4.2 is a plot of the 11

and 25 month moving means for the summer period and reveals an increase

of almost 100% in the summer flows, when one compares the extreme ends

of the line. This method is useful in indicating the direction of change but Dawdy as and Hatalas (1964) suggest, "even a smooth trend obtained by

the method of moving averages be by cannot represented ... a mathematical (nor equation can its significance be tested). If a mathematical trend is fitted data, to the a simple relation should be used unless logic 1

______I 159

s Time series. --ý- 11 month moving mean. """""""" 25 month moving mean. V G

ý1ý

ýý"ýýý~ýýýýýý

Ot\-tItI" "T" 1 ý" " 1130 fail IDS? IDSS 1934 Isis IDtSI IU? ISS" litt 1950 lilt fit: 00,101 1944 /DIS I9"4 9147 1/11 September

Figure 4.2 Trend in the summer (Apr-Sept) runoff from the Canon's Brook analysed by moving averages. 160

indicates otherwise. The simplest expression is a straight line".

In this case logic backed by supplementary evidence from the spread of urban area over the catchment (Fig. 3.13) and the growth in the number of dwellings in the basin (Table 3.9) points to a near linear or straight line relationship.

The most reliable method of fitting a straight line to the data is by using a least squares mathod (Blalock, 1960). The equation of the regression line is:

Y- a+bX 4.3 e ...... where, Ye is the estimated value of the dependant variable Y (runoff

in this case)

X is the independent variable (time in this case)

a and b are the regression coefficients.

A significant upward trend was defined as one where the slope of the regression line (b) differed fr m zero sufficiently for an F test to show that the difference would be likely to occur by chance only once in one hundred occasions, i. e. p 0.01.

Table 4.2 summarizes the significant results of the trend analysis of both monthly runoff and some of the related hydrological and meteorological variables which are listed separately in Table 4.3. Records for 216 months were used in this exercise because at this stage of the study the details of the growth of tie town were not available. Whilst this assumption must affect the degree of trend detected, it is a conservative error which makes for underestimation of upward movement. It can be seen from Table 4.2 that there is a significant upward trend in runoff when all the data are used, but that there are morq marked upward trends if the spring and suer periods are treated separately. There is no significant 161

Table 4.2.

Results/of a linear regression trend analysis for the period October 1950 to September 1968 on the variables listed in Table 4.3. Only trends which were significant at the 1% level are given here.

Variable Period of Analysis b

Monthly runoff, Canon's Brook Whole year 0.0021

Monthly runoff, Canon's Brook April-September 0.0027

Monthly runoff, Canon's Brook April- June 0.0027

Monthly runoff, Canon's Brook July-September. 0.0028 162 Table 4.3

Meteorological and hydrological variables tested for trend during the period Oct 1950 - Sept 1968.

------

Monthly rainfall at Eastwick Lodge/Terlin'gs/Sports Stadium Gauge. Days per month with measurable rainfall at ...... Potential Evapotranspiration in the Canon's Brook basin. (Penman estimate for a short green crop). Mean monthly temperature.

Mean daily hours of sunshine. Monthly water yield from the Canon's Brook, Elizabeth Way Gauging Station.

Monthly water yield from the R. Ash, Mardock Mill Cauging Station.

Monthly water yield from the R. Robing, Redbridge Gauging Station. Monthly rainfall at the Eastwick Lodge/Terlings/Sports Stadium gauge which was in excess of 0.02 inches per day. 0.04 0.06 0.08 0.10 0.20 0.30 0.40

0.50 .... go ..

Percentage of monthly rainfall at Eastwick Lodge/Terlings/Sports Stadium gauge in excess of 0.02 inches per day 0.04 0.06 0.08 0,10 0.20 0.30 0.40

0.50 """. f4"r 163

upward trend in the winter flows in the Brook.

It may be argued that the upward trends in runoff are the result of climatic changes or minor meteorological shifts during the limited period of study. The absence of trend in any of the meteorological variables, Table 4.3, suggests that any hypothesis that the upward trends in the runoff of the Brook are the result of meteorological changes are invalid. 4

Multiple Regression Analysis.

The multivariate statistical proceedure of multiple regression analysis enables the calculation of a statistical relationship between a dependant variable, y, and a series of independent variables, x1... xn. The form of the relationship is:

bnxn Ye 'b1x1+b2x2+... + +a...... 4.4 where Ye is the value of the dependent variable estimated by the regression equation

bl - bn are regression coefficients

a is the intercept on the Y axis

x1 - xn are the independent variables.

When calibrated on the initial three year rural period, an equation relating rural runoff from the Brook to other meteorological and hydrological variables may be used to predict what the runoff from the

Canon's Brook would have been fron 1953-68 had there been no urbanisation.

The application of thh multiple regression model to the present problem is not without itsipitfalls. The model is distinct from the related correlation model and within the regression model measures of correlation have very little meaning (Ezekeil and'Fox 1959). The regression 164

model is free of the limitation, which applies to the correlation model,

that variable scores should be strictly random samples from a normal

universe (Ezekeil and Fox 1959), the only statistical requirements being

that the residuals be random with respect to time and that the "formulae

apply only to a universe in which the distribution of the values of

independent variables remain fixed in successive samples" (Ezekeil and

Fox 1959 ). The former property may be tested by the coefficient

of autocorrelation or von Neuman's ratio. The significance of the

equation may be tested by use of the F ratio which analyses variances

due to the regression and the residuals.

The variables used in the derivation of predictive equations

for the rural water yield are given in Table 4.4(a) together with their

means and standard deviations for both the calibration and study periods.

With the exception of runoff (y) the distribution of the variable scores

does not change significantly from the calibration to the predictive

periods. Table 4.4 (b) presents the results of the regression analysis

in the form of the four best predictive equations. In spite of the

equations being statistically significant and the residuals random, there

is a wide variation in the Stan and error terms. The equations which

have the lowest standard errors are those which include data from the

Rivers Ash and Roding. Equations 4.7 and 4.8 which incorporate only meteorological variables have much larger error terms. The R. Ash and

R. Roding are not ideally suited as control catchments, the former

differs lithologically from the Canon's Brook whilst the latter has,

itself, undergone some urbanisation. Consequently it is not entirely valid to include them in predictive equations for the Brook. However, t when the philosophically satisfying equations 4.5 and 4.6 are used to Canon's predict Brook rural runoff for the 1953-68 period the results 165

Table 4.4 (a)

in Reprcaaien Studv of Wirer Y1 ! Variables used the Multiple -Id Elitabath Way (Variables are assessed over monthly periods at eithe r the Stadium Mot. Sta tion gauging station or the Eastvick lodge/Tstlings/Sports unless other visa stated) Code Variables Calib ration 1950-1968 U its Mean S Dev n Mean S Div . y Runoff Inches 0.66 0.80 0.59 x3 Reinfall Inches 2.00 1.16 1.99 1.07 X4 Days with Rain 14.97 3.18 16.56 3.08 xS Potential Evapotransp- Inches 1.33 1.01 1.74 1.16 / iration (Penaman) X6 ýI Mean Temperature CO 9.4 4.83 I 9.48 4.87 X7 Daily Sunshine Hours 4.12 2.06 3.93 1.94 X9 Rainfall in excess of 0.02 per day Inches 1.99 1.16 1.97 1.07 1.91 1.07 X10 .. 0.04 ...... 1.93 1.16 1.85 1.07 x11 of 0.06 ...... 1.87 1.13' x12 0.08 " 1.78 1.05 ...... 1.82 1.13 1.71 1.03 X13 .. 0.10 ...... 1.72 1.13 1.31 0.99 X14 0.20 . '. .. .. 1.30 1.02 115 .. 0.30 .... of 0.97 0.90 0.95 0.87 x16 0.40 0.70 0.81 ...... 0.68 0.80 0.50 0.51 0.66 x17 ...... 0.39 0.33 : 19 Percentage of rainfall in excess of 0.02 ins. per 98.1 2.1 96.4 3.7 day x20 ...... 0.04 94.1 5.4 92.2 9.5 x21 ...... 0.06 89.8 7.5 87.7 13.1 x22 .:.. .. 0.08 86.1 13.4 83.6 16.0 x23 ...... 0.10 80.8 16.8 . 79.3 17.7 I x24 .. 0" "" 0.20 56.3 23.0 56.2 25.3 X25 ...... 0.30 38.9 23.3 26.7 24.9 x26 .ý". .. 0.40 24.7 23.7 26.6 24.9 *27 ...... 0.03 13.2 17.8 18.9 22.0 X28 Runoff from R. Ash Qlardock Hill) Inches 0.47 0.41 0.36 0.34' *29 Runoff from R. Roding (Radbrý g9i ) Inches 0.55 0.64 0.53 0.57 X31 Antece A month's 1.17 1.99 1.07 rainte ý Inches 2.04 *32 Anteceden\t month's runoff'. / Inches 0.65 0.65 0.80 0.59

Table 4.4 (b)

Predictive equations for Canon's Brook rural runoff calibrated on the period Oct 30- Sept 53 (36 cases)

St. Error Cotff of Equationýý ! ratio t n. -ýý of E. Autoeorr

6.5... 0.83x29 " 0.17x16 f 0.07 y " 119(2,33) 0.24 "0.11

4.6... y " 0.26x16 " 0.51x28 -0.003x25 0.51x29 0.09 " " 63(4,31) 0.23 0.10

4.7... y " 0.09x` " 0.37x., -0.11x6

-0.82x10 + 0.60x1, " 0.72x16 0.009x2S 0.09x1 " '0.26 18(8.27) 0.31 0.20

6.8... y " 1.79x3 " 0.09x4 " 0.40x5 -0.10xb "2.69x10 "0.69x1`

, 0.51x16 +0.29x17 -0.008x25 16(10,23) "0.10x1 40 109 0.29 0.04 166

are totally inconclusive because the errors of prediction mask an

changes in runoff. Figure 4.3 shows the results of using equation

4.8 predictively. Dawdy (1969) states that "the standard error of

is the prediction ...... somewhat greater than. the standard error of

estimate, for it includes both the measure of lack of fit of the data

used to fit the model and the measure of error in the fitted parameters".

This would suggest that if the standard error of prediction were

calculable in the case of Figure 4.3 it would be in excess of 0.29 inches,

and this fact would go some way towards explaining the inconclusive

results shown in the diagram. In other similar studies in Pennsylvania

(Jones 1966), North Carolina (Swank and Miner 1968 and Hibbert 1969),

Kansas (Sharp et al. 1960), New York (Black 1968), and Long Island

(Seaburn 1969) the normal practice has been to establish at least two

experimental catchments, one or more acting. as controls. Whilst not

always stating precisely the nature of their regression equations and

the associated error terms, the authors seem to have achieved better

results than is possible for the Canon's Brook using meteorological

variables only. This lower level of accuracy must be ascribed to the

absence of a good control catchment. Further, although the use of the

regression model has sidestepped the unmanagable assumptions of the

correlation model, there remains the question of the logicality of the

equations and predictions. Sharp et al. (1960) stressed the illogicality

of many multiple regression equations involving transformed variables

and relationships which are seemingly the inverse of reality. The

equations in Table 4.4 (b) were produced by the stepwise multiple 11969) regression program Brm02R (BMD which at each step introduces into hiving the equation the new variable the greatest increase in variance

explained. The user may specify a cutoff value for the F ratio so that no variable is entered which does not contribute significantly to the 167

3s 'ýý--"' ý--" GAUGED 3 I.«»--+ --+ PAEDICTED z"s r L2 Eit ýt ý1ýr1 ý ý. :ýý; ý tý ý'': V .1 ,! ý " i11 boo t fiý Y ýý "65 `"i3 , SI 'Sä `Sa `37 Se 'bo "61 '62 '6 '64 "E4 16 '67 'St `32 Jon Jan Jon Sept Jan Jon Jon , Jan Jon Jon Jon Jan Jan Jan Jan Jon Jan Jan : Coubrauon Period

Figure 4.3 Runoff from the Canon's Brook predicted by the multiple regression equation, 4.8. 168

predictive power of the equation. The stepwise process may be halted by the user before the cutoff value is reached and this option was used in the case of equation 4.7. In spite of this proceedure it is still possible to calculate illogical equations as in 4.8. Here there is a positive relationship between runoff and evapotranspiration and a negative link between certain rainfall indices and runoff. Further, although the equation has a low standard error of the estimate for the calibration period, flows of below zero are calculated for months 22 and

36 of the series. This again is illogical and enhances the view that the employment of only meteorological variables in the multiple regression analysis is inappropriate.

A impasse be if complete may avoided only one accepts the equations . containing data for the Rivers Ash and Roding and then treats the results with some caution. Figure 4.4 shows the results of using equation 4.5 to estimate the rural runoff of the Brook. The graph shows that there has been a progressive increase in the flow of the Brook over that which would have been expected had there been no urbanisation and from the predicted rural flows and gauged records annual estimates of the impact of urbanisation have been prepared for Table 4.18.

Digital Simulation Model.

The power and flexibility of the electronic computer has been used increasingly by geographers and hydrologists in their attempts to investigate and model the dynamic complexities of the real world. The formation in California of a firm of consultants, Hydrocomp Inc., dealing exclusively in hydrological simulation studies'(Linsley 1969, Hydrocomp 1969, Hydrocomp 1971), the ongoing research into digital simulation models, Institute at the of Hydrology, Wallingford (Nash and Sutcliffe 1970), and

rr' 169

d 1 I

Figure, 4.4 Runoff from the Canon's Brook predicted by the multiple regression equation, 4.5. 170

(BGRG the British Geomorphological Research Group Symposia in 1968 1968)

importance and 1970 (BGRG 1970) all reflect this growing of computers and simulation. The IASH and UNESCO Symposium on the use of computers in hydrology held in Tucson (LASH/UNESCO 1969) brought together some

64 papers from a wide range of international contributors and concluded

"the in hydrology (was) of a decade in which use of ... computers one

in (had) been " the areas ... which a great advance made.

Digital simulation"by electronic computer is employed in this study in an attempt to overcome the absence of a good control catchment, to avoid the statistical and logicality problems of multiple regression and to gain an insight into the functioning of the hydrologic system of the

Canon's Brook both before and during urbanisation. Two versions of the basic model were used, a "RURAL MODEL" and an "URBAN MODEL". The first simulates the hydrological cycle of the Canon's Brook in its rural state and is calibrated on the records for October 1950 to September 1953.

Comparison of the predicted rural runoff and gauged runoff from the urbanising catchment for the post-calibration period up to September

1968 gives estimates of the effect of urbanisation. The URBANMODEL is calibrated on the period October 1950 to September 1960 and. simulates the urbanising of the Canon's Brook. This model givcs an indication of the hydrological effect of paving a once "natural" land surface and more importantly enables the prediction of what might happen if certain meteorological conditions occurred or if certain development policies were implemented.

The investigation of the water yields of Canon's Brook by simulation techniques is discussed in a series of sections indicative of stages in the development of the research. A consideration of simulation techniques, 171

the available data sources and the required output cads naturally into

a discussion of the building of the model, the proc sses modelled and

the fitting of the model to reality. The model's ensitivity to errors,

its internal consistency and its functioning are th, n examined before

the results are given. The findings are set out ii three parts

reflecting the RURAL MODEL, the URBAN MODEL and the flow regimen of

the Brook, the latter including the results'of both the simulation approach and the more traditional analytical methods. 4

The conceptual basis of the model is the water balance equation:

Runoff(t) a Rainfall(t) - Evapotranspiration(t) - [Storage(t) - 4.9 'Storage(t-1)) ...... wher4 (t) represents unit time which in. the case of this odel is one day., The model calculates a water balance for each day for each land use in the catchment and from this a prediction of runoff is wade for each day of the study period for both the RURAL and URBANmodels. The land uses in the RURAL and URBANcomputer catchments are given in

Table 4.5 together with the parameters which characterise the land uses. The albedo, defined as the proportion of total insolation reflected and scattered by the earth's surface without absorption

(Willett and Sanders 1959), is of importance in the calculation of the heat budget portion of the equation which estimates evapotranspiration.

The figures used are derived from Sellers (1965), Kung et al. (1964) and

Barry and Chambers (1966). Sellers shows in tabular form (page 21) the likely upper and lower limits of the albedo for various types of surface,

data the coming from a range of authors. Rung, Bryson and Lenschow (1964) undertook some 16 flights totalling 24,000 miles in the United States in order to measure systematically the surface albedo over various types of the earth's surface. Barry and Chambers measured albedo over different 172

Table 4.5

Land Use Characteristics of the RURAL and URBANmedals

Land Use Albedo Root Constant Percent of Rainfall Maximum Storage Inches Intercepted of Intercepted Inches " Rain -

Domestic Cdns. 0.05 and Nurseries 0.25 2.2 10% " Root crops 0.25 3.0 10% 0.05 Grassland 0.25 3.0 10% 0.05

Cereals* 0.25 5.5 10% 0.05

Woodland 0.19 6.0 12.5% 0.05

Bare Soil 0.18 0.5 0% 0.00

Paved Area 0.15 0.0 100% 0.01

RURALMODEL

Year 1 2 3 4 5 6 7 89 10 11 12 13 14 15 16 17 18

Dom. Cds. b Nurse. 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 Grass & Root Cps. 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48 48

Cereals* 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34 34

oodland 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11

Bare Soil 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Paved Area 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

URBAN MODEL

DomeUse 6 6 6 7.5 9 9 9 9 9 9 9 9.0 9.5 9.5 10 10 10.5 11.0 b Nurse.

Crass & Root Cps. 48 48 48 47.5 47 47.6 48.3 49 49.7 50.4 50 SO 48.5 48 46.8 45.5 44.7 45.4

Cereals* 34 34 34 31.9 29.8 27.6 25.5 23.3 21.1 19 17.6 15.9 14.5 13.0 12 11.5 11 10 Woodland 11 11 11 10.5 10 10 10 10 10 10 10.5 11 11.7 12 12 12 12 12 Bare Soil 1 1 1 1 1 1 1 1 1 1 1.5 2 3 4 5 5 5 S IPaved Area 0 0 0 1.6 3.2 4.8 6.2 7.7 9.2 10.6 11.4 12,1 12.8 14.2 1 1 *See text for details of the seasonal change from cereals to bare ground 173 1 types of cover in Southern England from the ground and from a light aircraft, and their values are used where the other authors support

the numerical value of the index. The root constant is a theoretical concept developed by Penman (1950b) and represents "a measure of the amount of water readily available within the root range" for transpiration to continue at the maximum rate. The figures are those given by Penman except for woodland whose root constant has been reduced from 8.0 inches to 6.0 inches to prevent excessive drying of the soil and a long term decline in runoff amounts through the study period. The root constant for cereals, 5.5 inches, applies only to the months April to September, for during the winter period the land is considered fallow and treated as bare soil with appropriate albedo and root constant.

The data for the percentage of each rainfall intercepted by the different land use types, with the exception of paved area, were derived from figures given by a number of workers. Penman (1963) states that in the case of vertical interception "a few qualitative generalisations

but the becomes in are possible, subject ... chaotic attempting to make them quantitative". He goes on to present the results of over a dozen studies which show that interception varies with rainfall intensity,

duration, rainfall type of tree or crop, season of the year, density of plants or tree crowns and method of measurement. Ward (1967) presents in form graphical much of the data quoted by Penman and states that the "data is sparse" except for wooded areas. The factor for the woodland

Canon's of the Brook was taken from White and Carlisle (1968) who

interception by studied mixed deciduous woodland in north-west England.

They 12.4% measured a mean rate of of the annual gross rainfall which does not appear to vary systematically with the seasons. The figures for the maximum amount of intercepted rain which may be stored on the vegetation i° 174

by Penman (1963), are rather arbitrary. The work of Merriam, quoted

0.005 in. suggests that the "figure for surface storage is ... on

figure 0.03 to annual rygrass". Linsley, et al. (1968) give a of

0.06 inches for interception storage on a forest canopy, 0.003 in.

for Monterey pine and 0.008 in. for bluegrass. The interception

known; characteristics for the urban paved areas are little published

data is scanty and applies only to specific types of surface rather (1960) than to urban areas as4a whole. Tholin and Keifer in their

be 1/16 inch, study of St. Louis Assumed storage on paved areas to

but also used 1/2 and 1/8 inch in an additional computation series.

The basic conceptual framework described above for the simulation

of the water balance of a catchment is a slight modification of the work '-. "month by of several previous writers. Penman (1950b) estimated the

month changes in (water) storage (in the Stour, Essex) from 1933-1948"-

He calculated changes in storage in the catchment from a knowledge of

river flow and a theoretically calculated estimate of evapotranspiration

and showed that the calculated changes in storage were in close

correspondence with observed well levels. The land use of the Stour

basin was fitted to a tripartite classification which had areas remote

from the stream with a root constant of 5.0 inches, intermediate areas

with a root constant of 8.0 inches and riparian zones drawing water

directly from the watertable. Since 1962 the Meteorological Office

has issued information on the foil moisture conditions in all of the

major British catchments (Grindley 1967). The method used to monitor

soil moisture changes is that used by Penman in 1950 and Grindley states

that it is assumed that each of the catchments has the same land use as

Penman assumed for the Stour because it is inexpedient to apply a root

constant to individual areas within specific catchments. In California 175

James (1965) has attempted to estimate the effects of urbanisation on

flood peaks. The Stanford Watershed Model IV (Crawford and Linsley 1966)

was calibrated on the period July 1959 to June 1963 and then used to

simulate the hydrology of the catchment from 1901 to 1963 with various

degrees of urbanisation assumed. The exact methods used to model land

use other than paved areas are not described by James although he does

describe the catchment as having 8% urban area, 31% cultivated fields

and 61% grazing land in 1961. It appears from the report by Crawford

and Linsley that their model divides a catchment into small segments of

homogeneous nature and that the flow from each of these sub-catchments

is routed through the channel system. It seems likely that James chose

to divide Morrison Creek into sub-catchments which had relatively uniform

land use, but this is not entirely clear from his paper.

A rather different approach has been taken by the Institute of

Hydrology in their attempts to simulate the flow of the River Ray by a

model entitled S. M. 2; (Mandeville, O'Connell et al. 1970). This model

was based on the work of Penman and recognised three zones in the

catchment. There was continuous evapotranspiration at the potential ce rate from the riparian area si the roots are able to draw upon an

inexhaustable reservoir of moifture. The rate of evapotranspiration in

the wooded zone never fell below the potential rate because the soil moisture deficit never exceeded the root constant. The remainder of the computer catchment was given over to grasslandzemote from the stream.

The model had three parameters, T, A and D;

where, T was the ratio of potential evapotranspiration to open st. Lr -

evaporation and was set at 0.8;

A was the proportion of the catchment covered by, grass

remote from the river; D was the root constant for this grass area. 176

The area of trees was measured and remained fixed whilst the riparian

zone covered an area which was equal to the total non-wooded area

of th/ catchment minus A. The model, when running, optim4sed the

values of A and D to give the best possible prediction. The logicality

of such a move must be questioned since it seems likely that both

parameters have ä physical existence measurable in the field. Indeed

Mandeville et al. state that "the value of either (A or D) might be

obtained from examination of the basin, though the assumption that the model parameters bear one to one relations with corresponding basin

characteristics could well be misleading". Such a method of simulating

land use is at odds with one of the principles established in an earlier

paper by Nash and Sutcliffe (1970), who stated that "although simplification

of a basin is necessary, especially in terms of variability over the area,

it is desirable that the model should reflect the physical reality as

closely as possible". This method of modelling land use in a catchment

seems inappropriate to the Canon's Brook situation since the whole aim of this model is the simulation of land use changes through time as they actually occurred and were recorded by maps and photographs. The building of the Canon's Brook simulation model is described in the following section.

Building the Model: The Hydrological Processes Simulated.

A schematic representation of the functioning of the model is shown by flow a diagram in Figure 4.5. The representation of the model as a series of links between dyn, 3mic processes enables one to disagregate it and consider the individual processes modelled. The sources and reliability basic of the meteorological data input to the model; namely daily precipitation, monthly mean temperature, actual vapour pressure, saturated vapour pressure, wind speed and hours of bright sunshine have been considered 177

Daily Prot. pit a bon. Monthly funshMf, iempero turt, NOpour Pressure and Wind Soted Aibedos PoOt Constants and Areas Of *CCh Land Uft typt

0 Q .Y g 0 q 00 Jf ~Y 4

raWmmy Evooprotan from Monthlyl Monthly I I i Monthly Totenaal pottte'ntoI pootent'al Oprn Worte w. tI AibtdO Of Evapo- Evooo- 1 Evopotronsoirot on I Evopo- II Evopo- II 11 i Urbon Ano as tronsoirotion tronsprot. on tronso, rot-on L------J 11.2=1=0, "- II ------II ---T--- II ---T-

InterceptPon intercept On Interceplgn IMercepU" Storage Storage Storage Storage Storage

j! ; 9, Interception I: Interception interception interception "n011 ¶1 : ýrpaess ssý+sssa ss I p 'a==c =r p ýaa ýýA : aaý + pýao i100% I) r---L"1 r"---L, Runo11`, 1 1 1 1 1 11 ( Iý I Inldnrouon 1I Innnrotwn I I Mhnrotwn MI nrot on lnf. nrotson taxR""O/I L-s. 1. L-.. 1 1after Re%ef%v. II -J ----J -J 1 ------., . ------il II II IMoisturtIupper \ IMO. er upper upper UPP#CsoiloN Soil Soil So,, soil II bistuMI\ SWM Mo-sture Nbaturt II Stara " Storo II

Soil soil Sofft Sogt Soil MO, ºture Moisture Mo-sturt montan awatur* II Storage storage Storage Storage St )rag* II

II -----1 r -----1 PercolationII Percolation 1 Percolotson(1 Percoiobon11v, rcolotion 1 II L'----, J L---- JL L --+--J ----J ----J II II

II -º Ground Water Stomp. ------II

. .... iýow.. "Mj4 r'- -ý- -'1 Atluol Flow I Fit oft e Durotfon Doily flow Durations L :. L---I---J -Mode -J «curves . ««!

ttM Gouged fit ' of x ý. Monthly Flow Model Flows L------J

r------1 I Not Loss*$ from t I Evoporotion and I Trenspirotion I L------J

Figure 4.5 Flow diagram of the computer simulation model. 178

1 in Chapter 3. The origin and numerical values of the input parameters,,

albedo, root constant, interception rate, interception storage and

proportions of the catchment in each land use, have also been considered.

The monthly potential evapotranspiration, defined as "evaporation

from an extended surface of crop, actively growing, completely shading

the ground, of uniform height and not short of water" (Penman quoted by

Ward (1971)), was calculated by what is widely known as Penman's method.

The technique and related equations was first published in 1948 (Penman

1948) with major supplementary papers in 1949,1950 (a and b) and 1963.

The version adopted for the simulation was that advocated by the

Meteorological Climatological Services Branch in their notes for users

of the Penman method (Meteorological Office undated document).

There has been considerable discussion of the assessment of evapotranspiration and the debate continues. Ward (1971) has recently presented a very lucid and well informed review of the topic. He weighed the macro view of climatologists and hydrologists against the sceptisism of the botanists and concluded that "there are certain elements of the basic Penman philosophy which remain applicable".

Nevertheless this conclusion was given a rider in a statement by Lee that "if indeed 'wicks', kind" plants are ... they are wicks of a unique which have some considerable influence over the rate of water loss from them. The problem of checking estimates of potential evapotranspiration by made empirical formulae against field measurements was considered by Ward (1971). He suggested that when adequately sited and maintained, irrigated the evapotranspirometer is preferable to the open evaporation pan since it gives an estimate of potential evapotranspiration from a vegetated He surface. quoted the work of a number of writers, notabl7 179

Rijtema and Chang, who show close correspondence between Penman estimates

of evaporation and potential evapotranspiration and measurements made

with pans or. evaporimeters. Holland (1967) after an extensive review

of the measurement of evaporation and evapotranspiration in Britain,

distinguished between winter and summer estimates. With regard to the

"chaotic" winter estimates Holland suggests that Penman caters

realistically for the problem of condensation whilst the direct gauging methods comparing evaporation gauge and nearby raingauge data, "take full account of evaporation but no account what ever of condensation".

Consequently he suggested that the Penman estimates will be systemmatically lower than the direct gauging. His analysis of the summer situation revealed a fairly good correspondence between the various methods, his ý" only reservation being that Penman's use of direct radiation rather than surface temperature produces a slightly earlier summer peak than the direct techniques. One could go beyond the two recently published reviews of Penman's method and consider in detail the whole literature but the value of such a review of the textual critisism of Penman is doubtful for two reasons. First, it is not possible to compare the results from the empirical formula with error-free measurements of potential evapotranspiration because no instrument exists which can work to such standards. Differences between estimates and measurements are as likely to be the result of errors of measurement as'conceptual errors inherent in the formula used for the estimate. Second, the demonstrable value of the Penman technique as used by the Meteorological Office, River Authorities, and other researchers is sufficient justification for its use in the simulation model under consideration.

The Penman method is based upon the' dovetailing of the aerodynamic and energy balance approaches so as to obviate the need for a knowledge 180

of the surface temperature of the evaporating surface. The energy

balance equation is expressed as follows:

E+K- Rc (1-r) R. 4.10 - b ......

where, E is the energy used in evaporation;

K is the energy used in heating the air;

Rc is the short wave radiation from the sun and sky;

r is the surface albedo;

Rb is the long wave back radiation from the earth's surface.

The short wave incoming radiation (Rc) may be approximated from Ra,

the amount of radiation which would reach the earth's surface in the

absence of an atmosphere, and a knowledge of the actual sunshine hours

over a long period such as a month. The resulting equation is as follows:

Rc Rg(0.18 N) 4.11 = + 0.55 ......

where, n is the actual hours of'sunshine per month;

N is the maximum possible hours of sunshine per month.

The use of this empirical equat on gives rise to inaccuracies if it is

applied over short periods but it is considered adequate when the time

interval is a month or more. The amount of long wave radiation, Rb, lost

by the earth is dependent upon the temperature, humidity, and cloudiness.

Without an atmosphere the earth would radiate as a black body at a rate 4 equal to PT where r is stefan's constant, and T is the temperature in absolute degrees. The Meteorological Office prefer to follow the work of both Budyko and Brooks who suggested that Penman was incorrect in assuming that a natural suýface radiates like's black body and therefore i apply an arbitrary constant of 0.95 to the estimated black body radiation.

This has to be further modified to allow for the blanketing effect of the atmosphere. The resulting expression is:

6.09 Rb 0.95 (0.56 0.09eä ) (0.10 N - PT4 - + ...... 4.12 i81

where, ed is the air's vapour pressure in mm of mercury.

The foregoing equations facilitate the estimation of the net radiation balance of the surface (H) thus:

(1-r) 4.13 11-Rc -Rb ......

Although the net radiation balance of the earth's surface can be calculated, it is not possible to solve equation 4.10 because K can only be calculated from a knowledge of surface temperatures. However, the use of the aerodynamic approach dgscribed below obviates the need for such information.

The energy used in evaporation is equal to a function, fl, of the wind speed, u, multiplied by the saturation deficit of the atmosphere.

This latter quantity is equal to the difference between the saturated and actual vapour pressures of the atmosphere. Ef1 (u) (es 4.14 - ed) ......

The energy used in heating the air is equal to a function, f2, of the wind, speed and difference between the surface temperature Ta and the air temperature, Ta. Thus

K 'y f2 (u) (Ts Ta) 4.15 - ...... ' where is an empirical constant used to maintain consistency in the units. If one assumes on the basis of empirical work that the two functions are equal and equat thus

f1Mf2-f (u) 0.35 (+u/100) 4.16 - ......

if and one accepts the detailed reasoning given by Penman (1948) and the Meteorological Office it is possible to calculate the theoretical amount of evaporation which would take place from a surface at the

i same temperature as the air, Ea, as follows

Ea - 0.35(es ed) (1 + 4.17 - u/100)......

r" 182

The final equation to calculate the potential evapotranspiration, Et,

is as follows

ýt -ax+ Ea 4.18 ......

Q where, is the slope of the saturated vapour pressure curve in

mm of mercury per F0 at Ta.

Despite the reservations expressed earlier about the relevance 4 of gauged measures ofevapotr, anspiration to estimated figures, it is

necessary to check that the Penman method had produced figures of the

right order. This is complicated by the paucity of gauged data on

potential evapotranspiration in S. E. England. The Nature Conservancy

has lysimeters "by had a network of in the British Isles which 1965 ... (Green passed ... the embryonic stage" 1970). The isoline maps

published by Green for this network for the years 1965,1966, and 1967

are of limited value as his interpolations appear to be highly subjective.

The gauged data nearest to Harlow are that for Huntingdon and these are

used in Table 4.6. The data for the Nature Conservancy lysimeter at

Alice Holt (near Farnham) have been taken from British Rainfall 1961.

Holland (1967) presents the results of gauging open water evaporation

from a standard sunken pan at Rothamsted for the years 1961 and 1962,

and this again is included in Table 4.6. Evaporation from a sunken pan

be cannot expected to be exactly equal to potential evapotranspiration from grass. Indeed, Holland states in a discussion of the underlying

pattern of potential evapotranspiration that a study of the five stations which possessed both tanks and lysimeters in 1961 showed that the ratio

of evaporation from the tanks and evapotranspiration from the lysimeters "between was about 1 and 1 1/4". -

/ 183

Table 4.6

Comparison of PenA* estiantes of Potential Evapotranspiration from Grass (r - 0.25) in the Canon's Brook catchment and published gauge data

4 Date Canon's Brook Rothamstead Stanstead Alice Holt (estimate) (Pan) Abbots (Pan) (Lysimeter)

1961 J 0.27 0.11 0.99 0.10 F 0.55 0.43 1.12 0.30 M 1.31 1.68 1.57 0.20 ýý\ A 2.26 1.70 1.89 2.00 M 2.45 3.46 2.88 2.90 J 3.67 3.79 2.94 4.50 J 2.81 3.69 4.33 4.90' A 3.01 3.33 4.15 3.70 S 1.65 2.19 1.76 2.70 0 1.37 1.40 2.08 0.50 N 0.43 0.86 0.92 0.50 D -0.06 0.30 0.79 '0.20 1962 J 0.25 0.49 F 0.63 0.47 M 0.94 0.70 A 3.20 2.02 M 2.50 2.62 J 3.90 4.37 J 2.50" 3.16 A 2.70 2.98 S 1.60 2.30 0 0.82 1.16 N 0.37 0.20 D 0.03 0.10

q Canon's Brook Nature Conservancy-. (estimate) 1luntingdon (Two Lyaitteters)

April - Sept 14.4 17 1965

April - Sept 15.9 16 1966 April - Sept 16.1 17 1967 184

Inspection of Table 4.6 shows that in general the Penman estimates of potential evapotranspiration from grass in the Canon's Brook catchment are lower than the pan estimates of evaporation, especially in the summer. Fiore significantly, the potential evaporation gauged by lysimeters at Huntingdon for the summer periods of 1965,1966 and 1967 agree very well with the calculated estimates in view of the fact that the two sites are some 40 miles apart. There can be no absolute test of the accuracy of the estimates made by the formulae in the evaporation section of the simulation model, but the circumstantial evidence presented above is sufficient to justify some confidence in the results.

Grindley (1967) showed that moisture accounting on a daily basis was more precise than using monthly time intervals. He indicated that in a very wet month with all the rain concentrated into the last few days; whilst the monthly system would probably not show any soil moisture deficits, the daily system would enable small deficits to build up during the 25 days of dry weather and so would begin to inhibit potential evapotranspiration in a manner closely analogous to the real world situation. However, as has been shown, it is not feasible to compute Penman estimates of evapotranspiration on a daily basis if one is using the hours of bright sunshine as a surrogate for incoming short wave radiation. A Meteorological Office procedure has therefore been adopted involving the allocati n to each day in a particular month a proportion of the estimate of he potential evapotranspiration for that month. The proportions are weighted so that seasonal changes are accommodated, as for example more evapotranspiration may be allocated to the end of April than to the beginning.

Interception was simulated by removing to the interception storage land fixed of each use a proportion of daily rainfall until the storage was. at full ca acity: The storage was

,' 185

depleted at the potential rate. The relative simplicity of the algorithm

was necessitated by the dearth of useful empirical studies and the over

riding need for simplicity in the model. Penman (1963) likens a tree

to an umbrella, "for a while, at the beginning of a storm the protection

is complete or nearly so, but gradually drops begin to seep through the

canoe until the protection is zero". He goes on to comx4nt unfavourably

uponºthe common practice of quoting interception rates for whole rainfall

events when patently the rate varies during a storm. lie prefaces his

review of quantitative estimates of interception rates by saying that

the subject is "somewhat chaotic". lie*says, of intercepted water "most of (it) is re-evaporated and becomes part of the evaporation term in the hydrological balance sheet". The fact that a little of the intercepted water finds its way to the ground as stemflow, 2.1% of throughfall in the case of oak (White and Carlisle 1968), was not thought to be sufficiently important to merit the added complexity it would cause within the model. Consequently, the storage was depleted simply by evaporation.

The need for simplicity in the model and the lack of suitable data precluded the inclusion of parameters which took account of varying intensity and duration of rain and wind speed.

Rainfall which was not intercepted was allowed to fall on either a riparian or non-riparian soil surface. The riparian area was considered to be bordering the area the stream channel where the soil is permanently field at capacity and from which runoff always occurs after rainfall. Intuitive reasoning by Penman (1950b), field measurement by Betson (1964) and by (1969) conceptual argument Kirkby has pointed to the existence of such an in area any catchment. An initial run of a rudimentary version of the itself, computer model suggested the existence of such an area. In dry summers, when the soils of the catchment had substantial moisture

/ 186 deficits and consequently would not be expected to yield runoff, there was enhanced streamflow from even the lightest rains. Intuitive field mapping, air photo analysis, and consultation with the Lee Conservancy suggested that about 8% of the Canon's Brook is riparian.

Rain falling on the non-riparian part of the catchment was allowed to transfer to the stream, upper soil moisture storage, soil moisture storage, or ground water depending upon the prevailing moisture conditions in the root zone of the soil. It was in this infiltration function that the greatest amount of simplification and generalisation took place in the progression from hydrological reality to computer simulation, but, as subsequent sections show, this generalisation was justified by the usefulness of the simulation results. Phillip (1969) has suggested that "the understanding of the basic mechanisms of I

(the hydrological micro-processes of the cycle) ... is ... well advanced ... The macrohydrologist ... seems often in the posture of flatly denying that (of the pieces a truely physical model of the hydrological cycle) are there. It is my impression that somewhat more physics could be embodied in the models with profit". Alternative views have been expressed by "macrohydrologists". Nash and Sutcliffe (1970) say that "the deterrent (to fully a aýsalytical model) is the complexity of the boundary conditions rather than any essential difficulty in the physical laws". A more general point was made by Dawd (1969) who wrote that "if a simple model will suffice, none more comple is necessary". A simple approach to infiltration has been adopted-in the simulation model; it produced acceptable results and suited the available data.

When the soil associated with a particular land use was at field capacity almost all the effective rainfall, i. e. total rainfall minus

r 187 i r losses, was transferred to the stream as overland flow. Once the developed soil a moisture deficit, all rainfall, except that intercepted, leached went; into soil moisture storage until field capacity was again.

So long as the soil moisture deficit remained small, percolation occurred from the soil moisture reservoir to ground water storage according

to. the following relationship

PERCO Kp + (Kq ACTSMD) 4.19 x ...... where, PERCO is the daily amount of percolation to groundwater from

that land use (it was never allowed assume a value

less than 0.0);

Kp and Kq are parameters optimised to values of 0.015

and 0.010 respectively;

ACTSMDis the land use's soil moisture deficit. (This was

never allowed to assume a value greater than 0.0).

Once the soil moisture deficit exceeded the root constant then the rate of evapotranspiration fell below the potential rate as shown in

Figure 4.6. When precipitation fell on a soil whose moisture deficit was in excess of the root constant, the soil moisture reservoir was filled from the top downward by the upper soil moisture storage reservoir. This upper level storage was filled until it was equal to the prevailing soil moisture deficit, at which time the situation

field reverted to one of capacity. This simple situation is complicated little by a the seasonal transition from cereals to fallow in certain parts of the catchment. At the time of this changeover the upper soil moisture storage of the cereals is added to the soil moisture reservoir because the fallow land cannot extract moisture from as great a depth as the roots of the cereals, see Figure 4.11 for an illustration. The upper zone soil moisture was depleted at the potential rate of evapotranspiration it since was considered that all roots have ready access to in moisture these upper levels of the soil (Grindley 1967). 9 " / 188

27 'eet eentlant e_M ý""` lCtraei. " woa "ý ýý. 1"

ef 3 0' W /t ff Ratl constant ., ý.. (a nt Woes) 4, it ý. º- Root constant ! "! '

(Law frost)

oJ sf 1%e0t ýtettant Oe" if Isara for) t >>I .. 0 fý>>7ýäý7tf p 14 POTENTIAL SOIL MOISTURE DEFICIT (IN INCHES)

Figure 4.6 The relationship between potential and actual soil moisture deficits.

0 f ýj 189

The ground water storage reservoir was depleted by evapotranspiration

from the riparian area and baseflow. This latter channel input was

variable and dependent on the level of the ground water reservoir as

follows:

BASE Kr + (KS STORE) - x ...... 4.20

where, BASE is the daily amount of baseflow, Kr and Ks are parameters

optimised to 0.005 and 0.010 respectively;

STOREis the current amount of water in the ground water

reservoir (this had a median value of 0.0)

The two channel inputs, overland flow and baseflow were summed

for each day for each land use as if the whole catchment consisted of .S that particular land use. The predicted runoff for the non riparian

part of the catchment was then calculated as a weighted mean of the

runoffs from each of the different land use types. The weighting

used was proportional to the percentage of the catchment for each land

use in each year. The runoffs from the riparian and non riparian

areas were then summed to give an estimate of the runoff for each day.

These daily predictions were used in two ways. First, monthly water yields were calculated by summing the daily runoffs for that month.

Second, daily the runoff in inches was converted into a mean daily flow

in cusecs which was then incorp rated in a routine which calculated and flow printed duration analyses or each month and for each year.

The described model may appear unduly complex to some and simplistic

to others and will certainly appear to be subjective to all except those involved in also simulation experiments. The major strengths of both the techniques and the particular model is that it mirrors the; dynamic interactions known which are to exist in nature and that it produces

i 190

results which are both valuable and meaningful. The problems of model building in hydrology are probably best summarised by James (1970) who observes that at this point, both the art and the science of watershed modelling become manifest. The science comes in the theoretical derivation and empirical verification of equations describing such hydrologic processes as infiltration or flood wave movement. The art comes in reviewing the large body of available equations and supporting data and selecting and combining appropriate expressions into the single unit which will give the best results".

Calibrating the Model and Testing its Fit against Reality.

All hydrological models must be fitted to reality by optimisation of parameter values over a set calibration period. Early models, developed in the sixties, used subjective fitting and testing techniques for calibration, while later models use more thorough, rigourous and quasi-objective methods. Linsley and Crawford (1961) discuss the development of a simple daily soil moisture accounting model in which initial estimates of parameter values were made by inspection of charts or through experienced judgement. They say of their results, which are shown as a time sequence of g uged and simulated flows, that "the Tcomputed agreement between actual and flow is quite good". James (1965) in his discussion of the use of the Stanford Watershed Model IV for the estimation of the effects of urban development on floods, reminds that

"the final test as to whether a given set of (parameter) values is is adequate whether it produces a synthetic hydrograph that matches a recorded hydrograph. for the same spot ... Experience will certainly speed 'satisfactory the trial and error proc s (of calibrating parameters) ... matching should result with less than 10 attempts". Dawdy and O'Donnell (1965) "automatic explored objective methods of finding numerical values 191

of the parameters of synthetic hydrologic models". They argued

that as hydrological processes become better known, so models will

become increasingly complex and, while not denying "the power and

advantages of the operator's skills, subjective trial and error

proceedures will become impracticable". More recently O'Connell,

Nash and Farrell (1970) have employed an optimisation process developed

by Rosenbrock which "involves successive changes of parameter values

(a) according to ...... preconceived rule ... which takes into

account the results of previous steps and in particular whether or

not a change improved the fitting". Similarly, James (1970) has

developed the Stanford Watershed model still further and reported an

algorithm called OPSET which makes for "self calibrating watershed

models".

At an early stage in the development of the simulation model for

the Canon's Brook, a subjective trial and error fitting procedure was was chosen for three reasons. First, initial runs of the model showed

that trial and error calibration was quite adequate. The relatively

large potential instrumental errors and the modest demands made of the model made the chances of ace ptance of the developed model very high.

Second, there were relatively few parameters to be optimised and therefore manual techniques were feasible and practical. Finally, digital the simulation model marked the first involvement of the writer

both with modelling and programming and so the added complexity of automatic optimisation was not considered desirable.

A number of methods were to test P used the model against reality in each of the trials undertaken di Bring calibration. Graphical output of both flows gauged and predicted provided a rapid visual check on persistent in or systematic errors simulation. Numerical results of ý 192 the gauged and simulated flown and the difference between them were expressed in both absolute and percentage terms. Inspection of these I results was generally sufficient to indicate the direction and magnitude of changes needed to optimise the parameters. An overall indication of the adequacy of the model was given by the standard deviation of the residuals and the percentage of the total variance explained by the model. The'criteria of goodness of fit was that the standard deviation of the residuals should not be greatly in excess of 0.3 inches which was the likely error in gauging discussed in Chapter 3. The proportion of variance explained by the model was calculated in a manner suggested by Nash and Sutcliffe (1970). They argued that their sum of squares,

F2, is analogous to the residual variance in a regression analysis; thus

n 2 ti FZ (q I" 4.21 q) ...... 1 where, q is the gauged flow; t q' is the simulated flow.

They suggest that the initial variance of. the flow data may be ca]4 fated from;

n_ Fo (q 4.22 - q%2 ......

q is where, the mean of all of all of the individual q values. The percentage efficiency of the model, R2, (analogous to the coefficient of determination in regression) is calculated thus;

F2 2 R2x 100 4.23 F ...... 0

In the absence of gauged data on soil moisture conditions or ground water a third rather subjective method was used to test the model. This involved checking that the soil moisture reservoir regained field in capacity most land uses in all normal years and that the ground water 193 table rose and fell around its predetermined median value of 0.0.

This final check prevented any long term decline injeither the water content of either storage locations. Ideally the model should have been provided with a few years of data prior to 1950 and allowed to

"settle down" to normal functioning before the calibration period was reached! In the event this was not possible because the meteorological records for Eastwick Lodge began at the end of 1949 giving only a few month] for the model to adjust itself. "Consequently, the conditions in tie catchment on the first day of simulation, October list 1950, were, ' defined by the author from a knowledge of the Monthly) Weather

Reports for the previous years and inspection of the performance of the catchment in the initial part of the study period.

Error and Sensitivity Analysis.

A qualitative comparison of errors involved in hydrologic modelling and analogous errors in standard statistical analysis has been presented by Dawdy (1969) and is repreduced in Table 4.7. He further suggested that, since measurement and sampling errors are present in all three types of modelling error, one can apply random errors to input data in order to examine their effect on an optimised model and its predictions.

The rural version of the Canon's Brook simulation model was tested for its sensitivity to errors in both rainfall and evapotranspiration by the method detailed by Dawdy (1969). Each value in the input series

daily of either rainfall or estimated daily potential evapotranspiration was given random errors with a mean of zero and a standard deviation of between ten and seventy percent. An IBM Scientific Subroutine

GAUSS (IBM, 1967) called was used to generate random normal numbers with 194

Table 4.7

Qualitative Comparison of Errors involved in hydrologic modelling with analogous errors in standard statistical analysis (after Dawdy, 1969)

Source of Error 4 Size of Error Statistical Analog

Data a Measurement and Sampling error

Comparison of measured Standard of to simulated during error for fitting the estimate period used "a -b

Comparison of measured to simulated during a Standard error of period not used for prediction fitting a+c 195 a mean of 1.0 and a standard deviation of between 0.1 and 0.7. Data input when multiplied by a GAUSSsupplied number, introduced the desired error. The results of the separate analyses of rainfall and evapotranspiration are shown in Figure 4.7 (a) and (b) respectively.

At the time of this experiment the variance of the simulated figures for the calibration period was 0.09 inches. Errors of up to 30% in the rainfall changed this figure but little, whilst with larger errors there was a significant decline in the goodness of fit of the model.

The reason for this almost certainly lies in the fact that introducing errors into rainfall alone, denies the existence of a clear link between rainfall, cloudiness, sunshine and evapotranspiration. Consequently in some months when the evapotranspiration estimate refers to a relatively dry sunny month, the random errors in rainfall may produce an adjusted rainfall which could only have come from a damp cloudy spell. The introduction of random errors into the evapotranspiration estimates had no significant effect on the variance of the model during the calibration period. This probably results from the simplification involved in estimating daily potential evapotranspiration from the monthly figure.

The introduction of random errors interrupts the regular sequence of daily estimates provided by the assumptions described in the previous

but section the substitute series of daily estimates is probably not unlike the real situation when hot dry days can often be followed by mixed or rather wet periods.

The likely errors in the rainfall data for the Canon's Brook were in Chapter 3 considered and the work of the Institute of Hydrology on the relative catches of standard and ground level gauges was described. In order to examine the effects of the underestimation of rainfall at Harlow as described in ChartQr3 , the rainfall for each month was increased in the proportions observed at Wallingford. Figure 4.8 shows

ý' 196

OC ,-O Cd of 4-b Ct

Ný ý0 O Lr d0

OC U Od a dö

ý-N

ßLvor

C)ca X Nd

he calibration period in the rainfall, input. ". 4 Variance for the calibration period. b. C 0 O i a In 1.0 / to 0 ýI dd t a%. - r, O .C bb 0 O C bü

Co. Nv

0ö bb Ly "'L öd a co O Öý«, d calibration period v ý the evapotranspiration 0LL

O D"1 0.2 0-3 0.4 N Variance of the calibration

'Figure 4. Simulation model error analysis. 197

vc 100 . d The variance of the optimised E model without simulated errors 80- "-, c

.00 dvý 60 `ý. ° r.- 0q H cU`- o N 40 v

Dn 20 Zn üd ca ox 4-0(S 0 0 02 0.4 0.6 Variance of the calibration period

'Figure, 4.8 The impact of a correction factor for the catch of standard rain gauges on the variance of the model during the calibration period.

i ,' i 1ýG f i 1 198

I that when the correction factors were applied with odification then

the variance of the simulated figures for the calibration period

doubled from 0.09 inches to 0.18 inches. The introduction into the

rainfall data series of random errors with standard deviations of

between ten and seventy percent, together with the ystematic corrections

is represented by the vertical axis of 4.8. It is clear that the

addition of progressively greater random errors to the extant systematic

correction reduced the fit of the model still further. The increase 'input in the of rainfall to the model also resulted in an absolute

increase in the streamflow predicted for the calibration period, the

meanldeviation of the simulated and gauged flows rising from 0.11 inches

to 016 inches. It would seem therefore that the model tloes not

possess sufficient self-regulation to accommodate the augmentation of

rainfall in this fashion. Consequently if the adjusted rainfall were

employed in the_ simulation it would be necessary to modify the parameters

controlling' evapotranspiration so as to avoid an excess outflow of water

from the catchment by way of the river.

This analysis suggests that the rural model and the associated urban version are not uniquely calibrated to a single sequence of data.

The rural model has a certain resilience in the face of random errors in either evapotranspiration or rainfall, but the incorporation of

in systematic errors the rainfall record showed that parameters would

be then need to re-optimised. More importantly, no modification to

the calibration procedure would be generated by random errors in either data of the series and the parameter values, similarly, appear to be stable under these circumstances.

/ 199

Results of the Simulation of the Rural Hydrology of the Canon's Brook. )The first run of the model was undertaken with a very rudimentary vers, &on of the one shown in Figure 4.5. It lacked the ficilities for y interception, groundwater storage, baseflow and riparian area runoff.

The model was also uncalibrated being wholly based on the rules and equations suggested by other writers. The results of this run are shown in Figure 4.9, which shows that the general pattern of the simulation-is correct but with frequent large deviations between the two sets of data. During the calibration period the winters seemed to have an excess of flow whilst the summers exhibit a runoff deficit.

This suggests that storage of some of the winter excess occurs. The very large positive residuals during the winter of 1950-51 suggested that the starting conditions were inappropriate. The efficiency of the initial run of the model, R2, was only -9.15% and the standard deviation of the residuals was 0.67 inches. The residual variance

F2, was high at 16.11 inches but it was a marked improvement upon the straight forward comparison of rainfall with runoff which gives an

F2 of 96.5. Nash and Sutcliffe (1970) suggested that the efficiency of a separable model part, r2, may be judged by the change in R2 which results from the addition of the new model part. The equation they give is as follows:

2R2 2- R2 r1...... 4.24 1- R2 where the 1&2 denote subscripts conditions before and after the insertion of the new part-of the model. On the basis of these equations the addition the of evapotranspiration and soil moisture calculations to a simple rainfall runoff model improved the R2 from -560 to-9.15 per cent. The efficiency of these additions was 54% and so justified their continued

f 200

/35- GAUGED f """ t "» PREDICTED ti

.ai" `ýi Ö, Y'1' ý1 ;j; r+ : tý ttit}1

;s t 52 *M *54 ss "aa 'Si 158 '59 '60 "at '62 "w es "es "aa w 'ea Ual Jon Jon Jon Jon Jon Jan Jon Jan Jon Jon Jon Jon Jon Jon Jon Jon Jon S"I ! cabrotioh Period

Figure 4.9 Run 1 of the simulation model.

/ 201

inclusion. This early success spurred further experiments with other

routines and processes.

The second run of the model incorporated the groundwater and

baseflow routines and improved starting conditions. The efficiency

of this second stage model was 31% while the standard deviation of

the residuals fell to 0.52 inches. The efficiency of the groundwater

component was 37% and so justified its continued inclusion. A third 4 run of the model, which now incorporated interception properties,

increased the overall model efficiency to 45% whilst the interception

routines showed an individual efficiency of 20.5%. Further attempts

were made to improve the model by inclusion of a more complex

infiltration function which allowed surface runoff to occur after

heavy rainfall, and by inclusion of an algoritLm which changed interception

and evaporation rates when deciduous trees would have been without leaves.

However the individual efficiencies of these new parts was so low that

they were not considered worthy of inclusion. One further change was

made to the model before final optimisation and operation since a

marginally depressed estimate of potential evapotranspiration produced

an excess net runoff figure during the calibration period. This depression appeared to result from the employment of saturation deficit data from the Eastwick Lodge meteorological station where readings were 9.00 taken at a. m. each day At this time of day the deficit would be

rather smaller than during most of the rest of daylight hours when

evapotranspiration was in progress. To overcome this deficit a constant 0.90 of was applied to all of the'estimates of actual vapour pressure at

the meteorological station.

The final optimisation of the complete model gave a maximum efficiency of 80.0% and a minimum standard deviation of the residuals 202

ý.

«-ý GAUGED I

a ý'a

; "a Rrý Pov" tia "as bý 6$ $. $ Jan Jan Jan j don ion Jon An . I- --sCallbrotlon -- Mriod------; --- -- I

Figure 4.10 The final run of the RURAL model.

i' a - Fr r. Owl" 203

of 0.28 inches. A lower degree of error in the simulation would have

been desirable but it proved impossible with the available data and the

subjective optimisation procedures used. It was, however, within the

0.3 limits set by instrumental accuracy. The results of the optimal

run of the rural model are shoum in Figure 4.10 where it can be seen 6 that despite the relatively large statistical error terms the visual

fit of the predicted to the observed flows is good for the calibration

period. The increasing gulf between the two sets of data for the

subsequent period can be ascribed directly to the urbanisation of the

catchment.

Confidence limits were calculated for the predi ons by

establishing the frequency distribution of the residuals and calculating

their mean. Table 4.8 uses b th the Chit test and the Kolmogorov-

Smirnov test (S,e4gel 1956) to test the distribution of the residuals

against that which would have been expected if the distribution were

truly- normal. It is clear from the statistics that even when a large

margin of random error is allowed, by using the 0.20 probability level,

is there no significant difference between the observed distribution and

a normal curve. Table 4.8 also shows that the mean of the residuals 0.03 inches. was Ideallythis should have been zero but sampling and

observational errors must account for most of this deviation. ' A 't' test

was used. to test whether or not the mean of the residuals was significantly different from zero. Both Blalock (1960) and Siegel (1956) discuss the

problems and assumptions of the test and it seems that it is not possible to meet the assumption of independent observations rota normal

populations with equal variances in this case. However, Blalock suggests that "we must assume a normal population when using at test" and consequently it is here used with caution. The statistic calculated is 204

Table 4.8

An analysis of the frequency distribution of the residuals for the

calibration ' period 'of ' the 'rüraI 'version 'of ' the 'sinulation model.

Mean St. Dev. St. Dev. -1 St. "Dev. -1 St. Dev. 0.03 in. +1 +1 ------

Observed 3 3 12 7 7 4

------N------

Expected Dis trib . 5 5 7 7 5 5 from normal curve.

------

Chi2 6.1 Critical value a p-0.20 is 7.30 with d. f. of 5

K-S statistic D-0.11 Critical value at p-0.20 is 0.18 with d. f. of 5

Thble'4.9

Confidence 'limits , for 'the 'predictions 'of ' the Rural Model

68.3% of the predictions are within 0.28 inches of the true, value.

95.4% 0.56 ...... j...... 99.6% 0.84 ...... 0.0.:...... 205 1

t equals 0.27 with 35 degrees of freedom. Since the critical value

of t for a two tailed test at the 0.20 probability level is 1.35 we

cannot reject the null hypothesis and must conclude that there is no

significant difference between the observed mean of 0.03 and the

expected mean of 0.0. It follows from these two tests and the

relationship which exists between the normal distribution and its

standard deviation that the confidence limits of the predictions are

as those illustrated in Table 4.9. 4

The detailed functioning of the optimised Rural Model is presented in Figure 4.11 which shows in graphical form the variation of runoff with soil moisture conditions and ground water levels. The diagram is a static representation of the dynamic equations contained in the computer eyýpig t model and expresses quite clearly the variation in hydrology between the various surfaces. The idea for the diagram is not original since the Ministry of Agriculture, Fisheries and Food (1967) presented a imilar figure for soil moisture variations under grass at Cardington from April 1964 to February 1966. This is i reproduced as Figure 4.12 and shows a remarkable similarity to the

Canon's Brook graph, when one considers that the former is slightly I more sophisticated in having multiple layers of soil.

The consistency of the results of the simulation are checked in Figure 4.13 which shows Maccumulated total runoff from the model and the River Ash. As such it 'is directly analogous to Figure 4.11(c)ß but does not show any changes in proportionality as did this earlier analysis.

The actual results of the simulation exercise are given in Table 4.10 difference which shows the between the gauged runoff and the predicted for rural runoff each month, each seaso and each year. The 206

8 Ma V hc R(tdStur! BARE SOIL SOepC'toil (white erect) 4 6 1yrLºý. eý y y Romoll LEY GRASS- oý_I , Sol moisture GARDENS (writeoet.c, t 41 ! rea)

Runoff PERMANENT OefC swre t GRASS (write w ea)

0 2 of nrA" sy . "M A AIR AAA J-- Runoff 01 `1 Soil moister* CEREALS Cafitit 21 (wºwte are0) 4; "i

1 k ý_ Ll lid Runoff 0 -A A L WOODLAND p 9i hMvte 2, area)

Ground A16- Aghý water Ift- A%- Aa- storao (datum n rbtrary) 1951 1956 1260 1065 1888

Figure 4.11 The functioning of the simulation model during the study period expressed as runoff and soil moisture conditions for each land use and groundwater storage. 207

h Y

u c

ýý

AMJJA5ONDJFMAMJJASONOJF 1964- 1965 1966

Figure 4.12 Soil moisture conditions for Cardington for 1964-66 calculated by the Ministry of Agriculture Fisheries and Food. (after Ministry of Agriculture, Fisheries and Food, 1969).

.ý" 208

1 100 LL LL ZU 80 Z

J 2:, 60 +< OQ

jj 40

20 O Utl.

0 20 120 140 . 40 60 80 100 CUMULATIVE TOTAL RUNOFF FROM THE RURAL MODEL (INCHES)

Ash Figure 4.13 Double mass analysis of the annual runoff from the River and the RURAL model. 209 ý`

Table 4.10

The increase in the water yield of the Canon's Brook as a result of urban- isatidrl'asge6sed'ag'the difference'between the gauged runoff and that ''simulated by'the Rural Model.

" Water Year. 0NDJFMAMJJAS

------00.12 1951 -0.08 00.51 00.10 -0.24 00.27 -0.11 00.07 -0.08 -0.03 -0.22 -0.09 1952 00.10 -0.78 -0.28 -0.34 00.11 -0.33 00.33 -0.03 01.02 00.08 -0.06 -0.02 1953 00.04 01.01 -0.36 -0.11 -0.27 -0.01 00.06 00.41 00.16 -0.12 -0.06 00.00 1954 -0.08 00.14 00.08 00.01 00.14 00.17 00.11 -0.11 -0.05 00.08 00.07 00.13 1955 00.12 -0.42 00.09 00.49 00.12 00.38 -0.02 00.19 -0.03 00.04 -0.02 00.02 1956 00.11 00.14 00.19 -0.13 00.28 00.24 00.09 00.03 00.05 00.09 00.33 00.51 I. 1957 00.78 00.16 -0.11 00.32 -0.14 00.18 00.05 00.09 00.07 00.03 00.08 00.08 1958 00.18 00.35 00.43 -0.02 -0.63 00.18 00.18 00.09 00.43 -0.29 00.31 -0.21 1959 -0.45 -0.19 -0.38 -0.40 -0.02 00.30 00.39 00.10 00.07 00.07 00.19 00.12 1960 00.07 00.18 -0.09 00.09 00.08 00.14 00.11 00.15 00.06 00.16 00.15 01.00 1961 -0.26 -0.05 00.05 00.20 00.01 00.30 00.38 00.45 00.19 00.24 00.18 00.31 1962 00.39 00.49 00.01 00.77 00.30 00.49 00.46 00.30 00.25 00.42 00.27 00.43

1963 00.48 00.58 -0.20 00.41 00.21 01.08 00.57 00.38 00.19 00.21 00.11 00.23 1964 00.35 01.12 00.42 00.15 00.30 -0.02 01.08 00.32 00.40 00.53 00.14 00.06 1965 00.08 00.15 00.20 00.43 00.21 00.64 00.51 00.31 00.21 00.45 00.47 00.26 1966 00.20 -0.85 -1.01 00.23 -0.04 -0.11 01.35 00.38 00.42 00.40 00.42 00.36 1967 00.69 00.19 00.05 00.36 00.26 00.43 00.71 00.92 00.31 00.32 00.36 00.33 1968 01.00 00.94 00.81 00.96 00.36 00.28 00.46 00.58 00.48 00.77 00.75 00.96

/ 210

Table 4.10 (continued)

Winter Total Summer Total Annual Total Water Year Increasp Increase Increase (Oct - Mar) (Apr - Sep)

N-----NM------ýýN-N-NNN ------N---- N------

1951 0.22 0.68 -0.46 -1.52 1952 -1.21 00.32 1953 0.74 0.29 0.45 1954 0.69 0.46 0.23 1955 0.96 0.78 0.18 1956 1.93 0.83 1.10 1957 1.59 1.19 o. 401-,. 1958 1.00 0.49 0.51

1959 -0.20 -1.14 0.94 1960 2.10 0.47 1.63 1961 2.00 0.25 1.75 1962 4.58 2.45 2.13 1963 4.25 2.56 1.69 1964 4.85 2.32 2.53 1965 3.92 1.71 2.21 1966 1.75 -1.58 3.33 1967 4.92 1.98 2.95 1968 8.35 4.35 4.00

----NM-MN------M N--NMMM----MM-N N------

i 211

table shows that as a result of the construction of the new town there ral Fý4. has been a progressive increase in the water yield of the catchment 2+S ý" i4ý

Water year 1958-59 appears to be something of an exception here,

largely because of substantial over-estimation of the winter runoff.

It is likely that this turning point error in the simulation results from the previous wet year when there was 32.34 inches of rain, some

8 inches over the study period average.

Table 4.11 presents the regression equations designed to predict

the effect of urbanisation under different weather conditions. There is clearly a strong relations ip between the annual increments in flow

"' and the percentage of the catchment paved equation (4.25), but this relationship is improved when two summer rainfall variables are added. Equation 4.26 suggests that the impact of urbanisation on annual water yields is greatest after two dry summers. This is wholly reasonable and in line with the view taken in Chapter 2 that whilst there is little difference between the hydrological behaviour of wet clay soils and impervious surfaces there is a marked contrast between soils with a large moisture deficit and urban paved areas. This is supported by the fact that there is no significant relationship between winter increments in flow and the paved area of the catchment. there appears to be a firm link between the urban aýea of the basin and the

increments in flow summer but this cannot be furthe r improved by the addition of more variables. In spite of the clear statistical 4.25-27 significance of equations one must question the magnitude of the b coefficients. If a value of 100 for I is su stituted in equation 4.25 then it suggests that the complete paving of t e catchment would increase by runoff about 32 inches. This is patently illogical when the average rainfall of the basin is around 24 inches and there was 212

Figur1 4.14 The increase in annual flow as a result of urbanisation and its relationship with rainfall

ýI

Y J 1.

1 « U) n

LL!^ 0)

O* 4

D (D co V) "o 6 t0 30 W N m . W u') co _ ý.., 0 0: c °rn Z0 Q 3 C) Iq u1 r- W (D r N co M z la) ýý 0 In ll1 ýº a) U"I M co N J ý Lr) 0) o)ý 0 r- L (r0((D *cp N In '' D) (1) ýi r . - LL 0 (/) 2 q- r- co 25 * ZN LL2 U v Lf) Iv rn 10 Q(A ZUZ M coU) ý"' ... i r L W Z J Vi 1 L < co =v WZ * v O * * °' 20Z ZQ * * Q 0 * * * Jý QZ DQ Zm Z Qr Q _1 1S -01 2 3 45 6 78 9 10 11 12 13 14 15 16 17 PERCENTAGE OF THE BASIN PAVED AND S EWERED

/ 213

-' Table 4.11

Regression analysis of' the' increase in water yield in the Canon's

Brook consequent upon urbanisation and related variables.

Variables Included

Annual increment to flow (ANNUAL) Percentage of the catchment paved (I) Summer increment to flow (SUMMER) Actual Evapotranspiration for each Winter increment to flow (WINTER) year from grass (ACTET) Summer rainfall (SRAIN) Effective rainfall : Total Winter rainfall " (WRAIN) rainfall minus ACTET (FRAIN) Antecedant summer rainfall (ASRAIN) Antecedent winter rainfall (AWRAIN) Annual increment to flow with 68% confidence limits (CONFA) Summer increment to flow with 68% confidence limits (CONFS) Winter increment to flow with 68% confidence limits (CONFW)

Degrees of No. Equation F. ratio Freedom

4.25 ANNUAL m -0.37 + 0.32 I 26.1 1,16

4.26* ANNUAL - 7.39 - 0.50 ASRAIN - 0.15 SRAIN + 0.38 I 25.2 3,14

4.27 SUMMER--0.12 + 0.19 I 72.7 1,16

4.28 CONFA - -0.30 + 0.14 I 10.6 1,16

4.29 CONFS - -0.21 + 0.10 I 28.6 1,16

4.30 CONFW - 2.00 - 0.10 WRAIN 7.7 1,16

* In the 4.26 case of equation the addition of the variables was tested for the increase in the variance explained by each and it was found that both added significantly to the equation when the 0.02 probability level was used.

Attempts were made to add-further appropriate variables to each of the but equations none added significantly to the explained variance when the 0.02 probability level was used. 214 about 5 inches of runoff when the basin was. wholly rural. There appear to be four possible reasons for this type of error. First, the computer simulation model may have given an estimate of rural runoff which is much too low and which resulted in an inflated estimate of the effect or urbanisation. Second, the relationship may not be truely linear, and it might be wrong to extend the regression line much beyond the end of the data. Third, the value given for the slope of the regression line or plane can only be considered an estimate which has a distinct sampling distribution with a known standard error.

Finally, it is possible that the flow in the Brook has been artifically inflated by either instrumental error or the addition to the stream of water which did not originate as rainfall within the confines of the

Canon's Brook catchment. These problems are evaluated in the next section on the URBAN model where similiar difficulties arose. ' In the interim to achieve positive conclusions from this study it would seem appropriate to use the confidence limits calculated for the simulated rural flows and to consider only those flow changes which are greater than a given confidence level. One standard deviation or 68% confidence level is chosen because a wider limit'may mask a number of small increments in flo4 likely when the catchment is in an early stage of development.

The results of using the 68% confidence level as a threshold for considering differences between gauged runoff and simulated rural runoff are shown in the second part of Table 4.11. These rather more conservative estimates of the effect of urbanisation show a strong link between flows enhanced summer and impervious area, a rather weaker link between increased flows annual and paved area and a negative relationship between enhanced winter runoff and the actual winter rainfall. This latter be interpreted relationship may as a weak link between river flow and 215

urban impervious surfaces, for as previously argued, the effect of urbanisation will be most marked during relatively dry spells.

Alternatively it could be argued that this relationship equation

(4.30) represents the inability of the model to cope with variability in winter rainfall amounts. The numerical value of both ANNUAL and

CONFAhave been entered in Table 4.18 along with the results of the other analytical methods and will be considered further in the concluding section. 4

Results of the Simulation of the Urban Hydrology of the Canon's Brook.

Two changes were necessary in the RURAL MODEL to convert it into an URBAN one. First, the land use characteristics of the basin were changed in accordance with Table 4.6, so that instead of uniform land use throughout the study period, there were progressive changes as assessed in Chapter 3. Second, the urban land use was made to function in a rather different way to the normal pervious soil surfaces.

Although it intercepted and retained rainfall in a manner similar to the vegetated surfaces, it did not store moisture in the soil, it only lost moisture through evaporatim when there was water in the interception storage reservoir, and it did not allow any moisture to percolate to the groundwater table. 11

The calibration of the URBANmodel was tested against the initial ten years of record from October 1950 to October 1960. The standard deviation of the residuals was 0.24 inches and the efficiency of the model was 79.6%. As such it was marginally better calibrated than the RURAL model and therefore no further optimisation steps were deemed necessary. The frequency distribution of the residuals from the r 2 calibration period is given in Figure 4.1G. Chi analysis confirms

I 216

Figure 4.15 Graphical plot of the results of the URBAN model simulation and gauged flow of the Canon's Brook 1950-1968.

-1

4 217

40 Calibrated ý[ý " tý\\\Y ýI. ndiýýnd Wem . _. = L . 30-

20-

10-

0 4T 3T 2P IT O 1P 2v 3,r 4r+ "96 "72 "48 "24 O "24 "48 "72 "96 Class Intervals in Inches and Calibration Standard Deviations

r for calibration Figure 4.16 Frequency distributions of the residuals the MAN and prediction periods of the model.

i ,,

N 218

the visual impression that the residuals are not normally distributed.

Inspection of the frequencies which would have been expected under a normal distribution show that the simulation residuals are clustered around the mean, 0.0, more than might have been expected. In these circumstances it is not easy to state confidence limits for the predictions, except to say that one standard deviation, 0.24 inches, embraces some 78% of the residuals occuring during calibration.

The results of the simulation of the urbanisation of the catchment are shown in Figure 4.15. In spite of the good fit during calibration, the prediction period 1960-68 displays a significant disparity between predicted and gauged flows. Moreover, the disparity between the two estimates increases with time. Figure 4.16 shows the frequency distribution of the prediction errors superimposed upon the distribution of calibration errors. The mean of the prediction period residuals is 0.169 inches greater than the 0.0 of the calibration period and their standard deviation is 0.44 inches, but interpretation of these results is made difficult by the skewed distribution.

There appear to be four possible reasons for the error in simulation.

First, the assumptions and operation of the model may be at fault. This seems unlikely since the Rural Model functioned well compared to the River Ash, Figure 4.13, and the assumptions with regard to the paved areas are wholly reasonable and in line with similar studies elsewhere. Second, it is possible that in the estimation of land use in the is in catchment error and that there has been a considerable underestimation of the percentage paved. This suggestions arises from the fact had been that there more of the catchment paved in the 1960s then the runoff would have been greater and consequently the residuals would have been Whilst smaller. feasible, this hypothesis seems unlikely in

0 219

the light of the thorough and painstaking analysis of Chapter 3.

Third, the flow of the Brook may have been enhanced by the addition i /xoenous of water by factories and/or householders. Tlis effect

should be minimal in view of the carefully designed separate sewer

system of Harlow and would appear to be negated by the recant personal

communication--6f the Corporation's Engineer in which he suggested

that there was some evidence of storm water entering the foul water

sewers at times of high rainfall. The theory of stream augmentation

by piped water is given some credence, however, by inspection of the

outlets of the surface water drains which always seem to have trickles

of water in them and the fact that a market garden in the extreme

west end of the basin sometimes irrigates crops. The amount of water

involved in the leakage from irrigation and the continuous dribbling

of the drains would seem to be inadequate to explain the rather

substantial errors found in the simulation exercise. Finally, the

total water yield of the catchment as estimated by the flume may be

in error to such a degree that the simulation errors may be a result

of aer-estimation of flow by the Lea recorder. Chapter 3 explained

the reasons for the flume's inaccuracy at low flows and mentioned the

accumulation of sediment which was removed from the stilling well on

29 October 1969, and which would probably have caused some over-estimation

of flow. Analysis was also undertaken of the likely degree of

inaccuracy in the flume, as compared to the specially designed low

flow station downstream, and it was found that confidence limits of

+ 0.3 inches were appropriate for the monthly estimates of water yield. If these confidence limits are applied to the prediction period then

there are scarcely any predictions which fall beyond them and so one

might assume that the model is a tolerably accurate predictor. However, the increasing difference between the predicted and gauged figures as time 220

jrogressed from 1960 onwards is too problematical o be dismissed so easily. From May to September 1968 the flow o the Brook was recorded by punch tape recorders at both stations and the tapes were analysed by the Water Resources Board. If one calculates the total water yield of the Brook by using the punch tape results for the lot flow wir when the flow was less than 9 cusecs and the punch tape results for the flume when the flows were high then one has an estimate of flow which is free of the errors associated with the 1 design of the flume and its chart recorder. Table 4.12fgives the water yield of the catchment as estimated by the model, the flume chart, and the punch tape recorders. It would seem from the very short period of record shown in Table 4.12 that the results of the ",, simulation of the urbanisation of the catchment are very good in so -1 far as they agree well with a precise estimate of the flow in the stream.

The two models, RURAL and URBAN, have now been built, calibrated and tested. They can now be used in an experimental fashion to predict what might have happened to the water yield of the Canon's

Brook between 1950 and 1968 if the land use constraints on the system had been different from those described in Chapter 3. Table 4.13 and Figure 4.17 depict what the runoff might have been had the catchment been completely urban, wholly grass covered or one of five intermediate stages. The riparian portion of the catchment model was, of course, retained for these hypothetical predictions.

Table 4.13 shows the not unexpected increase in water yield as paved surfaces extend over a previously grassland catchment. Figure 4.17 presents the results graphically and suggests a convergence of the 221 10

Table 4.12

The water yield of the Canon's Brook May to September 1969

Aggregate Simulation High Flow Station's of the Punch Tape Date Chart (Flume) records model for (inches) (inches) both stations (inches)

May 68' 0.37 0.78 0.52

June 68 0.58 0.74 0.51

July 68 0.64 1.05 0.85

Aug 68 0.64 1.01 0.77

Sep 68 0.70 1.23 1.08 222

Table 4.13

The annual water yield of the Canon's Brook, 1950-1968, with seven hypothetical land use types.

Percentage of Grass 100 95 90 85 75 50 0 the catchment covered by: Paved Surfaces 0 5 10 25 50 100 . :: 15 1950-51 11.22 12.0 12.85 13.66 15.29 19.37 27.51 1951-52 8.53 9.12 9.70 10.29 11.45 14.37 20.24 1952-53 6.62 7.33 8.04 8.76 10.19 13.75 20.90 1953-54 4.41 5.18 5.97 6.74 8.31 12.23 20.05 1954-55 7.47 8.16 8.85 9.55 10.94 14.28 21.37 1955-56 5.35 6.26 7.18 8.08 9.90 14.46 23.57 1956-57 6.86 7.52 8.18 8.83 10.15 13.44 20.02 Water 1957-58 10.66 11.67 12.69 13.70 15.73 20.82 30.99 1958-59 10.11 10.53 10.94 11.37 12.20 14.29 18.48 Year 1959-60 6.53 7.42 8.29 9.20 11.02 15.41 24.28 1960-61 14.71 15.30 15.89 16.49 17.68 20.64 26.57 1961-62 8.28 9.02 9.76 10.50 11.99 15.68 23.09 1962-63 6.60 7.25 7.90 8.55 9.85 13.08 19.57 1963-64 6.78 7.46 8.13 8.81 10.17 13.54 20.30 1964-65 2.54 3.51 4.49 5.46 7.41 -12.29 22.04 1965-66 : 1.71 12.39 13.16 13.89 15.34 18.98 26.24 1966-67 8.33 9.01 9.68 10.37 11.72 15.09 21,86 1967-68 4.61 5.49 6.37 7.25 9.01 13.40 22.18

i r

/ 223

" "% gross and 9% loved woo "y " 49% gross "n" 15% ". vn. "r*. " 751. gross end 85%G paved "n" y8 " 50 f. gross W 501 paved woo . 100% . ve Ms" """".

°6 V 10" ,0 1s 20 x Amwol runoff M IncM" for " portly wOmN""0 Conon'rt YroW

a 'Figure 4.17A graphical plot of runoff from completely grass covered Canon's Brook against the runoff from the same catchment in various stages of urbanisation.

1 it 224

yields of the different basins as flow magnitudes increase. This idea

was tested by fitting regression lines to each of the sets of data in

Table 4.13. The runoff from the partly urban basin was the dependent

variable whilst runoff from a wholly grass covered basin provided the

independent variable. Table 4.14 gives the results of this analysis Tendency, from hich three points are apparent. First, there is a

revealed in the coefficients, for the water yields of the various

basins to converge as runoff amounts increase. Second, the amount of

runoff which would occur from a partly urbanised basin, at a time when

a similar wholly grass basin has no outflows, is almost directly

proportional to the paved area. This second finding depends heavily,

however, upon the basic assumptions of the model. Finally, the

variability of flow around the line of best fit is greater with increased

paved area, probably reflecting the more direct relationship between ý`

rainfall and runoff in the urban areas than in the analogous grasslands.

In conclusion, it would seem that the use of confidence limits

in the further analysis of the results of the RURAL model are vindicated.

Further, it would appear that the simulation of the urbanisation of the

catchment has been successful and that data errors are almost wholly to

blame for the lack of fit of the model during the test prediction period in the 1960s.

The Flow Regimen.

The total water yield of the Brook has been shown to increase with the spread of urbanisation over the basin and the question may now be how asked has this increased flow affected the flow regimen of the stream?

There do not appear to be any widely used procedures for the analysis of changes in the flow regimen of a stream. Standard 225

hypothetical Table 4.14 Regression analysis of the water yields fron the seven

versions of the Canon's Brook Basin

Dependent Variable* Constant Coefficien Std. Error P Ratio df

Annual flow from 95/5 Basin 0.91 0.98 0.01 7456 1.16 Annual flow from 90/10 Basin 1.80 0.95 0.02 1717 1.16 Annual flow from 85/15 Basin 2.70 0.94 0.03 743 1.16 Annual flow from 75/25 Basin 4.52 0.82 0.06 241 1.16 1.16 Annual flow from 50/50 Basin 9.01 0.80 0.11 " 47 Annual flow from 0/100 Basin 18.05 0.59 0.23 6 1.16

* The independent variable throughout is the predicted annual flow from a wholly grass covered catchment. The figures for the basin indicate: percentage of the catchment with grass cover/percentage of the catchment paved. 226

hydrological texts like Chow (1964), Linsley, Kohler and Paulhus (1958) and Wisler and Brater (1959) do not discuss the subject beyond the establishment of a single flow duration curve for a stream. Crippen and Waananen (1969), who are almost alone in attempting to describe the effects of urbanisation on a river's regimen adopted two approaches.

First, they listed the number of days each year that the Sharon Creek, an emphemeral stream before urbanisation, was flowing and compared this with two other streams which were not urbanised and which remained ephemeral. Second, flow duration curves for the Sharon Creek before and after urbanisation were compared with contemporaneous curves for other streams. Whilst originally similar, the curves diverged widely after urbanisation of the Sharon Creek, (see Figure 2.2(b)).

Three approaches have been used to investigate flow changes in the Canon's Brook. Ogives of the accumulated total number of days that each flow was equalled exhibit changes of flow frequency with time. Time series analysis of median and quartile flows demonstrate changes in overall flow distributions and finally comparison of recorded flows with those simulated by the rural model give a good indication of the nature and magnitude of changes at each flow level.

The mean daily flow data or the period January 1953 to September

1968 derived was from analyses of the original charts undertaken by officers of the Lee Conservancy Catchment Board. The data for

October 1950 December 1952 to was derived from the charts by the author using the same method as the LCCB. Instrumental failure resulted in insufficient data being available to define the flow regimen of the in stream November 1950, February 1951, May and June 1951, and October,

November December 1951. and These breaks necessarily complicate the

i 227

interpretation of the results, but they do not negate the essential value of the early part of the record.

Accumulated total number of days with each of six flow levels plotted against time is shown in Figure 4.18. An unchanging flow frequency would have produced a straight line relationship whilst an increase in the frequency of a given flow would have reduced the slope of the line. A decrease in the frequency of flow would produce an increase in the slope. Figure 4.1$(a) shows that there has been a marked reduction in the frequency of flows of 2 cusecs from 1955 and again from 1960 onwards. This fact coupled with the absence of flows of 1 cusec from 1955 confirms the view that in the case of the

Canon's Brook, urbanisation has tended to increase the magnitude of the low flows. Figure 4.10(b) suggests that there has been little change in the frequency of flows of 3 cusecs whilst (c), (d) and (e) exhibit marked increases in the frequency of flows of 4,5 and 10 cusecs respectively. Figure 4.18(f) suggests that, when one allows for the absence of data for a few months at the start of the period, them has been little detectable change in the frequency of flows equalling or exceeding 15 cusecs.

Descriptive statistics may be applied to the frequency distribution of flows for each of the 18 water years under consideration. The quartile, median and a modal flows for each of the water years are plotted in Figure 4.1,9 and are listed in Table 4.15. Since it has already been shown that there are no significant overall trends in the weather affecting the a ea it seems safe to assume that the overall upward trend in the lev#l of flow in the Brook is the result of urbanisation. There are, however, two reservations about Figure 4.19. 228

a. b.

0 0 in w o- . i2 ý E 0

M n 5 c

Ö 0

0 0F E z

c, umurauvu 1u uu numvsr UT uuys Cumulative total number of days

C. d. 0 0 In in ob 200

CM C C o E Eloo

0 0 4- z xE z 3

0 300 600 900 1200 0 Cumulative total number of days Cumulotivt totot number of days e. f.

Ob ö 15-

N L 10-

w O %E LY G E z d3 z

Cumulative total number of days Cumulative total number Of days

Figure 4.18 Accumulated total number of days with Canon's Brook mean daily flows of: - (a) 2 cusecs, (b) 3 cusecs, (c) 4 cusecs, (d) 5 cusecs, (e) 10 cusecs, (f) 15 cusecs and over. 229

Upper quartile flow N N " Median flow 10 f Lower quartile flow \ý ® Modal flow

13-

" 1100"

1 "-" "" o

A 2]

XM X54 I ------11 52 56 '58 '6Q '62 '64 66 68 ------October

Figure 4.19 The changes in the modsr3, median and quartile flows of the Brook, 1950-68. 230

Table 4.15

Statistical Summary of the Changing Flow of the Canon's Brook

Modal Quart. Annual Rainfall Median U. Quart. L. Water Year Flow Flow Flow Flow Cusecs Cusecs Cusecs Cusecs ------N------NN -- --N - - ---N- -- N --N--N

1950-51 28.79 2 2.5 5.9 1.6

1951-52 21.55 2 2.4 4.9 1.6 Data Missing

1952-53 21.83 1 1.6 4.6 0.9

1953-54 21.43 1 2.0 2.8 1.0

1954-55 22.26 2 3.4 5.7 2.5

1955-56 24.80 3 3.7 5.6 2.6

1956-57 21.55 3 3.7 5.2 3.3

1957-58 32.34 3 4.5 6.0 3.5

1958-59 19.22 3 4.5 6.5 3.6

1959-60 25.41 3 3.9 6.0 3.2

1960-61 27.98 4 5.8 10.0 4.5

1961-62 24.27 4 5.3 8.0 4.4

1962-63 20.73 4 6.4 8.0 4.1

1963-64 21.34 4 4.8 7.5 4.0

1964-65 23.30 3 3.9 5.5 2.3

1965-66 27.40 3 4.8 10.0 3.1

1966-67 23.01 4 6.1 9.5 4.4

1967-68 23.43 5 5.4 8.1 4.8

ý- NMý- --N--NN--N -N-N - 231,

The x axis is time whilst the hypothesis relates changing flow to

urbanisation. Also, the variability of the flow of the Brook is

largely a result of yearly fluctuations in the primary phenomena

like rainfall. Figure 4.20 attempts to reduce the importance of

these two factors by plotting percentage of the catchment impervious

against streamflow standardised for variations in annual rainfall as

follows:

RF 4x SFkt kt 10.0 4.28 a ...... Rainfall t

where, SF is the standardised flow

RF is the recorded flow

k signifies the statistic of flow be it median or one of the

quartiles w

t signifies a particular year.

The data plotted in Figure 4.19 can be summarised best by three

linear regression lines whose exact format is shown in Table 4.16(a).

The F ratios for the three equations all exceed the 0.01 probability value of 16.12 with (1,16) deg ees of freedom and consequently we may conclude that all the rel tionships are highly significant. The slopes of the individual regression lines may be tested by comparison of their b coefficients with at test described by Ollsson (1965).

Use of this test reveals that even at the 5% level there is no significant difference between the slopes of the three lines and therefore we can conclude that there is no significant tendency for the lines to diverge with increasing imperviousness in the catchment.

rý The regression equations of Table 4.16(a) can be, transformed algebraically to give the'predictive equations of Table 4.16(b). 232

r K rd d3 d V 3 0

v2L O 0 C O N1

0 02468 10 12 14 16 1 Percentage of the catchment paved

Figure 4.20 The changes in the median and quartile flows of the Brook, 1950-68, standardised for annual rainfall variations.

/ 233

Table 4.16

Anal sis of the variation in flow in the'Canon's Brook an e spread "of urbanisatiön

(a)

SFmedianst 0.98 0.09 1t (F1,16 4.29 - + - 29.8) ......

" (F1,16 4.30 SFU + 0.11 It - 24.2) ...... Quart s t'1.86

SFL 0.66 0.07 It (F1,16 26.0) 4.31 Quartst - + - ......

where It is the percentage of the catchment impervious in year t.

(b) It 0.98 Rainfalls 0.09 Rainfalls ' 4.32 Median Flows . + . ... 10 10

1.86 Rainfalls 0.11 Rainfall It U. Quart Flow . +.. . t 4.33 s - ... 10 10

L. Quart F1owt 0.66 Rainfalls 0.07 Rainfalls It 4.39 _, .. + . " ... 10 10

0 234

Equation 4.32 has been used to construct the family of curves shown in Figure 4.21, which depict the estimated median flow in the Brook given certain assumptions about the percentage of the catchment impervious and annual rainfall.

This analysis has shown four things. First, there has been a marked increase from 1.5 cusecs to 5 cusecs in the modal flow of the

Brook with the urbanisation of the catchment. Second, the middle flow range, characterised by the median, has increased a little under

200% with 16.6% of the basin paved. Third, the low flows have increased in magnitude, a conclusion which runs counter to the intuitive discussion and reasoning advanced in Chapter 2. An attempt has been made to summarise the latter in the flow duration curves of Figure 4.22. Finally, although there is circumstantial evidence for an increase in the range of flow found between the quartiles, this is not statistically significant.

The simulation model may be operated using data recorded on time interval of one day. This facilitates the calculation of mean daily flows and the presentation o monthly and annual flow duration analyses. The results may e printed out, along with the actual recorded flow durations, in thirty divisions, using 29 categories with

1 cusec divisions, and an open ended category which included flows of

30 cusecs and greater.

The similarity between the simulated distribution of mean daily flows 2 and the actual-dis ribution may be tested by means of the Chi statistic. Table 4.17, presents the mean distribution of both simulated and gauged flows for the initial three year calibration period tt and gives the chi statistic as 11.1. The critical value of Chi with 235

Equation I. Malion flow .0 98 x annual rainfall " 0-09 410n . rviouf 10 10 o

Eaudtloe 2. Median Ilow. 2,35 . 01 R N. imp. rvlouf

to-

ýtton Eck' -. I". ".. ,ýtott. I",

I- /a}on fr + 30 ro beý-""-'EQ%)G ýý" 1%0,4 19yý' ,. c infdtt 6- E4 I, d ö ö }t%IýIý M: 2 O11 r IM VOW . ý''"ý tT, Eouot1On ý inlall 'ý fºow 'o M. dioý F

0 Of400 ,0 ,Z 114 10 ,s Percentage of the basin paved

y "ý

Figur 4.21 The estimated and projected median flow of the Canon's Brook and their relationship to rainfall and the percentage of the catchment paved.

f 23G

,ý

a U dQ a. 2501. impervious U

S C

4 3 O 10% impervious (. 3 r

Rural

o 125 10 '20 30 40 50 60 70 80 90 95 Percentage of time flow is equolled or exceeded

Figure 4.22 Flow duration curves for the Canon's Brook during a year of average rainfall with three degrees of urban development estimated from a regression analysis of the quartile flows in Table 4.16.

C 237

Table 4.17

Chi2. Analysis of the Simulated' Flow'FrequencY'and Actual Flow Frequency durin the'Celibretion Period. (A three year mean is presented rather

than three year totals. )

Class Interval iSimulated Frequency of Actual Frequency of Cusecs Mean Daily Flbws Mean Daily Flows

0-6 276 1 261

7- 10 10 21

11 - 15 4 9 16-20 15 4

21-29 2 2

30 and over 9 6

Chi2 s 11.1

df .-5

Critical value of Chit at 5% level is 11.1

Critical value of Chit at 20% level is 7.3

0 238

5 degrees of freedom is 15.1 if one uses the 0.01 probability level,

and coisequently one can accept that there is no significant difference between the two frequencies. There are, however, two reservations.

First, had the class intervals been chosen in any other fashion, the hypothesis would not have been acceptable at the 1% level Second, if he actual total frequencies for the-three years are e ployed in 2 preference to the three year mean, then again the Chi statistic is so large that it suggests that there is a significant difference between the. two frequencies. The overall conclusion flust be that

the model's performance is modest statistically during the calibration period.

.\ A representative selection of the results of the simulation of the flow duration of the Brook by the Ru al Model is shown in Figure

4.27. Figure 4.23(a) shows that, in spite of failure of statistical methods to show a significant relationship between the gauged and simulated flow regimens for the calibration period, when the results are presented in the form of standard flow duration analyses on probability paper there is relatively little difference between them.

There seems to be a slight overestimation of the frequency of low flows during the calibration period, but reference to (b) shows that during 1953-1954 the simulation was very precise for 98% of the flows.

It would seem from these two graphs, which represent the pre- and early urban states of the catchment, that a certain degree of random error be can expected in the analyses but that it will rarely exceed 5%. The data for 1955-56 confirms this view for flows of over 8 cusecs, but there seems to be a marked increase in the frequency with which the low flows of about 3 cusecs are equalled or exceeded. The remaining parts of the figure confirm that there has been a progressive tendency 239

"r t" r w-w. "w wr w w. w

trrr r. wwwý. r

ý( w"www ""1 S. 4/"rww w" w"""www w" 4r 4' r f. r . ate - "w. wn .. r per w"r w

ýr rw l "wn. w.. w. w"+"" w1

i~ M1M 7

" "

«""www O" 0f. " wý . """"""1""" N 1""11 flow "www w" 1" 0""01#00 M """"" t1,

Figure 4.23 Simulated rural flow duration curves and gauged flow duration curves for the calibration period and alternate water years from 1953-54 to 1967-68. 240

for the frequency of very low flows to decrease with further urbanisation,

and for the flows of between 4 and 14 cusecs to increase in frequency.

Figure 4.24 relates the percentage of time that given flows are equalled or exceeded, in excess of that which would have occurred had the catchment remained rural. It is clear from the diagram that urbanisation has brought a marked increase in the frequency of flows of 3 cusecs and greater and that even flows of 10 cusecs and greater, accounted for 14% more time with urbanisation in 1968 than they would have done had the catchment remained unchanged. It is more difficult to draw firm conclusions about the changed frequency of flows greater than 10 cusecs. This difficulty arises because they form such a small percentage of all flows that relatively small errors in the simulation may give spurious results. In view of this difficulty, a more detailed discussion of these flows is included in Chapter 5 where the frequency of floods as small as 10 cusecs is considered.

Conclusion.

The water yield and flow regimen of the Canon's Brook for the period October 1950 to September 1968 has been examined by a variety of analytical techniques. These varied approaches have highlighted some of the general problems' attached to using secondary data and the simulation exercise has exposed some weakpesses in the Canon's Brook

Flume data. The results of five assessments of the hydrological impact of the urbanisation of the Canon's Brook are shown in Table 4.18. The table displays not inconsiderable differences between the various techniques employed. Probably the most reliable estimate is furnished by the simulation model with confidence limits applied. The double mass analyses are likely to be the least dependable figures whilst the multiple regression and crude simulation results are intermediate. All of the techniques reveal marked increase in the water yield of the 24.1

100-1 3 cusacs Qu cý "---- 4 cusacs üý """""" 6 cusacs aCY 80 ilL 8 cusacs "-" 10 cusacq

00 a3 d

r Lt 40 1fi L 'O Ob

ZV VXOb 20

.. b

-oC,. 7. o..._. _. V to a a. to

-20-t 0 14 16 0 Percentage of the basin paved

Figure 4.24 Accumulated total difference between the gauged flow durations and the simulated rural flow durations plotted against the percentage of the catchment paved.

i 242

Table 4.18

Summary table of increases in the water , field of the Canon's Brook

consequent upon urbanisation; calculated by several techniques.

(Units are inches. )

., 4 w w 4.1. 3W O äi 0 d 4) U dd $4 13 to 1 >, a) cd cd c0 . -1 a O to 'a 0-4 GO >H N '.% . -1 (n r-1 w C) ". 1 O M cd CJ :3 ý1 1f "rl e-4 r-4 "rl co 1-4 N If ý'. " 1J D. dN V-4 N 0) N ý.' 0. N"' W C. G) col ý., >-* 0 044 >ý 0N .C 4) * 0) C: N 1 r1 " e-4 - rl r-1 p..4 ," Gi a C+ N 1J 14 U .G t-4 on . -4 0 004 ao *r4 aCao . s. 1 ý. Nb ".. to oaM o ai -4oo"a 0) a a a m r. m 3 Aý $4 < C/ x a4 w .o oc

1950-1 - - - 0.22 0.23 0.0 28.8 1951-2 0.0 21.6 - - - -1.21 -0.54 . 1952-3 0.74 0.77 0.0 21.8 ,- - - 1953-4 - - 0.87 0.69 0.00 1.6 21.4

1954-5 1.87 - 0.22 0.96 0.16 3.2 22.3

1955-6 1.47 1.47 0.73 1.93 0.26 4.8 24.8

1956-7 ` 1.85 -0.96 2.31 1.59 0.52 6.2 21.6

1957-8 2.62 3.09 0.18 1.00 0.02 7.7 32.3

1958-9 2.26 1.89 3.38 -0.20 -0.26 9.2 19.2

1959-60 1.72 2.38 1.69 2.10 0.71 10.6 25.4

1960-1 3.70 '6.11 4.26 2.00 0.28 11.4 28.0

1961-2 5.43 4.86 3.68 4.58 1.46 12.1 24.3

1962-3 4.81 4.34 3.69 4.25 1.78 12.8 20.8

1963-4 5.37 4.91 2.10 4.85 2.21 13.5 21.3

1964-5 3.15 3.34 1.60 3.92 1.07 14.2 23.3

1965-6 6.33 5.93 2.91 1.75 0.30 15.0 27.4

1966-7 6.3'1 5. b9 4.50 4.92 1.83 15.8 23.0

1967-8 6.19 5".97 4.09 8.35 4.90 16.6 23.4 243

Brook as paved areas increased in the basin. Exact relationships between water yield changes and urbanisation have been presented in the body of this Chapter, but is seems appropriate to state that with only 16.6% of the catchment impervious the water yield in a dry year has doubled and in a wet year it has increased by almost 50%. This increase in flow appears to have resulted in an increase in the magnitude of the low flows alone, but the contribution of urbanisation to flood }. ly loaf oNc6.311atý 44A 1:. flows is examined below. It seems thatk1lows of 1 and 2 cusecs, which were once common in the Brook have now been superceeded by low flows generally in the range 4 to 10 cusecs. However, it is in this latter area that one must be most sceptical about the quality of the gauged records. ý-- ,. 244

Bibliography.

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Betson, R. P. 1964 What is watershed runoff? Journ. Geophys. Res. 69. pp1541-1552.

Bio Medical Directory. 1969 Bio Medical Directory of Computer Programs. Univ. of California, Los Angeles. Faculty of Health.

Black, P. E. 1968 Streamflow increases following farm abandonment in an eastern New York Res. 4,6. " watershed. Water Res. pp1171-1178

Blalock, H. M. 1960 Social Statistics. McGraw Hill. 465pp.

British Geomorphological 1968 Computers in Geomorphology. Proc. of Research Group. Nottingham Symposium.

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Bruce, J. P. & 1966 Introduction to iiydrometeorology. Clark, R. H. Pergamon. 319pp.

Chow, V. T. 1964 Handbook of Applied Hydrology. McGraw Hill.

Crippen, J. R. & 1969 Hydrologic effects of suburban development Waananen, A. 0. near Palo Alto, California. U. S. Geol. Surv. Open File Report. 142pp.

Crawford, N. H. & 1966 Digital Simulation in Hydrology: The Linsley, R. K. Stanford Watershed Model IV. Dept. of Civil Engineering, Stanford. Tech. Report 39,210pp.

Dawdy, D. R. 1969 Considerations involved in evaluating mathematical modelling of urban hydrologic systems. U. S. Geol Surv. Water Supply Paper 1591-D l8pp.

Dawdy, D. R. & 1964 Statistical and Probability analysis in Matalas, N. C. hydrologic data: Analysis of variance, 'covariance and time series. Section 8.111 in Handbook of Applied Hydrology, Ed. Chow, V. T. ; iýG,ý,. ""A. II.

Dawdy, D. R. & 1965 Mathematical Models of catchment behaviour. O'Donnell, T. Proc. A. S. C. E. Hyd. Div. 4. Vol 91. pp123-137. r Ezekeil, M. & 1959 Methods of Correlation and Regression Fox, S. analysip. Wiley. 245

Green, F. H. W. 1970 Some isopleth maps based on lysimeter observations in the British Isles in 1965, 1966, and 1967. Journ. of Hydrology, X, No 2. pp127-140

Gregory, S. 1968 Statistical Methods and the Geographer. Longmans. 278pp.

Grindley, J. 1967 The estimation of soil moisture deficits. Met. Mag. 96, pp97-108. also. Not. Office. Document No. G. 14666/LG/12/68/30.

U. M. Gov. 1963 Water Resources Act.

Hibbert, A. R. . 1969 Water yield changes after converting a forested catchment to grass. Water Res. Res. 5,3. pp634-640

Holland, D. J. 1967 Evaporation. British Rainfall 1961. ppIII 3-34

Hydrocomp Inc. 1969 Simulation Continuous Discharge and Stage Hydrographs in the North Branch of the Chicago River. N. E. Illinois Planning Commission Report 56pp. t\iiO. '

Hydrocomp Inc. 1971 Simulation Newsletter.

LASH/UNESCO '1969 The use of analog and digital computers in hydrology. Proceedings of the_ Tucson Symposium. Two volumes. 754pp.

IBM 1967 System 360 Scientific Subroutine Package (360A-CM-03X) Version II Programmer's Manual. Document No. H20-0205-2

James, L. D. 1965 Using a computer to estimate the effects of urban development on flood peaks. Water Res. Res. 1,2. pp223-234

James, L. D. x.970 Watershed Modeling: An art or a science? " Paperlresent(d to the 1970 Winter Meeting of the A. S. Ag. Eng. in Chicago.

Jones, B. L. 1966 Effects of agricultural conservation practices on the hydrology of Corey Creek basin, Penn. U. S. Geol Surv. Water Supply Paper 1532-C.

Kirkby, M. J. 1969 Infiltration, thr ughflow and overland flow. Chapter 5. in Water, Earth and Man, Ed. R. J. Chorley. Methuen.

Kung, E. C., Bryson, R. A. 1964, Study of a continental surface albedo on & Lenschow, D. H. the basis. of fligtt measurements and Earth's North structure of the LWeather cover over America. Monthly Review 92,12. pp543-560 il 246

Linsley, R. K. 1969 Personal Communication.

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Mandeville, AN., 1970 River flow forecastirg through conceptual O'Connell, P. E., models. Part III - The Ray catchment Sutcliffe, J. V. & at Grendon Underwood. Journ. of Hydrology. Nash, J. E. 11. ppl09-128

Meterological Office The calculation of evaporation from - I4w40- " meteorological data. Two parts. 28pp.

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Ollsson, G. 1965 Distance and Human Interaction. A migration study. Geog. Ann. 47B. pp3-43.

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Penman, H. L. b 1950 The water balance of the Stour catchment area. Journ. Inst. Water Eng. 4. pp457-464

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I 248

CHAPTER 5

The Effect of Urbanisation on Floods in the Canon's Brook

Floods will always occur in years of excessive precipitation, whether the surface of the soil be generally cleared or generally wooded. George Perkins"Marsh

This chapter presents an analysis of the flood hydrology of the

Canon's Brook catchment and includes a brief discussion of an abortive

attempt at a time series simulation. The change in the magnitude of

the maximum monthly floods is examined together with the changing

frequency of floods of various magnitudes during-the pre-urban and

urbanising phases of the catchment. A large sample of flood

hydrographs is used to describe changes in the peak flow and shape

of the hydrograph and unit hydrograph with increasing urbanisation.

Finally an evaluation is made of the various factors including

urbanisation which control the rainfall-runoff relationship and

emphasis is placed on the effect of urbanisation on floods of different magnitudes.

Urbanisation changes the flood runoff characteristics of a basin

in two ways. First, the superimposition of impermeable surfaces such

as roofs and roads on the soil, inhibits the infiltration of rainfall and increases runoff to the river. Second, the relatively smooth

impermeable surfaces together with their associated dense network of

surface water drains and improved stream channels cause water to flow

out of a catchment more quickly than in a wholly natural basin. The

importance of both factors wil tend to be reduced during the passage of large a infrequent flood becau e at that time in a natural basin soils 249

tend to be saturated giving them a low rate of infiltration and, as current work suggests, the channel system may be greatly extended so that the drainage density is increased to the same order as that pertaining for the whole time in an urban catchment (Gregory, and

Walling 1968). Leopold (,1968) has discussed and illustrated

(Figure 5.1) the combined effects of urbanisation on a hydrograph and numerous attempts have been made in the United States to quantify the changes. By means of double mass analysis Harris and Rantz

(1964) showed that urbanisation in Santa Clara county, California increased the volumes of flood discharges and Seaburn (1969) used a pair of regression lines to show that the proportion of rainfall that ran off in the East Meadow Brook, Long Island increased substantially after suburban development. The unit hydrograph method has been used by Espey, Winslow and Morgan (1969) in Texas and Crippen and Waananen

(1969) in Palo Alto, California to discover that peak flows may be increasedby up to three times, the time of rise of the hydrograph may be reduced to a third of its rural figure and the lag time of the hydrograph cut by 409.. Wiitala (1961) found that flood peaks in a wholly urbanised basin in Michigan were three tines what might have been expected had the basin been rural, whilst Anderson (1967) reported increases in peak flows of between 2 and 8 times for urbanised basins in northern Virginia. Relatively few writers have considered the view that urbanisation night influence small floods more than large ones, probably because of the intractable analytical problems involved. (1965) James used the Stanford Watershed Model to simulate a long term continuous hydrograph (1905-1963) for lforrison Creek, Sacramento,

California for various degrees of urbanisation and found that "the ratio of flood peaks for urban conditions to those for rural conditions decreased from 2.33 for the (paean annual) flood to 1.57 for the (200 year) 250

Lag time 1 I I--Hydrograph of streamfiow

.c II

c I" Centre of mass of runoff and of rainfall ö U --Rainfall CL Time in hours a)

U /Lag time after urbanisation U l

b I 1 1 \---Original / I

/I -After urbanisation ` f

Time in hours

Figure'5.1 The effect of urbanisation on the flood hydrograph. (after Leopold, 1968)

t 251

flood". Martens (1968) used two different sets o arbitrary linear

assumptions and an unsubstantiated equation to prepare a graph which

showed that "the effect of imperGious area diminishes with increased

flood recurrence intervals becoming negligi4e for floods exceeding

50 yeas". A synthesis of results from these and other papers

undertaken in Chapter 2 confirmed that the effects of urbanisation

are smaller on large floods than small ones.

" 'The findingsof this Canon's Brook gtudy tend to agreý with

conclusions drawn from American work in particular strong support

is. 'given to the idea that urbanisation has a minimal effect on large

floods .

The only major human modification of the drainage of the area,

except for urbanisation, was the commissioning of a flood balancing

reservoir at Netteswell on 7th November 1954 (Figure 5.2). The pond

was originally designed with a small permanent water area and a

large storage capacity but subsequently the level of its outlet weir was raised to increase its amenity value. This latter action

reduced its flood storage capacity and so reduced its attenuating

effect on flood hydrographs. An analysis of flood hydrographs and unit hydrographs for the year before the completion of the reservoir and the subsequent year failed to reveal any significant changes in the flood hydrograph at the gauging station, which is not unreasonable since 1.5 only square miles of the basin lies upstream of the regulating weir.

The effect of the Netteswell pond will be ignored during the rest of this because analysis it seems to have had little effect on the outflow hydrographs for the whole basin and more importantly the effects which

have flood such ponds on hydrographs (Linsley and Franzini 1964) are 252

Stort

wxf Town Centre

Netteswef,

][ Gauging Station X Site of raingauge Watershed Built-up area 0 Miles 1 (from air photograph 1966)

Figure 5.2 The location of places and instruments used in the flood study. 253

the antithesis of the effects of urbanisation and so any estimates of be the latter for the Canon's Brook must be considered to conservative.

An Empirical Description of Flood Frequency and Magnitude for British The calculation of a flood frequency curve almost any

river is limited by the relatively short period of record available,

compared with the length required for statistically valid results.

The Canon's Brook with three years of rural record and 15 years of

is record covering a period of continuous urban expansion not amenable has to conventional analysis and so an empirical descriptive approach

been adopted in the main body of this chapter, but initially the

discussed suitabilities of simulation methods are and an abortive digital attempt at a time series analysis is reported. The value of

simulation techniques for coping with hydrological research problems,

where there is a relatively short streamflow record and a longer history of rainfall measurement, has already been demonstrated in

Chapter 4 and the suitability of these methods for the analysis of

Linsley urban flooding situations was shown by the work of Crawford and

(1966) and James (1965) reviewed in Chapter 2. There are three main

reasons why such an approach as not adopted for the examination of

floods in the Canon's Brook. First, the simulation model used in

Chapter 4 was not capable of extension to incorporate 15 minute or even

half hourly flows. The model was based on a daily water balance

approach, it lacked any sophisticated infiltration function which would

cope with short term rainfall patterns, and the programming design of

the model precluded any great extension in either the number of

statements or the memory required. Second, the continuous records of

riverflow and rainfall were held on the original paper charts and the

conversion of these into a computer compatible form was not deemed 254

feasible because there were about 1000 weekly gauging station charts

and arpund 6,600 rainfall charts. On reflection, it is clear that

data collection and initial model design must take cognizance of

long term goals rather than succumbing to early enthusiasm and haste. f , Fin lly, there appeared two better and more efficient alternatives, na9ely time series analysis and regression methods.

Time series analysis is now an accepted tool of the research hydrologist and rapid developments are taking place in the subject.

In Britain Dixon (1971) has reviewd the technique "for the practising hydrologist" and Kottegoda (1970) has looked at the suitability of short memory models for some English rivers. Notable practical applications of the method have been made by Hall and O'Connell

(1972) for the Swincombe reservior scheme and by Hamlin and Kottegoda

(1971) for water resource schemes on the Teme.

The assumption behind the method is that a time dependent sequence of data can be disaggregated into up to four statistical components; namely, trend, periodicity and a stochastic component which itself has a correlation structure or persistence as well as a purely random element. Once these elements have been identified and quantified, it is possible, with appropriate safeguards, to generate synthetic data which has the same statistical properties as the historic (1971) record. Dixon stresses that "the important point is (information) that the relevant can be determined, not by extra-

from duration polation a short record, but by a full synthesis of likely future events based on the statistical distribution of the original".

0 255

The strategy of the time series analysis of he Canon's Brook data is shown diagrammatically in Figure 5.3.1 was proposed to generate 999 blocks of data each of 18 years duration. The resulting 1000 blocks of data were then to be used to draw a rural flood(frequency curve from 3000 years of data and 15 similar curves.

each representing the degree of urbanisation found in one of the years 1954-68 and each based on 1000 years of data. Regrettably,

this analysis was unsuccessful and no useful results were obtained.

Be'ore discussing the reasons for this failure it is necbssary to consider the actual techniques used.

The data used was the maximum monthly floods for the 18 years, making a series of 216 data points. The'"Autocorrelation Analysis" route was then followed from the Imperial College model shown in

Figure 5.4. Initial experiments to detrend the raw data series from 1954-1968 only and related tests for consistency revealed that

there was a substantial change in the balance of summer and winter floods. Double mass analysis showed that as urbanisation progressed, so summer floods became proportionately more important than winter ones. Since there were also good hydrological reasons for this change (see Chapter 2) it was resolved to detrend the summer and winter seasons separately. In order to guard against the danger of generating negative flows and also to heed the advice associated with

Figure 5.4, two analyses were carried out; one used natural data whilst the other employed a logarithmic transformation. Detrending was undertaken by simple linear regression but only the summer data series had trends which were significant at the 0.01 level stipulated.

Only the first harmonic was significant (Hartley 1949) when periodicity was investigated and so it was apparent that only an annual cycle was 256

U D

999th Synth ti 18 yeor Record

HISWPIC Record

Oct S"t Oct soot 1053 WATER YEARS +aei 19e6

Figure 5.3 The strategy for the time series analysis of the maximum monthly flood data for 1950-68 for, the Canon's Brook. 257

Image removed due to third party copyright

" Figure'5.4 Methods of time series analysis. Source: Imperial College London, De,partment of Civil Engineering Post-Experience Course. 258

present. There was no significant periodicity in the standard deviations despite their erratic monthly fluctuations. Auto- correlation analysis for persistence used the correlogram method

lagged and Whittle's (1952) tables to test the significance of the

found be coefficients. A fourth order Markov chain was to needed for the natural series and a second order one for the log series.

Once persistence had been removed by the Markov models, the residuals The appeared to be purely random on investigation by correlograms. fitting of a distribution to the frequency curve of the white noise be was exceptionally difficult and only a modest approximation could 2 achieved using the chi method of testing for fit.

Synthetic data was generated in accordance with Figure 5.5, using an IBM supplied subroutine to generate random normal numbers which were then transformed to accord with the observed distribution of residuals. A digest of the results is shown in Tables 5.1 and 5.2. In the series generated from the natural number data series,

Table 5.1, where negative flows were generated a zero was inserted instead. This happened for around 20% of the time intervals.

With this correction, it is pparent from Table 5.1 that the descriptive statistics and frequency distribution of the synthetic data are broadly similar to those for the historic record. However, the zero requirement for negative flows crippled further progress.

With the logarithmic transformation of the raw data, there was no

"negative flow" problem but as is shown in Table 5.2, the simulated standard deviations, means and maxima were generally much larger than in the historic record a9d the frequency distribution was often bi-modal.

Again, no further progress seemed possible.

i 1 259

Image removed due to third party copyright

Figure'5.5 The generation of time series. Source: Imperial College London, Department of Civil Engineering Post-Experience Course. 260 Table 5.1

for 18 historic flood Descriptive statistics and frequency distribution the year blocks data. (Raw data in record of Canon's Brook and eighty nine synthetic of natural number form).

NV'IArt 111T11 04FAN t7AN fl'V "1'. l4'14 rAt T IM/ýNIFAf, t f%t1N1JIUT11Nl

0" I. 1111 161416011"1 1S" 7t", 13.411 7"rC0 S00"LO'S . I740 440 64 no A" If I. " I 1711+"tr %7. ý'º 1h"7' r, "n" u 1.1l-1"ti '" 164. +ti" 1" If" 70 I. D" A. n" 3 14171.114 73"36 93.10 A. 00 f17"SA A0 C6 0. 127" 600 10 14" 4" 40 40 0" 00 t ?. `111.41 e). b4 41.1.1 4"I, ri 41`1661 7.11 "" tell. 1"16 I. J1" 60 2" "" n" r" 4 21041.77 91.4% 117.06 n. A11 774.14 7.71, C. 111" 140 1" 740 I1" 46 7" I. 7" ? t. l. fool I. 1" 1" S 1714.71 r' "1f 1. '60414 h, "r 117.14 4.67 I. 4a" %. A. to 7. A 71111.21 1 "71 J^1.14 A. Aft A7tooOlt. W4 t" 410 $1" It 71" 11" 60 o" I. 7 Ih'11.71 N1. '' 41.3^ ".. (^" 1r1.14 1.41 " n" ny. %7" P. I'.. to I. I. A. A" 0 ? 1Ito. 13 91.71 1^A" IA n"r^ %W4.21 1.12 no Ir 1" ! A" 14" 21" 1? 0 10 74 1. 2. O 17"4". ? `1 41. )" 801,9. -. 3' 1(. 4.4+ 1.21 C. 11/" 4: " 1., . I. "" II. 1. Of /t" n. n" to ISM 048 7^09 MI"AI a. n0 let"44-12 . 74 0. t'7. ? t. 270 tl" 170 60 0" of 11 17744.113 ' """16 6I. I' I ""w" 47w"s7 1.7% 46 1'. 4" 0.1" +0" I"" P. 7. 7" M. 4" 17 71614.77 " e11.14 90.31 4I"0( 491.1% 1.114 1 C" $70 Sol. 160 20" 1. 6. 2" 0. 0" 11 74^''. O) 111.111 1'In"77 'irr ttiot"io ""«,. ^" 9s. h7. 0%. )... 9. O. it . I. 7. 14 23017.21 11"471' 1f14.11 0.41n 4}7.46 4.04 0" A1" S7" I 1" 17. Iq" A" 2" 7. 1" 1S 10111.7" ('. r"7t 1.0#" L" Ir4" "+" (" IS" 2" 6. 0" n. V. 16 11409.41 h+"01 7'. "n6 0.001 4^0""3%-1.4+ 9. 126" 44. A. I? " 4. ' 0. 7" 0. r" 17 17AR:. 42 Ap"11 '9.11 4. C^ 44""44 1.71 f" IL'1" ! O" ll. 11" 6. !" l" I. ^" It 2A%24.41 2A"4It 93.97 0"GD S) $"0IS 4.91 0. 64. 60" "S" 27" 14" 2" 20 t. I. 10 79.14.14 "+6"h% _ Nll"71 ",. r. 414.1, 1.16 '" 414" ". I" 170 s"r. At 40 7" 0. "/" 2A 14175.71 06.60 154.30 1. ' 0 41+4.74 1.4t 0" 117" 13" t4" 170 he 7" t" C"" 7. 7^97'. 70 - 'ý 71 O^. 94 r4.9y r"(o A11"*7 ?. ot to. 1'It" ". " 11" 71" 7. 60 It I. I. 72 16376.91 A6"'. f VA"SS r"1' 367.04 1"4% e" 99" Atop 260 72. be 1" 1" at I. 21 7.141.3? 117. '. / 41t"S4 ^. "' 112.1+ S"t6 r" .,,. 0" '61. 21. IS. , 4. Of A" 0. 26 ? 7I'13.46 116.41 106.91 '1.00 371.19 4.41 A, 94. If.. 47" 24" 10" 1" 6" 0. 1. ' _'r5"44 Z. 7S 1A6S9"Sýl 11.76 "l.: ^ 1.11.7; 1"I % C. Ill?. 44. 41, I7. ', 6" 24* Co. r"" 26 70691.01? ' 93.79 114.94 ' 1.40 736091 7.0 C. 106" 470 760 16. " S. 6" 3" 1" 1" 27 1'I, 7.17 ".. 14 '12")) ',. '' 677.1-1 7.1" ,. 1-4. 4%. »"' 1-1" "" 1. 70 I" :" 24 15167.34 71.13 - 11.95 ""110 445.74-0.77' 0. 1210 496 21"' 11" It 3. 1. 1" A" 70 167 2.1'1 6'. 2* Nl"2f -" " n. {1 74h. 7;. 1.2f ?" 174. 41" ? 4. I1" _ A" :" r" r" L" '- In 24721.1" 43.94 . 117""4 0. (91 600.41 2.6f 0. 110" 41 "' 76" ' 17" 9" As S" C. !" 31 2! 068.97 _. " 4.14 ' 02.67 1.4' "S6"t! 4. ^r I. "n" 4r"' +f"'ý tn. _ 11. " ]. 0" 1. 0. 17 714$0.99 ' '1? 1.14 "' 96. u "" 0.110 ~07. 1.90 0. 91" $A" 310 21" 1? It 1. I. I. 170(7.11 _ 7N. 7r " "76 _ " 1" +1 01.61 : "0^ 371"': '). S C" 114. Soot' ! h" 1! " >" 2" L. I. t4 22,11"AS"'-1'17. ßn 102 ' 1.00 S41.44 3.1S 1. t" 0" 1" "-ý _ ý "C6 00 oS" "S" ])" 7S, 9. " 14 7"It+O"AS 4A"N 40.66 '" - '" A. rC 4#00.45 1.47 ' C. 64. 57., 4%. 17. 9. 1. 1" I" I. lA 22416.1' ""1'13"'. 1""'_' "7.1% -' n"AA. 4.11.75 3.02 to 79" 540 43. ' 194 Ile 4" I. C" C. 11 1216+"46 " 36.13" " 77.49 -'" '"r, A 314.11-1.74 A, 11^. _ 4#4"" 71i 11" - A" 20 0. 0. 0" 24n«"41 111.72 1" IS''39' , ' " _ " " 91.611 - 0. r* 347.31 S. 32 Co* 700 39" 12" 7010 101" 7" 2.1 co. _ß" 10b 7114-16.40 " 11^. At 'C+"e4' " 3"^^ y i" r" 4S2"11 ! "46 roo 701" 4%" 4%. 1+0 176 i" 2" 40; 1.461.29 57.41' -' ! 7.10' 0.00 '74.61 2.05 0. 03. 52" 36" 20. 6. 4. 0. C. 0. -. ý. I^l. ` W, . 41, 22'1A, 1"+I.. ft Y4. i" _ 'I 4'/1.66 4.71N 4!. 6t" &Ae.. 1 )1" 1. 4" 1" 1" G" 42 10904.64 """ 61; 41) '" A7.47 ""' 0.40 451.4^ 2.201 0" 14'" St. lb: l2" I" 1" 00 1" 0" 41 101+An. 61 ý ý )f"ýlý~" ý ýýý . . _ " A9"ýt r"01' 40t"ti1'00CA G" 124" 410 t1.. . 161 " 40 I. 1" fC. 44 17100 111.44 "" 80 70-- 0.00' '- 320.61 1"ti CO l0! 496 I2" ? Oi l" D" 0" A" oe - "lt""'""ý " " ýS 207tr6.74 ' 4ý"'ýw'ý' Ihý"3) ` 11.01 " HT"! 1 X2.94+ - to lh4"ý SI" 71" 11" _ ?" _ 7" 1" f1" 1" 46 77706.96 ', 02.111 - 106.71 -- 0"00 516.62 7.79 as 49. 41" 2116 750 100 ' 60 20 2" I. "' 47 15211.17 44.14 94.00 '_ n. r"n 176"414 3.47 C. 1000 6Aý t%. ' IV* '0 r S" 1" C. C. 411 15420.36 " "71.39 74.76 0.00 341.17. 0.12 C. ItS. S7" 74" 1S" S" 0" 2" O" 0" ... »11.0'. '49 166.9.04 " 2.07.. _.. .. l '1S4"AV r:. 11 -'I 111"' 4)" Ile IS" _. olop'" . 1" 0. f" to "'. .' . SO 12AAIt")S 59.61' 67.21 0. Cn lTl"fO")"44 C. 110. "O" 260 S. !" 10 00 00 of ` ý"C S1 1'627.31 ! 6.70- .45 it c 111 . 0{ %"Or vý' lri4"' "7"' 160 +t" "" 1" 4" C"" to S7 190"5.9' 67009 - 19.74 A"r'o 444.61 2.17 at *67. 42" 41. 10" Ti 1" 0. 1" 0" " -_ß" 3! 10117.44 h0.40 44.04 4")i 1'57.7% i". " ' r" '. e" t8. 21" ' 21" "" S" 2" 1" C. A" S4 17330.71 54"10 " "1.26 C" 0A t1! 1C 0. 11'. 'O" 1o" S" S" 00 0. 0" ASS '74'11.41 "^7-). " 10" 9S; 4S~ 01.54'" "" '0. "'1 . - SS1"S, ' 3"11t ' C. 07" $t. "("" It,. A. " 1" " 2" I. 1" St 14146.47 449(11 60.00 p0114' 173")0 1.61 00 940 600 11" 1t'" 70 2. 4" C. 00 47 71110.9. IA°"+n 117.31 373.40 . r"" 40h1 A. "". "A. 'h" 11"' 17" 4" 7" 1" Is ' SA 2)061.16, - 47.46 -, 03.34" " 0603 346935 3.01 C" 117" 37" 10" 9. ' 2" I. two 00 40 21)11.72 1D6.2'º-ý I'14.71' "ß' 0"rw - 7.04 ' - 6! 4"w p". "1" 4S"' 1,10,1t"' '! " Ill 1" 2" ("" 1" hn '11"hi, II 1! "71 14.46 ""'' 4%4.45 1.1! ^" 111" 470 )1" I1" of at to I. r" Al 20647.04 95.02 1^4.9' "f. 9 4'. l. 6+ 7"4+ 0. 1^f. 17. 11. 11. I1. 7. 1" 1" r" " 62 104-61.23 9C"1" 64.90 C"3C 45.1"°7 2.43 0. 11. S2. 410 1:. 9" ?. 20 1" (, " 41 277''. %4 114"%1 _ 47.41 _ .". .. Co. . 40""to ". ""41 r. -A. 61" "n" 101. "" A" 16 2" r" 11704.21 ß_144.2I! 64 r '2.6s 0.1' 324.92 1.60 0. Or. S9" IA" 17" 6" 20 o" 0. I. f. 4 174.9. SO . " I^"? " 04.41 " r. (,. " 4"1"^t C"-. " C. 11: '"" J1" "_" I. 2" I. 1I. " 1. " .. " 64 1732005' ltl"t6 07.71 ' 0. Or' 717.1. 0.00 0" tr7" SS" 240 : 3. 1" 0. Cot _ p" 60 A7 11611"%7 5s, 14 AM"14 " %.. "a%. Ito-4.77 r" 17+" +. 7" "" 1" I. (" " 4'1" "t" AS 21407.44 144.44 "7.61 f. 07 S^4.6$ 700% I. I^1" 43" 1t" O" r. I. " 1"0 1r" s" " 11176.54 "" 114"! 7 94.44 4"Cf 4'. 0"4 60 . 'S C. 176" +. " 7h" 1'/" f" 40 +, to as 70 11960"! 7 4%. 1% $0.06 00AI" 411.74 1"+01 to 1A"" S! 7! Ile I. Poo' C. " " " It's 7" 7) 71444"^1 110, (0 1-4.77 r. rr 4,70,11 4.75 5" 77, . , 17" 411" 21. " Ill 6" 1. r. C. 7f 1ý 116", 1 C""ýý "w"tj 16951.65 "A f+ r" It"#* . d" 160 f" % 1. (I C. 71 1441'. 47.11 0400 r. " 600"t ý"'h to 1410 41" .. " D" I. 07" "#'" II. " P, I. " 1"9 7" 24'S7o, AA 44.67.. "A"r"r .1"r! 47.17 4,77 n" 41" S4" U. . 224 0. "" 1" I"' to 74 116! 6"'. 4 MAooat 4+"610 'I, hw 111.011 1-. 4% Ito, . Ve * 6116 \1" IA" 60 I. 1" n" C. 70 14671075 47611 42.30 0"'f fln"^6-1"'1 r" I`? 6T" 711" I" . "" 10 to r" 1" 17 1A1i1"At. 7-4"A{ 4n"40o t"ol. + 4'"4.111 r. 1" " , At Ile. "'. 1%" is. w" r. 1. to. A" 7r 19074.14 11101 "3.75 r": C 64,. "41 ;, C1 to It'? 41" 1%" 1-h" Ice t" 1" It r" 00"41 '" * . 10 7Jh"A"7n 0+"71 ". 61". 1+ +ol7" r" 040 S4. +1. 11" 170 26 If I. to An 1+697.07" 42.67 ' 78.00 n.: r 441. t7.7":! to I1. . " 4.6 301" To If I. :" I. r" "I 1411\"rb 01. 01"0.1 44'"45 10 It 111" "7$ 410 t. " 16" 1016 I. I. It r" to $7 )444%, 60 01.44 44,7% retook 6110x+ 7oorm f" 141" O'f" 12" 71" I" to A" I" to rl 1 t+7n"sn 1+.. r r9"h7 %0 . "1""O'" r"(1 1- tl. 4: Ih" 140 7" it I. 1" to M 197.12.3! Ah"44 1'0.40 " 0 91.61 0"a( 7.40 CO 145'. 0" $4" 77" 0" 44 10 t'" to 00- %S; 7 94 10,14# 04.97 4"rr 4#14,47 1""" , " , G. 410. In. 7f" Its I. I. It C. I. 116 21151.10 101.61 %) 104.17 0.06, 610. 4"Aa 4'" I. A. 010" 40" : 1. to 4" 1" 7" It "7 1701l+,. r tn"7f . 71.47 1"00 9A46.04.1h, tso 0. Ile. V). V4. to* I. to Is . 1, At e 09 11644.14 111.11 116.41 ii. " 717"\" 1". + P. t0" * " 61" n! " """ tr" It. 29 1" 1" rr 27425.41 11ý. T/ 101.14 ýý t+"r" wet+r "ýý ý, r. ý1" SS. "A. IS. 1. "" 7" C" , _ " " Table 5.2 261

historic Descriptive statistics and frequency distributions for the 18 year data. flood record of Canon's Brook and eighty nine synthetic blocks of (Raw data transformed logarithmically)

ý14I? 1MU'1 MAX 1. 'tPivV0C7 bl)1'1v111114 kL98E8 T014L PLAN STAN OLV .

26" d" l" 1. 0" 1* PEOL 161816000 7S"24; 75.6)3 26GLU St': 0V! C. 12d" AS. h" . 1" of 1 VIC1: A'i"NA 21R"! L 1049 1011.44 1.7( C. 141" t2" ». to As 1" CO "A6 16" 40 to is b" o" 7 70557.72 OS. tof 214.3. I. vs 2146.40S .1.116 l.. 14 If 146 be 20 2" 6" 3 215eI"I"! 91.11 22! to!? 25: '. 26 1"tI " 13)", At* IS" 11" I. " "314 1V'I" I Is II. 1" S. !" 1 4 79161.91 "h"37 167.09 1.49 1111.21 lost U" 479 " " As 2u"ll9.7I 9w3I 1i2"V5 1.4! 11101.21 ! "41 L" ill" "r" We 1" t" 6". as As ! 1*7" 114" 160 . M" . 90 79 16 P" he A 71776.44 IOn. w 141"Ro 7.74 9&h"71 ? "4 no 12. J". 2" 30 7 1)225.34 0'4.. 1 13'x. 1:. 2.1'# 941.4.. 1. %i 1. I... 10. 21. Sa " We ! 1" 1. at I. A" A 19449"45 on. 0% Ilh"AA 7.14.' %'7C,. 4p 1040 6. 440 1. 0, 3" 9 11224.14 71"Ti 111.92 '"+. 141. (. 7 !a '10 L" We 51. It* 1" is 40 "" '" 1640' ! 20" 10. . 20" 11" I" 1" 10 11C47.1 74.07 115.14 1.71 47h"C4 0.60. L" M" - .. 6. " 0" f" 11 10.34.47 OC. " 14 06 '2 1. º7 It)'. 77 1.7) C. ta4 " tl" Its It's S" 'I" 1. " . 1" I. 6" 17 21SNd"l4 91.05 174.03 1.51 1594".. ) ? "C6 Co 1370 '40 140 It. h" !" 7" 12 101.41 177.57 1. 'i' 14', 0.7. 7.24 C. 111" h" ! 1" e. . ",, /", '" 1"_,.,, 11 2? ')-;? . , 4 ° 0 14 2173s"co ' ICI. 07 149.08 1.63 1047.61 1.99 0. 1230 57. ISO T. 4. 2., 2" " 4 " 1i... `". "" t4 11171.71 71"". ^ 1. S"'1 1.41 0.'11.71 1.59 " I1. " 41" 14. ". ""_ . »4". _ 1" . 66 It . 16 179R2.2A 00.21 21b"04d 7.4.1 714^. "47 Coal D" 1.1" a1" 10" "" 4" "" s t"" I. 1 1T 71510.67 1(1.71 1)7.15 1.1'1 Ih7'1.7. 1"oo '. 1's. al" 114" e" " V" t" 31. IS" IC..... T.. 30 2" 1" le 218C8.70 101.43 152.71_ 2.06 1215.74 2.52 Co 111" . 6 .r. .' '" S"... 1"" "I, 19 1046'"14 Ai"N7 114.15 1.1: 7r. 1 . "l 1.1. C" 1.', " al" 14" 1." '" " _ , , 1747.5! 1.41 ' , 0. 174" 410 39. , f" ,... 3" ... 90 Co. " 1" 64, 20 19l0ý. 69 91"Tý 171.10 7.11 " . ". 21 21C43. '4 4109" 13P"1S 1.14 1534.0.6 2.1"'" L" 11'". . 1. 11.1 14", 7, 4" As, '", i" _ _, , 3 22 27002.28 125.01 723.24 2.15 10567.46 1. C1 0. 137" 400 11" 1C" 5" 3" 0" 1" " 171" 14" 11" 1"_ 1" 1" 1" S 71 21213.91 41.61 144.33 1.74 11? 4"21 7.34 to 411"0 1 , 222.48: 2.32 23/9.21 1.42 C. 124" 31" 26. 5. 3" O. ` , C" 7 4 " 24 20914.62 " 46.83 - . _ ... - " 23 14544.16 on 41 167. C1 1"'"0 1751.0.1 1"l" 0" 111. '4. 77 6"" "" 1" 1"_ : "" ý., ' 1.64 . Ill. to 26 ? SS41"ot 11+"AV )44.24 " 104 446""4'3 . L. 44. 27 '3257. ': 4 1t'?. na ; 117.1'4 3.3 "5.. 0.17 I", 4 ^" L. J. r". 14 " 1:. .. " 4" . "" 1 11 . x, ý 14718.43» Ad"14 $3.93 641.17-1.23. C. 1410 4 4 IC" 4" )" 1 " ' 1 ". 1. 28 074 0 11" " if to ,. It `" 29 7C0*4"30 ^606'1 .. 174.07 1.34 1121"'7 1.7" " 24f" "4" le" 4" " -'5" _ 10 9" 30 70966.54 07.07 '15?. 53 1.76 911.91 2.10 C, 142. 27. 16" S" 0" _11" ! . 1. 40 1" 11 7!. X. 7'1 el. fe 1±4.0" 1., 7 Ilr. "+t tool C" 124" 141. 1'. Y. "" ", ý ' ý. _ 1"_ _ I" 32 10404.59" 65.21 120.00 1059 904.04 1.13 Co 131" 44. '_19. V" G". 2w f" 33 10905.41 02.96 164. 'C 1.33 131'. 74 1.51 C. 14)" 13. 10" "" 2" S" 1.1" " 6" ' . : '' ' S" 102.9" 21 01.52 " 1.52 252 4 74 1.03 C. 1 260 46 16 1) 1ý 30 2 1"`. 34 22122.35 Y_ . , . " 0 " . . 6 1. 35 19032.76 44.11 148.24 .: 1.21 11: 5.40 IOS C. 171" 51" 1'. 7" "" 20 3. a" _ M All 76 157.61 w"69 2076.00 0.79 U" 126" sr. 1Ve, '"1S", 7".,. 10 1. to 2" 16 14.. 91.39'" ý" 11 16600.72 64.7.5 "1.25 1.47 U70.091-1.16 f" 145" Jo" 2/"" b" ""ý 1T I" ýý ý ý ' _I" 7 ý 38 20519.40 95.04 i44"6C '3. C6 921.61 1.49 0. 11C" 90" 9" 9". he ~ 1" 1"" 1", . " )"_ 39 10917.21 tl7.67 129.67 3.70' Q T"'fb 1.4.1 '" t'r" f1" 111: lt" ""_ 1" ". 1" !) " 1" ' 1 7 911.7 s 04 02 120 S3'ß 2 31 111'3 10 0 95 C 129 45 2J 12 ; ; 2 )' 2" 40 . " " . " . . 0 " " 0 " 4 . ,. . 41 2(064.7' 070.1.. 1'1.04 1.45 791.5* l"d" Of 11'"" 61" Va. 1. '"" S" 7" 4" 4" " ý 42 18643.72. A6"SO M129.9ºý 1.72 400.41 I. 24 C" 111" ýt)" rý6" 7. y 3. r, C. " T" _SU. , of : .ý To 43 197610iN. A? 27 142031, t"h.,, IMOT 1071, lJc"" "r. 14". '. L. 1!.,,,.,. "ý,. r. - ., ý,,, " 7 , l, 306.21 0.29,. ýC. 0" 12 . 18" T .. =ý5.4....7"_ 1" l 44 16645.27 77.04 91.34w « " - 2.91 ý. ' " 47" : tlv. _. " _" . ,,; ý. ý4". I. I. a" 49 22010.56 1'12. u4 190"'S 1"! 1 21'': "61 7.67 C", 1120 4. r., 1'" 4. 105.17 _ _q19.110 2"": , 44 22717.54 227.71' 2.06. 2195"! 3 1.02 0. 121" 40" ". - " . ,,., _1" , _9",..1. _6".1" 47 ! 1! 94.64 101"AS 146.95 l"S' 141'. 67 2.11. v" lit. ? b" 16" ILA Is To 1", ý. ' 1.04 _ ý''S. ' ' 2" ,, . 1", ý4" 40 19906.64 92. So 224.42 1"70 2421.53 l. 2"ß ]L 21" 4.. 3. ,i 49 15795.71 71.11 94.77 1. '1 7hý"4S"(. 34 Co 1111" 44" .. 18" 5* 40 ?0 4" 1" I. ' 7""84 1019.71 .. - _ 1.03 1140. otw. 60 _ 0 _ 10. ! "8. ýý" . 0... ý Y" )"ý: 1" SO 15340.50 " ., " 147" 40. . 31 19? 11060 00.95 166.56 1.46 1.37"04 1.21 C. 10" 16" 11" 1" 4" 3" 2" 1" "" 32 10914.07 92.20 172.24' ' , 1.72 2: 41.43 1.45. ý_ C. 1]: '406 '. ? 10 t,: 9i 6r'-: D..... 0": ýU"«." 4. " . , . 4". 51 1R040"CO 47.7h 144"45 1"tS I; F.is h'" 1.10 L" 117" "4" 17" h" 50 b"r ", "", ,: ý 54 17734.56, ' 82.44 153.98. _ 2.15. ' 1511.32 0.44 C. 149" 33. 9; 711. ±1. 3" 2". b" _. _'_ , . ._ _1".. AS 106001.14 46.13 110.19 2.107 r"t'! 1.16 C. 111" 4t" 7^" )1" ; 1" %" "47 't " ^ ý 36 18077.62 '" 84.09 142.94 1047 1367.1.7 0"07 C. 112" S? "10" 9"' )" 3" 2" U. 4. ST : 3031.711 VJ"74 144.5. ' 1"w' 1)41.47 1.1" 0" 12+" 4%. 'f" 11" I. 1" /""" I. I"" r " 33. 14504.79 36.37 195.09. 3.71 600.042 1.44, Co 124" 410 210 7" 4", 70, 20 20 " . " _ " S"r 2345o"2u 111i. 4T" 217.49 1. a"' 74"A. '. 3 ! to Il'" 'S. l4. Il",,, "". 2., S.,, +.1. "11 ý21" _ _+",,.. , _ 60 11444.10 101.99 278.51 1070 3613.23 L"? 4. C. 140" 11" ý, A"' )""" 1" 4". " . .! " Al 70 d: s"14 9.. 11 154. " 1"ý'' 1ý". "". ''' ý" 1 :. 1)1" '7" i'" r" +" " Is ý", " "" . , 67 17040.61 02.60 124.19 1.53 125 2"9 A 3.07 C. tilt 41" ' 14" 7". '14".. 1" to :" 1ýa. 141.17 . : . ý'"_ 45 : 746... 11 71 1"+1 1.114. ^" "7" C. t"" f1" 1 to As "i ) 8" M" _ .. ___ 6 4 40272"15 _ 84.49 179.97 10 !09 ' 2271-. 2 0.76 CO 1t0" " 47" 14" ~ I" 1".. .. 6" 3" 1" 1" 65 11011. CP 4: "12 " 121.11 4041 1". 4.,. 03 r" at f. 11'" 41.. It. It. I. 1. to " to 66 10667.64 56.67 130.79 2.36 11ý'I"S4 1"17 1." 1 61" 660 1A,6 be 3" to 1" w" 6" 67 111144o'#O 7.044 '1102' 1"", 4 S4'4S3"'"47 C. 1111" 1/" l+" 7" f" )" 1" !" 1" 113038 1" ' 69 14! 17.00 04.79 7061 641o, 40 1.71 C. 1%1" 16" l0. " P" 1" 1" G" O. 0.9 ? 1017.47 ' 164.54 %ft, 27 t"'5 A. lie", "" Its, C. It=" 1""0 1r" e. ý"_ "" ""_ '" '1. . . . 7C 176C4.71 01.51 134"70 2.21 1244"39 00#14 C. IJ60 4A" 2l" 1" 4" " 2" 1" 3" 5", 71 )4109 1""4 171441a 104' I'+! l'. 1.1" I. , 1., . 40, ! 1745.10 'NI"Cil '. " " 610 ""r . o"r" ", 72 19415o23 119.38 144.91 7.10 1817.01 1.16 C. 1420 10" 11" 9" 5" Is 20 00 5"" 71 16117.! 4 91"++ 110.4,3 1. `(' 7%1.1t. l. l! C" 1: 1" 4". 1'0 to i" 1". 1"", ." " Is t" . 74 21617.57 93097 161.81 2.70 1113.66 1"i? co 125" 49. 10. +. S. 20 as I. 48. 75 17444.70 4417 125.20 1.45 loss. RI 4005 t. " 1'1" 440 1'" +" +" 40 !. 1", 1".. . , 1SA"65 _ 1. 76 16687"79 77". 2 1""f 11.07.44 7"t9 00 1410 ! 9" 17" O" 30 10 1" to " 77 17123.70 711.24 105.24 2.51 A! 1.21 Si' 0. 111" fl" I4" 1.0 to 2" of 10 7", 78 l9f"^7.79 174.67 I0.!! I""'.. elo 77 1.1? "17 C. 11'" 4""" 1t'" 0." 1" 1" As I. 40 19 10156.47 4""0.4 111". ^7 2. C: 1>4"" 10! 6" 121. a! 2)4 It* J... 2". t.. ""- . " 1 . .: ".. . "n 10.0. /". 65 70.49 IoT"44 1.69 1774.11 40.70 0. 147. 41. it, 4. As 0" 1" I. 40 81 4004w, %7 81". 0 121.10 1007 'l1"^: 1.41 C. 1J. 11" :! 90 l.. J" Is is 'I". 0.7 1'0'3,0" 701.60 1.54 7ASe064 x"91+ " " . R""1S 0. 11. " a1" pie 7. 70 f" 10 n, of 4) 20#411.14 440"6 111.119 2.73 111'1.7. 1.0"' C. 11"0 410 1: 11 J" to 1" h0 04 6 º" . 11191.91 44.14 297.14 '"71 15)4.52 no%) C. 11.. 64. 14. 4" "" O" t"" 1" be A! 1141).: 9 RO. 60 103.61 '"13 '5'04.0 '"T2 ': " Me 470 1/" 1" 4" 40 2" 1" 10 AA 71624.11 10n. 3 It 101.76 1.40 1427.10. 1"62 C" 111" 41. 26. It. 2. 3" to 20 to RT 1h:.)1.: s 74.3') 13302' 1.17 II! ý"'": "o.: ' CO 1! ! 1)" "" to. I. At . \" %$ . -i" ". " RR 1001*"111 141.14 714.17 7.95 271+"I1 1.114 C" 114" 410 Iu. 16. A. 4. 40 to 11" 84 22401011 100.13 176.49 3054 1116"17 'sK f" 11). 44" ! 1. "" 40 40 :. to S6, . ti_. ., ,. *" " -. .,, .. 262

The reasons for these unsatisfactory results arc probably three

fold. First, a series of only 216 points is rather short and daily

data with over 6,000 items would have been better. Second, the naive

and elementary attempts to fit a modified normal distribution to the

random residuals, known as white noise, produced only modest success

and other distributions may have worked rather better. Third, the

series showed stationarity when subjected to analysis of variance, but it was not entirely consistent by the very fact that it covered

an urbanisation phase in the catchment's history. Moreover, the seasonal detrending was probably inappropriate in a situation where a distinct annual cycle of winter floods and summer low flows was

transformed into one. of summer flash floods and winter high flows.

The technique should not be maligned as a consequence of what was possibly its mis-application. Subsequent work may yet prove it to be valuable even in this context. One minor spin off from this analysis was an exhaustive analysis of daily meteorological data for the climatic station at which the rain gauges andcssoc:ated data from the surrounding Meteorological Office stations, using moving mean and regression analysis methods. This failed to reveal any significant (SsýTý6Ias changes for the period 1950-68. q. 2. "*A 1.3)

The plot of the maximum onthly floods (Figure 5.6) suggests four important effects of the urbanisation of the catchment. First, the number of floods, over a threshold such as 40 cusecs, has increased appreciably. Second, the seasonality in the sequence, with high winter peaks and minimal summer floods has been destroyed by urbanisation. Third, the very largest floods do not appear to have any relationship development, with urban but this conclusion must be qualified by a consideration of what the analysis might have shown had it"been undertaken 263

.Iir

Mý. M. iWnu 1 hood .. ch 11iMn ý«? 400 so mwm nvvkv R~

300 WU" N 2001

100

G 1fl71 1952 +cS4 705f 1055 IQZ M7 1956 WA f qa 19eo w61 +00aß 1D60ý1067 l Figure 5.6 Maximum monthly floods and moving-average for Canon's Brook, 1950-68.

r 1 264

in 1961. Finally, the mean maximum monthly flood, as expressed by

the 51 month moving mean, has increased from 41 cusecs to 91 cusecs,

a rise of 220%, during the period under scrutiny.

Further insight into the changes may be gleaned from an analysis

of the frequency of floods in various size categories during distinct

seasons, Figure 5.7. For the purposes of this analysis a flood

peak was defined as"the highest flow occurring between two periods

of normal dry weather flow of about 2 cusecs. Figure 5.7 shows

that the frequency of winter floods has changed very little with

urbanisation, but that in summer there has been a marked increase in

the total number of floods over 10 cusecs and those between 40 and

100 cusecs. Floods between 100 cusecs and 200 cusecs show a much

smaller increase. There were only nine summer floods in excess

of 200 cusecs and so it is difficult to isolate a definite trend,

but over half of they 'occurred in the last five years of the

eighteen year record.

. 'The'Flood Hydrograph

A more detailed analysis of the impact of urbanisation was begun

with the selection of 192 flood hydrographs and associated rainstorms.

The problems of analysing multi-peaked hydrographs and the strongly

skewed distribution of peak flows made strictly random sampling

inappropriate. Instead, floods were chosen on three subjective

grounds, First, rainfall and streamflow records had to be continuous

and free from obvious error. Second, single peaked hydrographs

were picked except in the rare cases where it was feasible to super- impose a direct runoff recession, derived from a similar flood during

the same season, onto one peak of the recorded complex hydrograph. 265

701 "N news ever 10 twco 70 M IY. M FM 10 ew«s !0 80- EO do. ft b

10 0 0- _6 100 XI te rbIO1 WwIem f0 W 1D0 Nra !" five" MfwM 40 Ir ores b

¢ p: W. lJ ý +o v PJ 4z all :1n

(,! Oe rboN Mt. *" too 200 gums flmý bo"P"db 100 &V 201) ftems `ý r z 0j . e" rreeý wem."oo sý,. u

i >o51 __ >oeo +oeý wee +ý vie Aeo Ica WATER YEARS WATERYEARS

Figure 5.7 Frequency of flood peaks for Canon's Brook, 1950-68. #ý ý" 266

Finally, every flood in the sample had to have one or more of the

following qualities; a high peak discharge, an intense rainfall,

a large volume of rainfall.

Critical rainstorm and hydrograph parameters were measured for 5.8. each flood in accordance with the definitions given in Figure

The graphs in Figure 5.9 depict the temporal changes in the half

yearly means for some of the measured hydrograph parameters, but

these can only be used as indicators of possible changes in the

flood hydrographs of the Brook because of the descriptive limitations

of the mean (Blalock 1960), the effects of non random flood selection

and possible temporal trends in the meteorology of the flood sample.

Nevertheless after urbanisation began in 1953, there appears to be

a significant increase in the percentage of rainfall which runs off

during the summer and a decline in both the lag time and the width

of the hydrograph at 50% of peak flow. Comparable analyses, not

shown in Figure 5.9, suggest that peak flows have been increased

especially during the summer months and the time of rise of the

hydrograph has been reduced particularly in the winter months. These

findings support the notions of Leopold (1968), but in addition it is

clear that seasonal effects are important and that the impact of

urbanisation may not be the sane in a moist winter catchment as in a

basin with a substantial summer soil moisture deficit.

Subsequent analyses will attempt to compensate for the sampling

variability and the effects of changing seasons and weather conditions

in three ways. First, matched pairs of floods, which represent rural

and developed stages of the basin under comparable hydrological conditions,

are described. Second, the concept of the unit hydrograph" is employed 267

Log Urn* A

P*ok OIKhOf9!

40 ý. OI ýif1 °°il ýilfM - "1 fid I 30 II .Gý I Tb0

Cc 1 "10

Ot il 12 13 14 15 0/23406 Turn m hours

Figure 5.8 The hydrograph parameters measured. 268

PERCENTAGE PAIN RUNNING OFR

". 1 no- t" r f0 rý1' -ýý r'ý 60. 1'. S"" ./1i1.,,

yl \ý7 "Z 30. " 1/1.1

to.

0 "f"" 50.91 59.00 WATLA Y(AAS

17 HYDROGRAPH LAO TIME

11 jfý 10

.J"

0 70"äl 50 00 8,. U . I WA1[R YCA*.

26] WIDTH OF HYDROGRAPH AT 50% OF PEAK FLOW 24 22

ie w 12 a "'.. I I.. .' e 1 r; 1 / by "p1 ..

-771 ~ 'T"'7ýIýT ylý Treý"ý 04 ýTTý rýT r'ý1 l 30-51- 59"e0 a. a WATCHYCARS

MEAN MR tu u.4LI1(API 4LPTCMaa)$Oo'4 r """" MEAN FOR WINTER (OCT06U " MAMCN) STOoMI

Figure /5.9 Changes in hydrograph with . parameters urbanisaton. 269 to standardise flood events to a given rate and duration of effective

rainfall, and mean unit hydrographs for three periods in the development

of the basin are compared. Finally, multiple regression methods are

employed to derive predictive equations for each of the hydrograph

parameters for floods of various magnitudes and for various seasons.

Comparison of matched storms

Three pairs of matched storms and associated flood hydrographs are described in Table 5.3. The season, meteorology and catchment conditions for each pair of events are very similar and so differences in the hydrograph characteristics may be taken as indicative of the effects of paving about 15% of each basin. The increase in peak flows following urbanisation is positively related to the soil moisture deficit in the soils of the catchment, i. e. the rate of runoff from an urban area is much greater than that from a dry soil but only a little greater than that from a soil at field capacity. Similarly the percentage of rainfall running off is enhanced by urban development in the summer months only, whilst all of the hydrographs are narrower, i. e. more flashy, after urbanisation than before.

Unit Hydrograph Analysis

The unit hydrographs used h re represent the direct runoff resulting from 0.2 inches of effective rai fall generated uniformly over the basin at a uniform rate for 1 hour. This rate of effective rainfall was chosen, in preference to the more usual rate of one inch per hour because only three floods in the sample had an observed rate greater 0.2 than inch/hour. Their rates were 0.25,0.27 and 0.34 inches/ hour and it did not seem profitable to attempt extrapolations beyond the observed range. The Unit hydrograph theory has been attacked but Chow (1964) gives a concise yet comprehensive review of its' problems and assumptions. The method has been used extensively in the field of urban hydrology; e. g. Crippen and Waananen (1969).

i 270

TARtt

O. t. ! .1 t. tat Oat., I.. A+. tq. r. tq. I of basis rook see 116.1 too Taff 1001 T1l bau 1.1.1.11 of l. least t1 "a u u.. t r1.0A las 04I. t. 1! of I. (1KF.. I C. (tw. ) IYM/q (0.... ) ýf. ) (0. ) (ºwt. ) +N 0.1.1.1! ./ "$. trt. + I""ttt N. It.. ß.f. (Mw. ) Os1. t. U "t1U1 oil (Mats) ) (*. t M. /4f "I (Iýib..

70.1.11 0.0 0.13 1 0.11 !. 0 0 0 0.1 0.1 1.0 1.1 1. f 1.0

13.1.11 '1l. 1 0.10 1 0.13 0.1 1 !! 1.! 1.0 11.0 f.! !. f 0.1

I 1 1.1.3 1.1 1.41 1 0.20 1.0 S K1 I. 1 4.0 11 0 0.1 1.0 3.1

8.1.13 13.1 0.11 f 0.30 3.2 -0 -13I 11.1 1.1 101.0 6.1 1.1 3.1

0. 4.12,90 0.0 e. 76 f 0.03 f. f fý 34 71.0 f f1ý. f fo. e fl. e t. e

10.1:. 01 17.0 e. N 1 0.06 0.0 1o0 tai 133 0. f Ul. f U. e 4. e f. 0

1 TMaa figures apº11 to U. w-º. Vd are of the basis .. y aai wert dative/ true 4 tally t»tu baled a1NVA1441»0 (Ibllta tile). 1 See figure 3.8 1« da(tatttaar. 271

Espey et al. (1969), Seaburn (1969), ASCE Task Force on Urban Hydrology

(1969), Chow (1952), consequently this paper concentrates upon the

presentation of results for comparative purposes rather than undertaking

a further review of the merits and demerits of the unit hydrograph

assumptions.

The selection of storms for analysis was constrained by the advice

of Chow (1964, p. 14-14) and a requirement specific to this study which

demanded that the peak of the unit hydrograph should not be more than

ten times that of the original flood thus reducing the danger of gross

error through over-extrapolation of the unit hydrograph assumptions.

The imposition of these conditions meant that only a few storms each

year were suitable for the derivation of unit hydrographs. Consequently,

it was necessary to calculate mean unit hydrographs from the unit

hydrographs of a number of years. Three periods were selected for

the calculation of a mean unit hydrograph. October 1950 to September

1954 represented a period before urbanisation began but included a year

during which the paved area occupied 1.6% of the basin. October 1957

to September 1962 represented a period when the percentage of the

catchment covered with impervious surfaces increased from 6.6% to 11.5X, but the paucity of suitable floods shortened this analytical period to

August 1958 - December 1961.1October 1965 to September 1968 saw the expansion of the paved area fr m 15% to 16.6%. The unit hydrographa themselves were derived manually according to the method given by Chow (1964, pp. 14-17 to 14-22) and the baseflow was separated by a horizontal line at the observed dry weather flow for each period since this clay catchment has an almost negligible baseflow component in its hydrographs.

I 272 r

5.4 The results of the unit hydrograph analysis are given in Table

Twenty-seven floods were analysed in all, eight for October 1950 -

September 1954, eleven for October 1957 - September 1962 and eight

for October 1965 - September 1968. The floods for the first period

were almost all of a winter type, for as can be seen from the flood

of 16.6.53 and from Figures 5.6 and 5.7, there were hardly any

substantial summer floods during this early period. The floods for

the two later periods contain a much greater proportion of summer events

because summer rainstorms are generally of the short intense variety

needed for unit hydrograph analysis and as a result of the building

development,. they gave rise to both substantial volumes of runoff and

distinct peaks in flow. Urbanisation appears to influence all measures

of unit hydrograph shape. The peak flow has increased 4.6 times, from

61 to 281 cusecs, with 16.6% of the basin paved. The time of rise

of the hydrograph has shrunk from 4.8 hours to 2.1 hours, but inspection

of the time of rise for 1957-62 shows that there was relatively little

change in the 1960s. The widths of the 1965-68 unit hydrograph at 25%

and 50% of peak flow are only a fifth of their respective values in the

1950-54 hydrograph. The findings of the unit hydrograph analysis are

summarised in Figure 5.10 where the three mean unit hydrographs have been

plotted from the parameters given in Table 5.4 The observed changes in

the unit hydrograph certainly result in part from the urbanisation of

the catchment, but the predominance of summer floods in the later time

periods, as opposed to the winter floods of 1950-54, may have caused the

narrowing and peaking of the unit hydrcrgraph. Evidence for this

derives from the peaks of the unit hydrographs for 13.12.61,1.12.60

and 1.1.59 which are very similar to the mean unit hydrograph for 1950-54,

and from the fact that the time of rise did not fall significantly between the second two periods which are both dominated by summer floods. 273

able 3.4 (NIT IIVDRO(RAP1I5 AND M?A$ (NIT HTDP(CPAPH$ (0.2 inches of effective tetptell Is one hour) ; (e) Unit Hydrographe for period October 1950 " September 1l31

Time Date Original Original volume origin ;t IM ra« of TOO T21 T30 T23 ? or rý or x eýnott Ro: Vv Oak u-1 4.3.32 8.6 30 2.3 76.0 17.3 8.0 4.23 19.12.32 6.416 0.0640.114 31.1 36 7.0 115.0 20.0 11.0 7.0 30.4.33 27 0.098 14.0 74 3.5 92.0 12.0 3.3 2.23 13.6.53 4 0.021 14.1 38 1.0 71.0 46.3 18.0 6.3 13.10.33 17 0.031 3.6 67 2.3 96.0 16.5 3.3 2.5 1.11.33 38 0.120 12.7 63 9.5 113.0 17.3 9.3 6.0 13.1.34 16 0.033 16.2 61 3.3 32.0 20.0 12.3 8.0 2.3 . 2.34 56 0.158 32.7 7e 4.3 90.0 13.0 3.5

Mobil Unit 1 4.6 hydrogreph 61 4.1 81.1 20.4 !.

(b) Unit bydrotrephs for the period October 1957 " feptesber 1962

Data Original original voluwe original UH Teei Tim of T00 T25 TStl TIS Peak o Aunorf Z Runoff 8 ýuie usece' (cueece)

18.8.58 33 0.033 6.6 193 3.0 36.0 6.0 3.3 2.25 22.8.38 108 0.091 23.3 237 0. S 106.6 4. S 1.0 1.0 3.9.38 151 0.173 40.2 174 3.0 43.0 7.3 4.3 3.0 1.1.59 36 0.064 52.8 83 3.3 30.3 U. S 8.0 3.3 28.7.59 76 0.039 32.9 359 1.0 29.0 3.3 1. S 1.0 27.7.60 28 0.023 6.4 244 3.3 20.0 6.0 3.0 1.0 18.8.60 35 0.033 20.5 187 1.3 33.3 6.0 3.5 2.3 4.9.60 41 0.033 6.3 247 2.0 21.0 3. S 3.0 2.0 1.12.60 29 0.073 30.3 70 2.0 110.0 12.0 4.5 1.73 14.9.61 77 0.092 12.9 168 4.0 66.0 7.0 4.0 2.3 13.12.61 21 0.071 70.3 60 1.0 81.0 13.3 8.0 1.0

lean Unit 184 2.3 34.4 7.6 3.6 2.0 ydrograph

atio of can UH to 3 0 0 48 0.62 0.37 0.40 0.44 an V11for . . 1930-34

(e) Unit bydrogriphe for the period October 1965 " Septesber 1964

Original Original Volume Original Time Dat. Ni t aY of T00 T23 TSO T)3 týJMyýa ) oR no t Z Runo!! RI o! 11 (cu"Oev) . mau w

22.6.66 276 0.189 13.4 292 3.0 34.3 4.0 2.0 1.3 24.6.66 77 0.031 42.6 213 1.3 42.3 4.3 1.5 0.73 20.7.66 66 0.050 16.2 343 1.0 40.3 2.3 1.3 0.3 24.7.66 38 0.028 11.6 321 1.5 43.3 3.25 1.3 1.0 15.11.66 106 0.104 34.3 193 its 60.0 5.0 1.3 1.0 20.10.67 37 0.023 20.8 280 1.5 31.3 3.0 1.3 1.0 10.7.66 129 0.049 22.1 366 1.3 , 24.0 3.3 1.1 0.73 14.7.66 261 0.223 25.7 234 3.1 33.0 6.5 3.3 1.3

Mean unit hydrotraph 261 2.1 46.7 4.0 1.6 1.1

Ratio of mean V to 0.20 0.34 mean Lit for 4.6 0.44 0.33 0.20 1950-34 274

280 i 260-

240-

220- " "ý'-1966-68 II 200-

180 in v 160 V ! 140 ýýI 11 120

100 '1-1958-62 II ýt 80-1

60-11

40 "ý"` 1950-54 20

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Time In hours

Figure 5.10 Unit hydrographs for three stages of development.

F 275

The work of Harvey (1971) on the rural clay catchment of the River

Tor in Essex lends support to this view. He identified "two distinct

from types of simple flood hydrograph resulting storm rainfall ...

one characteristic of winter runoff, and a shorter duration typo

(with a relatively high peak) occurring only during the summer. "

These results indicate the extent and direction of the effect of

urbanisation upon floods of the type chosen for analysis and they are

very similar to the findings of comparable American work cited earlier.

The rise in peak flow is somewhat greater than that found in the U. S. A.

for the same degree of urbanisation, but in any event one cannot

extrapolate directly from one basin to another and it is hazardous

to attempt to extrapolate these unit hydrograph findings to larger and

less frequent floods within the Canon's Brook.

The influence of both the seasons and overall flood magnitude on the effect of urbanisation on floods: A regression analysis.

Seven sets of regression equations were derived in order to test

the two specific hypotheses; first, that the effect of urbanisation declines as flood magnitudes increase and, second, that the effect of urbanisation on floods is gre er in summer than in winter. To test the first hypothesis, equations describing the relationship between hydrograph parameters and climatic, hydrological and land use variables were derived from three different samples of floods. The first contained 192 all floods, the second contained 38 floods in excess of 100 cusecs whilst the third had 19 floods all greater than 138 cusecs. The second hypothesis was tested by the calculation of similar equations for the 80 floods occurring during the winter months (October )larch incl. ) P - 112 floods and the occurring during the summer months (April'- September incl. ). The two hypotheses were tested together by the calcuAition of 276 ý

equations for floods over 100 cusecs for both summer and winter seasons.

Table 5.5 describes each of the variables employed in the analysis. e Ther, were eight dependent variables, six of them corresponding

exactly to parameters assessed in the unit hydrograph analysis. The

independent variables numbered sixteen, but only eleven of these were

entered in equations. The three variables, X4, x6, X9, which describe

the soil moisture conditions in the catchment before each flood were

derived from Chapbbr"4'5 analysis of monthly water yield which involved

the simulation of a daily water balance for the basin for the period

1950-68 (Hollis 1970). All the variables were transformed to

logarithms so as to normalise their distributions and a stepwise regression program, BIM02R (Brm 1970), was used to derive the equations.

Normally variables were only entered in equations when their contribution. \ was significant at the 1% level. However, in the cases of the under- lined variables and coefficients in Tables 5.6 and 5.7 variables were entered in the equations on the grounds of hydrological reality, even though their contribution was only significant at the 5% level.

The results of the analysis of the effects of increasing flood magnitude on the impact of urbanisation on floods is given in Table 5.6, where the'equations are presented in their natural number form. The further twenty-eight equations used in, the analysis, of seasonal influences cannot be presented here, but the following comments about the regression analysis apply to the testing of both hypotheses. All of the original logarithmic regression equations were significant at the 1% level, but the proportion of variance explained was generally rather low, and particularly poor for the equations describing hydrograph width. However, all of the meteorological and soil moisture variables were logically related to hydrograph parameters. The welter of detail in Table 5.6 277

Table 5.5 Variables in the Regression-Analysis

Dependent Variables

Y Peak flow in cusecs. y The percentage of the storm rainfall which ran off. (Impermiability

factor. )

Y3 The time of Rise of the Hydrograph in hours.

Y4 The Lag time of the Hydrograph in hours. (Centroid of Rainfall

to peak flow. )

Y5 The width of the direct runoff hydrograph at the base in hours. (TOO)

Y6 The width of the direct runoff hydrograph at 25% of peak flow in

hours. (T25)

Y7 The width of the direct runoff hydrograph at 50% of peak flow in hours. (TSO)

Y8 The width of the direct runoff hydrograph at 75% of peak flow

in hours. (T75) I Independent Variables

X1 The duration of the rainstorm in hours.

X2 The total rainfall in inches.

X3 The river discharge at the beginning of the rise in the hydrograph

in cusecs.

X4 The mean soil moisture deficit in the non-paved soil areas of the

catchment in inches at 9.00 a. m. before the flood. X5 The percentage of the catchment covered with paved surfaces which

were linked to the river by surface water drains. X6 The percentage'of the non-paved soils of the catchment with a soil

moisture deficit of less than 0.5 inches at 9.00 a. m. before the flood. X7 The average intensity of rainfall during the storm in inches per hour. X8 The intensity maximum of rainfall during a five hour period of the

storm in inches per hour. 278

X9 The percentage of the non-paved soils of the catchment with a

soil moisture deficit of less than 1.5 inches at 9.00 a. m.

before the flood.

X10 The duration of rainfall at a rate in excess of 0.08 inches per

hour in hours.

Xll The maximum intensity of rainfall during a two hour period of

the storm in inches per hour. 4 279

Table f. 4 7(M. 11: OtpMlet Luº1" ttwtl.. lld. d the tree Reell. f 07 1.1. 1 4.0. 1 1 trn1 . YA. r . q. nt (P. ha11.1=b1: º1" I hr1. N" {lu 1 . ýwtfY. ) l+{. rltrl4 regression Number trot 1M (t. r IM (rot IM O 1M 1"/ }'!. n ., Iq "jY. t/w) tq . q.. UN) (.1.4.. 41«1 04-41") Natural "Mahe I,

0014 7f' 1 on 0.76 0.16 144., 4,111 7. A flow 61.4 111 11" 10.13.00.0.41 . QYNl. ) u. 1 6,33 lw. a 7{ rl" tu. 7.. 1" . 77' .ý .1 7 eat e,/3 e.1$ 11.1 1,16 13$." tf rl" us. a.sý . {ý f 0.11 0.3s 0.10 43 56 36 It of 26a 111 r"0.41.11 s) . ' " 0. /1 0. // e. )s 40.0 ", 111 taW all . .1 it J4 S 0.11 "1.1 1, If t. nlls/ $). 6 38 T, - 7.33.12' . f{' 0. /S 0.77 "fI 1/ " 0.19 0.10 0.1) 31.1 7.16 6.7 T1.1.3f. 1tfý . fý'

1 0.30 7344 2,109 Time .1 3.1 lot Tý 0.3/. 111' . llý' 0.1$ 0.73 rl,. N h7are/rqº Lo ' T )! 0; 11' 0.01 0.78 0.10 61.1 7.31 (hour. ) 4103 5.36 / 0.16 44.4 1.16 11 T3.0.70.1=' . 71' 0.03 0.81 0530,031 Lot 1.7 111 1 2.11. tß" 110' ' 10 0.73 0.14 0.1) $4.8 4"167 ties a" . . .1 f 1 (Clnt, Yla of rein- 7.1 )0 ll 0.20 31.1 1.31 fall to fý 0.403. %2'4 . f/' 0.63 0.66 Peak) ). a1 11 Tý 0.17.7iß'" ll 0.40 0.11 0.72 13.1 3,11 0~0 º... 14.1 303 7,0.3 )) 0.39 60.0 3.140 M TS" . 111'4 . 77' 0.70 0.49 t 67irgnfº t)6.7 N TS" 72,70.41' 14 0.61 0.41 0.76 13.1 1. ){

(Anurs) 109.3 19 15 0.39 0.21 10.1 1.11 f7" 11 0.67

YIt% of 10.7 111 T4" 1.675.1 U 0.71 0.39 0.15 66.9 4.161 1/" ý 11rregraph .i .i I4rr., si of 10.1 0.14 61.7 70) SI T/" 1.17.71' . 16' )1 0.11 0.19 l ! "M rp 4.6 0.13 31.1 1.11 (Mira) it Tý 3.11.77 'a 11 0.11 0.11 N 7{ 09 3.110 width of 7.0 102 7 . 0.1". 14: '5). {'. . t1 0.11 i 0. a e. 34 106.4 º7. 7 ' . r. tr. f1, .3 foi of lot S. 1 31 0.11 Sa.) 7, )3 LL T7.0. {). 11'4 . i/ 70 0.87 0.131 1uM ry . 7.30 If T7 1.01.71 ' )l 0.1) 0. )) 0.1e 11.0 1.11 (hour. ) 11tH 3.7 lot 26 0.16 11.7 3.141 of T. . 77 0.17 0.59 at lit Yf 3.7 31 0.11 31.1 1.34 nY T1 0.47. x1'w. x/' . x)04 fl 0.01 0.1S LaMry. 7.73 It TI. 1.00.1 -, 56 74 0.61 ". 41 0.10 IS. 1 1,11 (fit) 1

The castle of 192 floods-sestaim all floods used is "to study The sample sf IS floods sMtei" floods is #stege of 100 guests The sgls sf if floods swtslw floods Is omega of 136 assets

2. The ssettibstLN sf each satiable to the gwtiw is Nplllus ss the 1i 1... 1 . r. q so"* the sociable I. W.. Ur1 she" Shod Its s.. uN. tlwls "Iplfitu. t st sal? W 51 level

ý- 280

makes it difficult to examine the effects of urban sation and so

Table 5.7 has been prepared as a summary of the work undertaken to

test both hypothcses. Table 5.7 gives the value of the exponent of

X5, the percentage of the catchment paved, when for a particular

sample/and dependent variable it was entered into the equation.

Cells with the words "No Relationship" indicate that for the appropriate

sample and dependent variable, the percentage of the catchment paved rot did make a contribution to the stepwise regression e nation which

was'significant-at even the 5% level.

Table 5.7 shows that urbanisation has a marked influence on almost

all hydrograph properties when all types of floods are considered,

(column 1). However when only floods ovei 100 cusecs are examined,

(column 2), urbanisation affects only peak flow and its contribution

is only significant at the 5% level. Column 3 reveals that for floods

of over 138 cusecs, urbanisation does not significantly influence any

hydrograph parameters. This latter rather arbitrary threshold was

selected so that two floods from the first four years of record could be incorporated into the sample for analysis. Clearly, the hypothesis

that the effect of urbanisation on floods declines with increasing flood magnitudes must be accepted, and in round figures a flow of 150 cusecs for the Canon's Brook seems to be the upper limit of rural floods which are modified by urban development. It is not easy to give a frequency flood to a magnitude of 150 cusecs, but equation 3 from Table 5.6 may be used in conjunction -with the Meteorological office studies of rain intensity frequency relationships in Britain (Holland 1964) to give an

If approximate answer. a pre-flood discharge of 2 cusecs is assumed and data from Holland (1968) taken for, rainstorms of six hours duration, the large floods, concentration time of then it would appear that a flow " 281

Table 5.7 The variation in the exponent of Xs, the percentage of the basin paved, with various dependent variables and sub-samples of floods.

Column 1234567

2 18 Sample All 192 38 Floods 19 Floods 80 Winter 112 Sumner2 20 Winter Summer Floods Floods over Type Floods over 100 over 1381 Floods Floods over 100 cusecs 100 cusecs Dopendent cusecs cusses Variable

No No Y 0.633 0.154 No 0.31 0.87 1 l relation- relation- re n- Peak Flow ate ship ship

No Y 0.56 No No 0.23 0.86 No 2 relation- relation- relation- relation- X rainfall " of ship ship ship ship running off

Y3 No No No No 0.11 No 0.48 Time of relation- relation- relation- relation- relation- Rise ship ship ship ship ship

No Y -0.39 No No -0.34 0.42 -1.59 4 relation- Lag Time relation- relation- ship ship ship

No Y 0.30 No No No 0.46 No S relation- relation- Time Das. relation- relation- relationship ship ship ship ship

Y No No No No No 6 -0.11 -0.13 relation- relation- relation- Width of relation- relation- ship ship ship ship ship Hydrograph at 25% of Peak

Y No " No No No No 7 -0.09 -0.11 relation- relation- Width of relation- relation relationship ship ship ship ship Hydrograph at 50% of Peak

Y No No No No No 8 -0.13 -0.13 relation- relation- relation- relation- relation- Width of ship ship. ship ship ship Hydrograph at 75% of Peak

1 138 cusecs was chosen because this was the highest threshold which still allowed the rural period, Oct. 1950 -. Sept. 1933, to be represented. 2 Winter season is October - March Inclusive. Summer season is April - September Inclusive.

3 Exponents written thus, 0.63, indicates that the percentage of the catchment paved made a contribution to the predictive equation which was significant at the 1% level.

4 Exponents written thus, 0.15, indicates that the percentage of the catchment paved made a contribution to the predictive equation which was significant at the Sx level. 282

of 170 cusecs in the Canon's Brook is equivalent to a rainstorm with

a return period of only five years. The general conclusion must be

that urbanisation has a very significant effect on small floods but

it does not seem to influence large floods of over 138 cusecs.

Summer floods, (column 5 of Table 5.7), are very greatly influenced

by urban effects but this influence is much smaller or non-existent

in the winter months,. (column 4). However, when only large floods of

over 100 cusecs are considered, the summer - winter dichotomy almost

disappears and as columns 6 and 7 show, there is practically no relation-

ship between the urbanisation of the catchment and changes in the

hydrograph during either summer or winter months.

S It only remains to examine the rows of Table 5.7 to assess the

nature and direction of the impact of urbanisation on those floods

which are significantly affect d. With only one exception, the

findings here are as they wer for each of the early analyses contained

in this chapter. The paving'of 16.6% of the catchment increased peak

flows by between 1.52 times for floods of over 100 cusecs (column 2)

and 11.5 times for all summer floods (column 5), but these relationships

do not extend to large floods as is shown by columns 3,6 and 7. The

percentage of rainfall which runs off is increased for those samples

which contain a majority of small floods and lag times are decreased

for the same samples. The time of rise is again somewhat problematical. I It shows no relationship to urbanisation for all samples except those

containing only summer floods. In these,. it appears to increase with

advancing urbanisation, but the exponent is only significant at the

5% level. The time base of the hydrograph iq increased for summer floods and the width of the hydrograph at higher flows is omewhat reduced. 283

The-most significant point however, with regard to the hydrograph )ariables width is the lack of relationship to urbanisation for all

samples except those containing all floods and all summer floods,

which are samples dominated by small floods.

Dis ussion

The quality of the results obtained depends on both the basic

data and the analytical methods used. The streamflow data were

derived from an instrument specifically designed to accommodate flood

flows and in spite of the movement of the climate station, it has

always been near the catchment. As far as possible standard methods

of analysis have been employed to ensure comparability of results and

an attempt has been made to avoid excessive levels of abstraction in

obtaining results. The findings must be qualified by a restatement of

the fact that the basin comprises a bedrock of London Clay overlain

by glacial boulder clays and a single bed of gravel. As such, it is

hardly surprising that urbanisation has had so little effect on large

floods, for clay basins in eastern England are characterised by a

"flashy" runoff regime with high impermeability factors during heavy

rainfalls. Moreover, the frequency of flooding expected as a consequence

of a deliberate underprovision of surface water drains is "once a year in

areas of new urban development" (Watkins, 1962). The limited effect

of Harlow on relatively large floods may therefore be a result of the

throttling of flow by the storm water sewers and the restriction of

the movement of the surface water to the river other than over permeable

surfaces or by the restrictive sewers.

It is most desirable that a return period be ascribed to the flood flow associated with the five year rainstorm. Unfortunately this is 284

not an easy matter since the probability of a flood flow is conditioned

by the 7ombined probabilities of rainfall amount, rainfall intensity,

soil moisture conditions, storm shape, storm movement etc. However,

it seems likely from practical experience (CIRIA 1973) that the return

period of a flood is much longer than that of the associated storm 4ay rainfall. A five year rainstorm over the Canon's Brook therefore

produce a flood with a twenty year recurrence interval. The Netteswell

regulating reservoir was neglected in the study because first, no change

was observed in-the hydrograph at the gauging station, second, it

collects drainage from a small part of the catchment and finally any

attenuating effect would lead to a conservative estimate of the effects

of urbanisation. The stability of the large floods during a period

of substantial urban development in the catchment cannot be ascribed

to the construction of the pond, because of the marked increases observed

in the peak flows of small flood events.

Many (Nixon 1959 Leopold, Wolman Miller 1964) authorities , and

have found that bankfull discharge is associated with a return period of

between 1.5 and 2.2 years. Consequently the discovery that floods with

a return period of perhaps 20 years have not been affected by the 16.6% paving of the Canon's Brook leads to the conclusion that floods of a shorter return period must have been increased and therefore the frequency of overbank discharges in the area downstream of the gauging station must have been increased to some extent by the construction of the new town.

Conclusion

The urbanisation of the 8.25 square mile clay catchment of the

Canon's Brook began in October 1953 and by September 1968 16.6% of the basin had been impervious covered with surfaces linked by surface water .. J ZÜ5

drains to the of the river. This town development r semi-natural channel

increased the mean maximum monthly flood by 220% and made summer floods

in the range 40-100 cusecs much more frequent. The calculation of

mean unit hydrographs for 0.2 inches of effective rainfall occurring

in one hour for the rural years of hydrological record and the three

most densely urbanised years of record, presented some problems of

interpretation. The unit hydrograph for the latter years had a peak

4.6 times greater than that for the rural period, whilst the time of 16 rise of the hydrograph and the width of the hydrograph at 50% of peak

flow showed contractions to 44% and 20% of their respective rural values.

These results were thought to apply only to the small frequent floods which were used to derive the hydrographs and they were not advocated as the complete answer to the quantification of the hydrological impact of urbanisation. More detailed and incisive regression analysis showed

that whilst the intuitive ideas of Leopold (1968) and the findings of

the unit hydrograph analysis were appropriate to floods and particularly summer floods, of under 100 cusecs, the effect of urbanisation on floods of about 150 cusecs, with a return period of around 20 years, was minimal. 286

Bibliography

American Society of Civil 1969 Effect of urban development on Engineers Task Force on flood discharges - current know- Urban Hydrology ledge and future needs. Proc. A. S. C. E. Journ. of Hydraulics Division 95, HY19 pp287-309.

Anderson, D. G. 1967 ' Effects of'urban development on floods in PN.Virginia. U. S. Geol. Survey Open File Report 39pp.

Blalock, H. M. 1960 Social Statistics. McGraw Hill, 465pp.

BMD 1970 Biomedical Computer Programs. Edited by Dixon, W. J., University of California Press, 600pp.

Chow, V. T. 1952 Hydrologic Studies of urban watersheds: Rainfall and Runoff of Boneyard Creek, Champaign-Urbana, Illinois. Univ. of Illinois College of Engineering, Civil Engineering Studies, Hydraulic Engineering Series No. 2,73pp.

Chow, V. T. 1964 Runoff. Chapter 14 in Handbook of Applied Hydrology Edited by Chow, V. T., McGraw Hill.

Clayton, K. M. 1957 Some aspects of the glacial deposits of Essex. Proc. Geol. Ass. 58(1), pp. 1-21.

Construction Industry Research 1973 Discussion during the Research and Information Association Colloquium on Rainfall, Runoff and (CIRIA) Surface Water Drainage of Urban Catch-ments. Bristol April 1973. (to be published)

Crawford, N. H. and 1966 Digital simulation in hydrology: Linsley, R. K. Stanford Watershed Model IV. Dept. of Civ. Eng. Stanford University Technical Report, No. 39,210pp.

Crippen, J. R. and 1969. Hydrologic Effects of Suburban Waananen, A. O. Development near Palo Alto, California. U. S. Geol. Survey Open File Report 126pp.

Dixon, D. J. 1971 A review of schocastic processes and data generation techniques for the practicing hydrologist. Water and Water Engineering, 75, No. 910, pp. 465-469. 287

Espey, W. H., Winslow, D. E., 1969 Urban effects on the Unit Hydrograph. and Morgan, C. W. In: Effets of Watershed changes on Streamf1o Edited by Moore, W. L. and Morgan, C. W. Texas Univ. Press, pp. 215-228.

Gregory, K. J. and 1968 The variatI ion of drainage density Walling, D. E. within a catchment. Int. Ass. Sci. Hydrol. Bu]1., 13, pp. 61-68.

Hall, M. J. and 1972 Time series analysis of mean daily O'Connell, P. E. river flows. Water and Water Engineering, 76, No. 914, pp. 125-133.

Teu. Hamlin, M. J. and 6 1971 Extending the record on the e. Kottegoda, N. T. J. of Hydrology, 12, pp. 100-116.

Harris, E. E. and 1964 Effect of urban growth on stream- Rantz, S. E. flow regimen of Permanente Creek, Santa Clara County, California. U. S. Geol. Survey Water Supply Paper 1591-B, 18 pp.

harmonic Hartley, H. O. 1949 - Tests of significance in r analysis. Biometrika, 36, pp. 194-201.

Harvey, A. M. 1971 Seasonal flood behaviour in a clay catchment. Journal of Hydrology, 12, pp. 129-144.

Holland, D. J. 1968 Rainfall intensity frequency relationships in Britain. First Reprint of Meteorological Office Hydrological Memorandum No. 33, 28pp. with Appendix of lOpp.

Hollis, G. E. 1970 The estimation of the hydrologic impact of urbanisation: an example of the use of digital simulation in hydrology. Dept. of Geography, University College London Occasional Paper No. 5.

Institute of Hydrology 1971 Research 1970-71.

James, L. D. 1965 Using a digital computer to estimate the effects of urban development on flood peaks. Water Resources Research, Vol. 1, No. 2, pp. 223-231.

Kottegoda, N. T. 1970 Applicability of short memory models to English riverflow data. Journal Inst. Water Engs. 24(8), pp. 481-489.

Leopold, L. B., Wolman, M. G., 1964 Fluvial Processes in Geomorphology. and Miller, J. P. Freeman, 522pp. 288

Leopold, L. B. 1968 Hydrology for urban land planning - A guidebook on the hydrologic effects of urban land use. U. S. Geol. Survey Circular 554, l8pp.

Linsley, R. K. and 1964 Water Resources Engineering, Franzini, J. B. McGraw Hill, 654pp.

Martens, L. A. 1968 Flood inundation and effects of urbanisation in Metropolitan Charlotte, N. Carolina. U. S. Geol. Survey Water Supply Paper 1591-C 60pp.

Nixon, M. 1959 A study of the bankfull discharges " of rivers in England and Wales. Proc. Inst. Civil Eng. 12, pp. 157-174.

Seaburn, G. E. 1969 Effects of urban development on direct runoff to East Meadow Brook, Nassau County, Long Island, New York. U. S. Geol. Survey Professional Paper 627-B, 14pp.

Waller, R. S. and 1970 Drainage and Flooding in the Shaw, T. L. Gloucester Region. Civil Engineering and Public Works Review, 65, pp. 368-369.

Walling, D. E. and 1970 The measurement of the effects of Gregory, K. J. building construction on drainage basin dynamics. Journ. of Hydrology, 11. pp. 129-144.

Watkins, L. H. 1956 Rainfall and runoff: An investigation at Marlow New Town. Proc. Inst. Mun. Eng., 82,8. pp. 305-316.

Watkins, L. H. 1962 The design of urban sewer systems. Road Research Technical Paper No. 55. 96pp. H. H. S. O.

Whittle, P. 1952 Tests of fit in time series. Biometrika, 39, pp. 309-318.

Wiitala, S. W. 1961 Some aspects of the effect of urban and suburban development upon runoff. U. S. Geol. Survey Open File Report, 28pp. 289

CHAPTER 6

The Sediment Yield and Channel Morphology of the Canon's Brook.

The immediate cause of river inundations insufficiency ... is the of the channels by ... occasioned partly their narrowness and partly by obstructions to their currents, the most frequent of which is the deposit of sand, gravel, and pebbles in their beds. George Perkins Marsh

In this chapter the impact of urban development and construction is work on the rates of erosion and deposition within river basins examined. The literature, largely American, is reviewed and emphasis The is given to the effects of urbanisation on the natural system. ý", few British published studies of catchment sediment yields are evaluated. Changes in the morphology of the Canon's Brook channel from 1956 to 1970 are studied in the light of the flood analysis of

Chapter 5 in an attempt to extend our present knowledge of the inter- relationship between channel shape and discharge. An investigation of reservoir deposits in the upper part of the catchment is reported and the mean rate of erosion fr the period 1956-70 and compared with estimates for other British c tchnents.

Urbanisation always occurs in natural or semi-natural areas.

If the erosional and sedimentological effects of such development are to be understood it is clearly necessary to have an appreciation of the functioning of the water - sediment - channel morphology system under natural conditions. Wolman (1967) has argued that urbanisation is only a further stage it the history of a once natural drathage basin and has suggested a cycle of sedimentation and erosion in urban river 290

SCHEMATICSEQUENCE: LAND USE, SEDIMENT YIELD AND CHANNEL RESPONSE J FROM A FIXED AREA

2000-

"41000- !EN -La 600 200 1000 1660 1900 1960 2000

Lond Cropping WoodS" Urbon Use Forest Grozing Construction Channel Stoble Aggrodotion Scour StobleIScour Bonk Erosion Condition A9grodotion

-Figure. 6.1 A cycle of erosion and sedimentation in a Piedmont river undergoing successive modification by man. (after Wolman, 1967) 291

channels. Figure 6.1 is Wolman's chronological sequence of changes

occuring in a catchment during a two hundred year period of deforestation,

cropping, construction activity and stable urban development. In

essence the figure depicts the effects of temporal changes in the

relationships between a complex mix of variables in one region in the

U. S. A. The effects of similar temporal changes in other localities

may be different because there the relative importance of variables

and the relationships between them may be different. Early reviews

suggested that there were simply a series of factors which affected

rate of erosion. Musgrave (1947) suggested that "qualitatively the

primary factors influencing the rate of erosion are known to be

flow ... rainfall ... characteristics of the surface runoff as affected by (1) degree (2) slope and slope length ... soil characteristics ...

and vegetal cover". Glymph 'k954) provided a more comprehensive list

of influencing factors and, 1 ke Musgrave, reviewed a series of multiple

regression equations which had been developed to predict sediment yield from ungauged basins in various parts of the United States. He concluded by that "sediment be saying yield ... appears to the result of multiple casual factors. Variations in the (statistical) significance of the individual casual factors from one physiographic area to another probably accounts for the observed` differences in sediment yield over the country".

Our increasing understanding of the inter-relationships in the environment and the statistical autocorrelation between variables (Shue Tuck Wong 1963) together with the growth in the importance of systems thinking, (Chorley

1962, Chorley & Kennedy 1971, Cooke 1971) has1suggested the formalisation

Figure of 6.2. It is apparent from Figure 6.2 tha when an area is urbanised a series of variables like river'f low, so 1 and the channel system are affected directly by the change but that in fact urbanisation has ramifications throughout the whole system becau e of inter-relations between all of the variables shown. In the ensuin review each of the 292

r------I URBANISATION I BUILDING RURAL-UR -i----- _____ ACTIVITY IC0NvERs10N-'---, -'1 1 ______I II _ r111 I1Iri " H [44LIMATE! I 1 11I1 11 1 L* rr LAND USE AND ý""ý 1I SOIL VEGETATION j 11 ýr

j RIVER RUN OFF ®-" - FLOW

CATCHMENT GEOLOGY- MORPHOLOGY I 1

CHANNEL SYSTEM .4 ------

CHANNEL EROSION EROSION ON SLOPES a AND AGGRADATION

CATCHMENT YIELD OF SEDIMENT

Figure 6.2 The major components and relationships in the catchment system and the effects of urbanisation. 6 293

links in Figure 6.2 are discussed in turn, initially the rural/natural

links are discussed and the impact of urbanisation i considered after-

wards. The literature is so extensive that a fullylcomprehensive

review is beyond the scope of this work and therefor) attention has

been focused on widely quoted papers and those whicl give quantitative

expression to particular links in the system.

The two most important components of the system are geology, which major forms a input of mass in the form of weathered rock, and climate

which provides both energy and water for the system. The major links

between geology and climate and the soils. and river variables i -vegetation öt will 4 be discussed for they are essentially undirection4l and unlikely

to be; greatly influenced by urban effects. It does seem valuable, however, to look at the subsystems of climate - vegetation - runoff - `.

erosion, and geology - erosion for these are relevant to the discussion.

Langbein and Schumm (1958) investigated the relationship between sediment yields and effective precipitation, this latter quantity being defined "the at amount of precipitation required to produce (a) known amount of runoff", and is therefore equal to runoff plus evapotranspiration. They took data for 74 sediment gauging stations of the U. S. G. S. and 163 reservoir surveys and discovered that "sediment yield is a maximum at 10 about to 14 inches of precipitation, decreasing sharply on both sides of this maximum in one case owing to the deficiency of runoff and in the increased other to density of vegetation". Most significant was the finding that the rates of erosion in the reservoir surveys were about ' double those in the sediment station records (Figure 6.3). They also found a between positive relationship annual precipitation and bulk density of vegetation and a strong negative relationship between bulk 294

I,ooO 1 FOAUT DESCAIMCAT ýC^ GRASGpASSLAkCS--rLAkDi-r- FOAUT WNVO I I- "00 '

v2 600

WOO

0a 60 io w "o ww "' crrcctncºPcunt. r4os. 4$ wupp

determined from Figure 6.3(a) Climatic yield of sediment as variation of 1958) sediment stations. (after Langbein and Schumm,

T -ý

». I

ýý h

Figure 6.3(b_)_ Climatic variation of yield of sediment as determined from reservoir surveys. (after Langbein and Schumm. 1958) i

ýi 295

density of vegetation and relative erosion measures. Douglas (1967) has re-examined Langbein & Schumm's work and commented upon Leopold's

(1956) work in the same area which showed that sediment yields could be increased by between two and fifty times by land use changes.

Douglas constructed a further empirical diagram relating suspended sediment yield to runoff and showing a peak of erosion at about 50mm

(2 inches) runoff and an increasing rate of erosion after 600mm

(24 inches) of runoff., The diagram has a form similar to that of

Langbein & Schumm's although the rate of erosion at the lower rainfall peak is only 1/3 as great. The increase in erosion at high rainfalls is beyond the limit of Langbein & Schumm's data. Douglas went on to correlate suspended sediment load with runoff for 26 basins in eastern

Australia, and found a coefficient of 0.5. lie was able to improve the multiple correlation coefficient to 0.7 and consequently the estimation of sediment yields, when the ratio of the square of the maximum mean monthly precipitation to the mean annual precipitation was used as a further independent variable. He concluded that marked seasonality increases the erosion from basins for it brings with it both intense rainstorms and attenuated vegetation.

Dawdy (1967) examined recent unpublished work on the controls of rural sediment yields. In the Potomac basin he found that sediment yields fell from 400 tons per mile to 40 tons per mile as forest cover increased from 20 to 80 pelcent. Similarly an increase in cropland i from 10 to 50 percent increased sediment yields three fold. The significance of ram climatic events in erosion and sedimentation has been reviewed by

Piest (1963) who examined records from 72 basins, largely situated in the humid S. E. of the U. S. A. - He found that for "lost watersheds more than half of the soil losses are attributable to the smal er storms that occur 296

more often than once a year". Wolman and Miller (1960) observed that

in the 2,280 sq miles of the Yadkin Basin in North Carolina, 90% of

the total sediment yield was transported by flows that occurred about

three days each year. Similar flows moved only 54% of the sediment

in the smaller Brandywine Creek of Pennsylvania. Swenson (1964) has synthesised the results of several other workers; over 10% of the sediment yield of three quaters of the 34 basins investigated by

Love over a period of 15 months was removed in 24 hours, whilst the " streams in Missouri and Wisconsin discharged 90% of their annual sediment load in about 10 days. Swenson also quotes data which shows that 75% of soil loss from seven experimental stations throughout the

U. S. A. occurred during only four storms per year.

An appreciation of the i ter-relationship between rock type, soil `"ý and sediment yield is important if one is to consider the impact of urbanisation, for this latter quantity will clearly vary from one geological region to another. A notable study of the variation of soil shear strength with the parent material is that of Chorley (1959), who used a penetrometer on the soils of the Oxford Clay, Upper Greensand,

Shotover Sand and Kimmeridge clay. He found that relatively high relief exists in areas underlain by sandstone and that these areas also possess the highest-soil shear strength. The lowland clay soils were associated with low shear strength aid very small permeabilities.

Yamamoto and Anderson (1967) derived indices of soil erodibility for 31 areas in Hawaii by means of laboratory anIlysis of the size distribution of water stable aggregates and suspension per cent Principal components analysis revealed that parent rock material "was tie most important factor in explaining the variation of water stable aggreg tes of soils in Hawaii". Copeland (1963) has commented upon the very great, elevance of infiltration 297

to soil erosion. Data from the Boise River basin was used to show that

there are considerable variations amongst mean infiltration rates on

granitic, basaltic and sedimentary soils, and examples were citied

where erosion had removed the permeable horizons and so reduced

infiltration by 70% on average. The unpublished work of Meeuwig

was used by Copeland (1963)to show the exponential relationship between

soil bulk density and erosion and the negative exponential relationship

between ground cover and erosion of soils.

On a more general level several Americans have investigated sediment

yields from basins on a variety of lithologies. Rains et al. (1952)

examined 35 small reservoirs in the Little Colorado Basin and found

that those on shale and soft sandstone collected between 2 and 8 times N more sediment than those on well indurated sandstone and coiglomerate.

Differences of 20 times were noted between individual basins on shales

and conglomerate. Allen and Welch (1967) found, that in experimental

river basins in Oklahoma, sediment yields varied between "29.7 inches

(and) for per 1000 years for a mixed geology watershed ... 1.32 a

sandstone watershed. " Similar findings to these have been reported by

Hadley and Schumm (1961) for 99 small stock reservoirs covering five major rock types in South Dakota.

The rates of erosion and aggradation in basins and the spatial variation of these rates clearly determine the overall morphology of catchments in the long term. However, the relationship is not a simple unidirectional one of the form "erosionamorphology" for there are a whole series of complex subsystems and feedback loops as Melton (1958) has demonstrated. He stated, on the basis of a correlation 15 analysis of geomorphic, surficial and climatic elements, "that there 11 298

is some tendency for geomorphic elements to control the processes of

erosion that shape them, but the control does not extend to those

external elements of climate, geology etc. ". "A change in some

governing factor results in changes in the topography that neither

return the governing factor to its original condition nor affect the

processes of erosion in the particular way necessary to restore the

original form of the land ".

4 At a much more practical level, where parameters are needed to

facilitate the estimation of sediment yields, Schumm (1955) has

investigated the relationship between the relief ratio, i. e. total

relief of the basin divided by the maximum basin length, and sediment

loss for 35 small stock reservoirs in the south-west of the U. S. A.

He found that "annual sediment 1os$ is a positive exponential function

of the relief ratio. " In the San Gabriel Mountains of California,

Lustig (1965) found relief ratio correlated poorly with sediment yield

because of variation in basin hape. He found high positive correlations

between sediment yield and six complex geomorphic factors, which comprised

transforms of two or more variables. The variables he considered were

basin area, mean ground slope angle, total stream length, bifurcation

ratio, total number of streams of each order, and the mean stream channel

slope ratio for streams of each order.

The importance of veg? tation in controlling erosion has already been touched upon in the context of the climatic control of vegetation. However, in most parts of many countries man is of great significance in

the ecosystem, determining as he does the land use'of agriculture areas,

the management of forests and the use of tools'such as fire. It is important Tn to appreciate the place of non-urban man the erosion system for when urbanisation takes place it is usually not on virgin land. 299

In a related study to the hydrological work of Jones (1966), Reed (1971)

found that the construction of diversion terraces, farm ponds, partial

reforestation and other conservation measures produced a 47% decrease

in sediment discharge from Corey Creek, Penn. Comparable results

have been obtained by Baird (1964) for 20 Texan basins, whilst Holeman

(1965) states that careful crop management and conservation measures

have reduced reservoir sedimentation by up to 70% in Baltimore County.

It would appear that such conservation practices merely serve to bring

the yields of cultivated areas down to their pre-disturbance level for

Striffler (1964) found in Michigan that wild land, comprising 26% of

the basin, contributed only 5% of sediment whilst the proportions for

the wooded and cultivated land were 44% contributing 23% and 10%

contributing 15% respectively. He attributed the relatively high rates of erosion from agricultural land to annual disturbance by ploughing and grazing. Copeland's (1963) review of the effect of land use changes on sediment yields showed with reference to logging, that some soil disturbance is desirable silviculturally but that the 15% of the area which is disturbed to considerable depth by tractor operations inhibits natural regeneration and increases sedimentation. Ile reported cases of streams with 490 p. p. m. of sediment during logging, 38 p. p. m. one year after and 1 p. p. m. a further year later; an unplanned logging operation produced a sediment concentration as high as 56,000 p. p. m., and the construction of a logging road produced "a streamf low sediment content 81 times greater than, the undisturbed watershed". The effect of fires, either natural or man managed, is rather different from logging in that the disturbance of the soil is slight but probably all the vegetation will be removed. This exposure of the bare soil to direct rain impact eliminates interception storage, permits overland flow of moisture and reduces the soils shearing resistance so that 0 300

gullying is more probable. Copeland reported the case of Boise, Idaho

in 1959, where during the summer 10,000 acres of rangeland above the

town was burned by wildfire, three subsequent intense storms cut

almost the whole of the land surface with gullies and covered pasture-

land and parts of the city with boulders and debris to a depth of

between 1 and 3 feet. Severe overgrazing has much the same effects

as fire and Copeland quoted comparable quantitative estimates of erosion

rates.

There remains only one aspect of the catchment sediment yield

system which demands discussion, namely the closely interelated system

of riverwater, channel network, erosion on slopes, scouring and

aggradation of the riverbed and the overall catchment yield of sediment.

The literature is large but Leopold and Wolnan (1957) have provided

such a succinct summary that it will be quoted in full together with additional references and more recent material. "Channel cross section and pattern are ultimately controlled by the discharge and load provided by the drainage basin". Leopold, Wolman and Miller (1964) presented simple two-variable equations which relate channel width, depth, velocity, and suspended sediment load to discharge. "It is important, therefore, to develop a picture of how the several variables involved in channel shape interact to result in ob erved channel characteristics. Such a rationale is summarized as fo] ows:

Channel width appears to be primarily a function of near bankfull discharge, in conjunction with the inherent resistance of bed and bank to scour. Excessive width increases the shear on the bed at the expense of that on the bank and the reverse is true for very narrow widths. II 301

Because at high stages width adjustment can take place rapidly and with

the'evacuation or deposition of relatively small volumes of debris,

achievement of a relatively stable width at high flow is a primary

adjustment to which the further interadjustments between depth, velocity

shape and roughness tend to accommodate"., Wolman's (1955) work on

Brandywine Creek and Harvey's (1969) study of three S. E. England basins

are good illustrations of these relationships. The bankfull discharge

of the Brandywine Creek was shown to be equivalent to the flood with a

recurrence interval of 2 years. This important relationship between

the one or two year flood and bankfull discharge has been further

illuminated in the U. S. A. by Wolman and Miller (196CD and Leopold, Wolman

and Miller (1964), in Britain by Dury (1958) and Nixon (1959) and in I\ Australia by Dury (1968). ti

"Channel roughness, to the extent that it is determined by particle

size, is an independent factor related to the drainage basin rather than

to the channel" Hack and Goodlet (1960) and Hack (1957) have demonstrated

these relationships for parts of the Appalachians. "Roughness in streams

fine howeve function dunes carrying material, , is also a of the or other characteristics of bed configu ation. Where roughness is independently determined as well as discharge and load, these studies indicate that a particular slope is associated with the roughness. At the width determined by the discharge, velocity and depth. must be adjusted to satisfy quasiequilibrium in accord with the particular slope. But if roughness also is a variable, depending on the transitory configuration of the bed, then a number of. combinations of velocity, depth and slope will satisfy equilibrium". Schumm (1960) has gone further and suggested that at constant slope and discharge, channel form will vary markedly with the relative

i 302

resistance of the bed and bank material. As the threshold of erosion

of the bank material increases, whether by addition of coarse or cohesive

sediments or by the presence of vegetation or bedrock, with no change in

bed material or discharge, the channel will be narrower. Similarly,

if the bed of the stream is given an armouring of coarse pebbles, cobbles

or sands by, for instance, erosion of the catchment after burning or soil

disturbance during urbanisation, then the channel will tend to become

wider, even with constant discharge characteristics.

"An increase in load at constant discharge, width and calibre of

load tends to be associated with an increasing slope if the roughness

changes with the load. In the laboratory river an increase of load at

constant discharge, width and calibre resulted in a progressive

aggradation of long reaches of channel at constant slope. " Schumm (1961)

has shown that width-depth ratio increases with a decrease in the silt-

clay percentage in transport and so channel widening is a likely result

of coarser sediment entering e stream.

"The adjustments of 'several variables tending toward the establishment

of quasi-equilibrium in river channels lead to the different channel patterns observed in nature. For example, the data indicate that at a given discharge, meanders occur at smaller values of slope than do braids.

Further, at the same slope, braided channels are associated with higher bankfull discharges than are meanders. An additional example is provided by the division cf discharge around islands in braided rivers which produces numerous small channels. The changes in slope, roughness and channel shape which accompany this diversion are in accord with quasiequilibrium adjustments observed in the comparison of large and small rivers. " 303

It seems reasonable to conclude that the systems approach to the

study of the functioning of the drainage basin-sediment' system

illuminates the complexity and inter-dependence and all parts of the

system. We know something about all of the links in a few areas,

but we are as yet unable to explain fully the functioning of drainage

basins in all areas. The system may be quantified in some aspects but

our theory only really permits ordinal scale prediction of the form

'if A then B will be somewhat higher, but if C then B might be rather

lower',

The Impact of Urbanisation on Catchment Erosion and Sedimentation

The process of urbanisation incorporates two important stages, as far as hydrology is concerned; the clearance of vegetation fron a site is often suceeded by a protracted period of civil engineering works, building activity and general soil disturbance, the post- urbanisation stage is characterized by extensive paved areas, a dense surface water sewer network and improved drainage lines. Both of these have an impact on the functioning of the hydrological-sedimentological system within any basin as illustrated on Figure 6.2, the effects of the links indicated obviously have ramifications throughout the system.

The effect of vegetation clearance and engineering works upon sediment yield from a forested basin in Oregon has been investigated by

Fredricksen (1963). When comparing the sediment yO 1ds from the 250 acre basin containing the 1.65 miles of logging road construction with those from two neighbouring basins; he found that yields increased by-

250 times immediately after construction but that after 2 months they are only slightly above those for 'the pre-construction period. During 304

the next two years there was a further tendency towards "normalcy"

but mass movements often produced extremely high sediment concentrations.

Highway construction in a suburban area near Washington occupying

11% of a 4.5 square mile basin has been studied by Vice at al. (1969).

They found that 88 storms in the three years accounted for 37% of

the runoff, 99% of the sediment and occupied only 3% of the time.

The disturbed area associated with the road building contributed 85%

of the sediment and the particles from this area tended to be twice 4 the size of those from the rest of the basin. When allowance was

made for below average precipitation, they found the rate of erosion

was "10 times above that normally expected from grassland and 2,000

times that expected from forest land. " Bullard (1963) has also

commented upon the detrimental effects of the sediment derived from

road construction and maintainance upon the aquatic habitat, water

qualify and the aesthetics of streams. Felton and Lull (1963) in

considering the reduced rate of infiltration prevalent in the

Sunnybrook River Philadelphia after urbanisation, comment upon the

exacerbated rates of erosion from disturbed soils and from the floods

of increased magnitude.

The Anacostia River on the outskirts of Washington D. C. has two

gauging stations. The upper one at Colesville collects water from a

wholly rural basin whilst the lower one at Hyattsville also receives

a contribution of flow from an expanding urban area. Keller (1962)

found that "a sixfold increase in suspended sediment discharge from

(the) urban growth area compared to that from (the) rural area".

Walling and Gregory (1970) used both Keller's technique of one river

gauged at two points and the paired catchment approach in a study in

Devon, the only published in Britain. They found work of this nature . 305

in each. case that suspended sediment concentrations were between 2 and

10 tines greater in the urbanising area than in con arable rural areas and occasionally 100 fold. Similar wide variations in "unit area yields (of sediment) from construction sites" has been reported by

Davis ad Yorke (1971). They found that land slope on construction sites and the proximity of the sites to defined stream channels explained most of the variations that they observed between drainage basins. A further study by the same authors (Yorke and Davis, 1971) r of tLe 1.7 sq. mile Bel Pre Creek, Maryland found that th; development of garden apartments and town houses on 15% of the catchment increased runoff by 30% and sediment yield by 14 times. The sediment yield froth the construction sites was 90 times greater than the yield expected from the area with pasture and woodland.

Guy and Ferguson (1962) in an investigation of the rate of sedimentation in Lake Bancroft near Washington found that the mean rate of erosion for the rural period 1915-1938 was 3.68 acre feet per year.

The subsequent period up to 1957 would have been expected to have produced sediment at 1/3 this rate "because of a large-reduction in cultivated land and better land use practices. " In fact the yield was

10.41 acre feet per year consequent upon the urbanisation of 68% of the basin. They went on to consider the eleven most important factors affecting the "urbanisation-induced sediment processes". The first nine have been considered earlier in this chapter, whilst the last two, "intensity and dispersion of construction" and "construction methods and street layout", "seem complex and in need of intensive study".

They commented upon the greater time spans needed for larger schemes and on the greater rates of erosion from mass housing projects compared with "custom built" dwellings. Guy (1963) undertook a further study in the 3 Ub

outskirts of Washington at Kensington where a 58 acre area was

urbanised between 1959 and 1962. He employed a paired catchment

approach and multiple regression was used for calibration. 121,000

tons of sediment-nischarged per sq. mile during the building activity

because of (1) the rolling topography (2) a friable soil (3) construction

of a street in the main drainage channel (4) the lengthy exposure of

extensive areas of soil and (5) the infiltration capacity of the soil

was below the rainfall intensity most of the time. He compared the

results with those for Lake Bancroft, 25,000 tons per sq. mile (Guy and

Ferguson 1962), and found that the rate of erosion can be considerably

greater than may be expected for average urbanisation around Washington.

There have been many articles which review sedimentation in an

urban context (Guy 1970, Dawdy 1967, Swenson 1964, Meade 1969/64 N

Dumper 1966) but the synthesis undertaken by Wolman and Schick (1967)

is outstanding. They examined the published and unpublished work on

erosion in the Washington-Baltimore area and constructed tables to show

the magnitude of the erosion processes under different conditions.

They found that for wooded basins sediment yields were in the range

200-500 tons/ni/yr, whilst intensive farming could raise this to

1000 tons/Je/yr. Building activity was found to increase yields to between 3000 and 150,000 tons/m&/yr. In this latter case there was a marked negative relationship between yield and size of area being considered; because on large sites there is dilution from tributary rural areas and storage of sediment in transit. Original empirical investigations on road cuttings in Georgia indicated rates of erosion

high as as 50,000 to 150,000 tons/ta/yr which supports the findings for the urban areas. 3UY

Wolman and Schick went on to examine the effects of increased

sediment yields on the stream channel. They reported considerable

aggradation in the channel itself in the form of banks, point bars,

increased size of riffles and in some cases blanketing of the

streambed with sediment much coarser than was transported previously.

A map of the Oregon Branch shows eroding banks in only a few places.

Wolman (1967) also considered channel behaviour in urban areas. Ile reported considerable deposition of material whilst building was in 4 progress but admitted that channels might be clear of sediment in 5 to

7 years except in local cases where regulation, trash or channel curvature inhibits this.

The post-urbanisation period has received less attention than erosion on disturbed slopes. Guy and Ferguson (1962) suggested that there is a tendency for widening of channels after urbanisation is complete because of the armouring of stream beds by coarse sediment derived from construction work, the depletion of silt and clay grades in suspension after complete paving of the basin and increased water discharges. Their reasoning however is intuitive and not backed by any empirical evidence. Wolman (1967) reported low and-average sediment yields from three wholly urban areas but admits that these were only sampled at low flow periods. He also detailed the unpublished work of

Brodsky who found very low con entrations of sediment in two small summer storms. The material 3ampled by Brodsky was almost exclusively granular and lacking any clay component. In discussing channel behaviour he "both said the expected increase in runoff from urban areas and the absence of sediment should contribute to an increase in channel erosion and to an increase in channel width. " His personal subjective observation of streams and impressions of time sequences of photographs tend to support this view. He argued that "because of the great 3Uýi

difficult variability of natural channels, it is to make statistically " adequate comparison of channel shape before and after urbanisation. for However in a plot of drainage area against bankfull channel width

25 rural rivers and 11 urban streams there seemed to be some support for his hypothesis. More positive support for these ideas is tobe found in the work of Goodwin and Dippen (1968) on a tributary of the

San Francisquito Creek near Palo Alto, California. They found that

headwalls steps in the channel profile, characterised by potholes and are receding rapidly upstream following the paving of much of the headwater region as a result of urbanisation. The more frequent and

6 foot relatively higher flows, since urbanisation, have made one headwall erode 5 feet upstream during the autumn and early winter months of 1967-68; aerial photographs of the time before urbanisation showed that no such rapid changes occurred then. By far the most substantial work on river channel enlargement due to urbanisation is that by

Hamner (1972). His empirical study related the imputed increase in channel cross-ectional area to detailed land use data and other information for 78 small catchments near Philadelphia. He found that unsewered impervious ars and development younger than 4 years or older than 30 years had little effect on channels, but large channel enlargements are found for sewered streets and other major impervious areas of over 1 acre.

Two major criticisms cast doubt on the validity of his findings and method. First, his evidence of channel enlargement, the channel enlargement ratio-R, was found from:

R- C/24.8A0.657 where C is the cross-sectional area in square feet and A is basin area in square miles. The constants were derived from an analysis of the

28 rural basins, and for these R-1. More variables should have been included in this calibration of a rural channel size relationship and a tJ ?1

coefficient of determination greater than 0.75 would have been desirable.

Moreover, the concentration on cross-sectional areas alone, was undesirable because of the inter-related areas of channel variables.

Second, "many irregular, (obviously) small streams are quite ...... limited only at a number of ... points could the channel be in quasi-

flow. Because equilibrium with the ... of this ... no attempt was made to obtain measurements that would provide a complete characterization ... rather, measurements were made only at points of apparent quasiequilibrium.

Elaborate for identifying ... criteria quasiequilibrium points were developed"!

In conclusion, one can say that even though the number of studies is small, there is strong evidence to show that the construction of an urban area increases both erosion on slopes and the sediment yields of rivers. The area of soil exposed and the degree of disturbance are influential factors as are the relative juxtaposition of building lines and drainage lines. The increase in sediment movement usually causes considerable aggradation ofythe channel bed with sediment coarser than that usually found in the semi-natural channels. After urbanisation is complete sediment yields are probably below pre-construction levels and floods are both larger and more frequent. There is evidence to suggest that these changes produce enlargement of channels and complete removal of building-induced sediment. from the channel s beds.

Work on Erosion in Britain.

The N. E. R. C. Census of Hydrology (1970) lists many active projects which are monitoring sediment movement, but few of t ese list any published There be work. appear to only a few published reports, and consequently they deserve brief individual treatment. 01V

The draining of the Strines Reservoir in 1956, following 87 years

of operation enabled Young (1958) to observe and measure the deposits

derived from this Millstone Grit catchment. He found that the

sediment had accumulated largely in deltas at the points where the two

contributing streams entered the lake, and he assessed the volume of

one by preparing profiles of the surface and probing to find the depth

of bedrock. He estimated the volume of the second delta quite subject-

ively without any measurements at all. The total deposition in the

reservoir was 3 million cubic feet which indicates an overall rate of

erosion of 0.5 inches per century. He believed that since the catchment

consisted of a plateau with deeply incised "V" shaped valleys that such

an overall figure was inappropriate. He considered a figure of 4 inches

per century from the valleys above more fitting and even divided this

into 1.2 inches per century from the steep vegetation covered slopes

and 6 inches per century from the bare bluffs. Young compared his

results with two French studies, two Alpine investigations, work on the

Danube's tributaries and several American studies and concluded, "the

values for river basins show a wide range, as is to be expected in view

of the large number of variables involved; but the majority of them,

especially the smaller basins, lie between 0.001 and 0.005 in. (per

annum). "

A similar study at the Cropston Reservoir, Charnwood Forest,

Leicestershire was undertaken by Cummins and Potter (1967). The

reservoir was emptied in 1965 for the first time in 95 years of use. The thickness of the mud on the floor of the reservoir was measured and isopach an map constructed. From this they calculated the amount of deposition making very careful allowance for drying cracks in the mud its and moisture content. They found this latter quantity of very 311

great importance since when the mud settles it incorporates about ninety per cent of water by volume. They calculated a mean annual lowering of the surface of the 4,400 acre basin as 0.00048 inches (0.048 inches/ century). This is a much lower rate than that observed by Young.

The reasons are twofold. First, and probably most important, is the very careful conservative measurement made by Cummins and Potter in contrast to Young's estimates. Second, actual differences in rate between the two areas must contribute something to the differential observed.

An investigation of the pattern of sediment movement in the

River Tyne by Hall (1967) used both reservoir and stream load analyses.

The Catclough reservoir on a tributary of the River North Tyne collects water from 15.9 sq. miles of moorland. Hall found that between construction in 1905 and 1960 the floor of the lake had gained, on average, a layer of sediment 0.845 feet thick. This approximates a basin wide lowering of 0.45 inches per century. Hall went on to assess the rates of sediment transport in the River Tyne at Bywell and in the River Derwent where it discharges into the estuary. He measured suspended sediment at the Bywell Gauging station and estimated bed load from the records of the gravel extraction companies. Solution load was measured by "only the most meagre of sampling and analysis".

The respective sizes of these three loads was 130,20 and 80 thousand tons per annum, which gives a basin wide lowering of 0.266 inches per century. All of the figures for the Derwent were "estimated" and here the equivalent erosion rate was 0.46 inches per 100 years.

The rate of sedimentation in a Wealden Hammer Pond catching sediment from 5.25 sq. miles of mixed lithology has been investigated 312

by ltennhaw (1971). Using documentary cvidence, he vag able to trace the history, the back to 1760 and by sounding the pond, he of pond i determined the extent of sediment accumulation. lie found the not lowering of the basin to be 0.48 inches per hundred years.

I Flexing (1970) has attempted a sediment balance of the Clyde estuary. Ito undertook u largo sorica of aoasureaanta of liuapendcd and bed load sediments on the Clyde itself and the tujor tributaries to the patuarica. lie did not aka any sttc. p t to reduce hiý data to . crosicnter s, but if one applies Hall's assumption of 1OO1ba per cubic foot than tho rates of erosion for tho various rivers are shoim in table 6.1.

The variations in ecdis nt production fron thrca Cant York*lhiro catchucnts has been investigated by Iaeaon (1970). Yields of suspended sediment were found to be 1.16 tons/year/sq ka fron the

Drevton Beck, a7 sq km clay and chalk catch cnt fed entirely by spring flow, 8.91 tons/year/sq ka fron the Catchwatar Drain, a 15.4 sq krs basin draining fairly flat glacial deposits, and 480.3 tons/year/ sq kn from the bodge Bock which is an 18.9 sq kts long n,Arrov catchncnt high in the North Yorkshire floors with deeply incised valleys. The sediment in the Drcvton Back was derived largely from bank erosion whilst in the lodge Dock, the moorland contributed most of the sediment. In the two larger basins the infrequent, (3 tiaoo a yntr) floods rc ovod over 90% of the suspendedsolids.

AB part of it project to assets the erosional affect& of building construction near Exator, Walling and Gregory (1970) have datarnincd sediment rating curves and inter-catch=ant rulaticnshipd for A Igc il rural 313

Table 6.1

}ublitehed rateoo of eroelon for Brftteh river beuins.

Author Date Location Rate of Erosion Re marke inches 2ýyaar of erosion 3jL over whola catchment per century Young 1938 Strinee Rea. 0.3 127 Total solid load sinus losses over dar CumminsS Totter ( 1967 Cropaton Rea. 0.048 12.19 Total solid load lnua losses over dam Hall 1967 Catcleugh Rea. 0.45 114.3 Total solid load inua losses over dam Hall 1967 Tyne, Syvell 0.266 67.6 Total load Hall 1967 Derwent, Eddybridge 0.43 117.0 Estimated total load Henahav 1971 Hammer pond 0.48 117.0 Total solid load sinus losses over du Fleming 1970 Clyde, Daldovie 0.146 37.0 Solid load y Fleming 1970 Laven, Llnnb; mane 0.088 22.4 Solid load Fleming / 1970 Whitecart. Hawkhoad 0.310 78.7 Solid los Fleming 1970 Kelvin, Killermont 0.082 20.8 Solid los Imeaon 1970 Drevton lack 0.004 1.01 It 100 lb "1 it3 Imeaon 1970 Catchuater Drain 0.033 8.83 It 100 The "1 it3 Iawaoa 1970 Hodge back 17.9 4570.0 If 100 The -1 it3 Cregory i Walling 1970 Exeter area 0.73 230.0 Suspended load Ceika 1868 Nith, Dumfries 0.265 67.5 Total load. Quoted by Douglas (1970) Ceske 1868 Clyth. Caithness 0.10 25.4 Solid load. Quoted by Douglas (1970) Woodward 1887 Cotavolda 0.103 31.6 Total load. Quoted by Douglas (1970) 314

basin which was due for urbanisation. They calculated that the rate 3m2/yr of erosion from this small catchment for 1968-69 was 230 m/k which

is equivalent to a lowering of the catchmant by 0.75 inches per century.

They state that "this measurement is substantially higher than rates

calculated for small catchments of the East Devon plateau" but so little

is known of the magnitude of erosion processes in Britain that we cannot,

as yet, condemn any measurement on the basis of a lack of agreement with measurements for other areas.

Douglas (1970) has reviewed the evidence for the controls of

erosion by careful land use management and has discussed studies of

erosion in Britain, most of which have been considered here. tie does

include, however, several stud es of solution load which has not been

considered here and a number f papers from the 19th Century by Geikie who found total erosion in the River Nith, Dumfrieshire to be 67.5 m3/kn2/year (0.264 inches/century) and solid load transportation from

the River Clyth, Caithness to be 25.9 m3/km2/year. (0.098 inches/ century).

It is difficult, if not impossible, to draw useful conclusion from this review of publighed work on erosion in Britain. The cases iI cover a very wide range of catchment types from the 834 square miles of the Tyne to the 0.2 square miles of tha Exeter example and from the small Wealden.pond to major Pennine water supply reservoirs. Overall catchment erosion rates for Britain vary between 0.048 and 0.75 inches per century and the average figure, for the cases c nsidered, is about 0.325. The very great degree of even local variat on in the rate is illustrated 6.1 in Table by the cases of the Clyde nd Yorkshire studies. It seen that no realistic estimate of the seni-natral erosion rates 315

for the upper part of the Canon's Brook basin can be given. It is a

small, clay catchment of low relief in Eastern England and as such it

shares few, if any, parameters with any of the examples cited.

The Channel liorphology of Canon's Brook.

Introduction.

The preceeding discussion of the functioning of the natural erosion-

sedimentation system considered the close association between the

channel morphology and discharge rates. Most notable were the findings

of both Nixon (1959) and Leopold et al. (1964) that the bankfull

discharge of streams was of an almost constant frequency. An appreciation

of the close interdependence of the hydrological and morphological systems

leads to the conclusion that any change in the hydrology of a stream will

in result a complementary change in the form of its channel. The INI.

increases in water yield and low flows discussed in Chapter 4, the

increased frequency and magnitude of floods up to the 20 year flood

considered in Chapter 5 and the linking of Wolman's work on coarse

sediment deposition during building activity with Schumm's ideas on

the influence of relative stabilities of bed and bank material on channel morphology necessarily lead to the hypothesis that the urbanisation of the

Canon's Brook has changed the channel, probably by making it wider and larger and capable of discharging more water than previously. The following section examines this hypothesis and tests it using data derived from engineering drawings andgmore recent field survey.

Data Sources

There are a number of ways of tackling the problem of channel change through time, each with its own particular merits and drawbacks. Early in the it di"covered present study was that the Engineer's Department of the Harlow Development CorporI ion had prepared in 1956 a series of drawings 316 of the channel of the Canon's Brook downstream of the confluence of the Todd and Parndon Brooks. Fascimiles of some, o the drawings

in 6.4. The drawings dated 1 4.1956 are shown Figure are . and shoe the long profile of the river bed and banks atla scale of

1/500, a plan of the reach at the same scale and 32isections across the stream at a scale of 1/240. Figure 6.5 shows tjhat the section of river for which there is morphological data is ideally suited to a study of channel change since it is in the lower part of the catchment and has, therefore, been influenced by a large amount of development; moreover, it is very close to the gauging station so that the flood flows in the reach are known fairly precisely.

he wealth of data available from the beginning of th(e study period and the possibility of a resurvey made it unnecessary to follow other procedures such as the direct measurement of bank erosion processes or the subjective mapping of areas apparently

aggrading. eroding or The resurvey was undertaken on the 14th

September 1970 by undergraduate students of the Department of

Geography, U. C. L. The channel sections were surveyed with quickset levels and staves but it was only possible to examine 19 of the 32 sections since recent thicket and hawthorn growth prevented access to the remainder. The channel plan was mapped on the same day using a compass traverse method. The long profile of the reach was not resurveyed as a concrete lining had been given to the channel in the gauging station flume, and culverts inserted at Elizabeth

Way, Canon's Bridge and Fourth Avenue.

Evaluation of Data

A series of difficulties and reservations need to be considered before results can be presented. First, the instruments and-methods 317

/(Y, HARLOW DEVELOPMENT CORPORATION ''

iL: JS i LLI --- t _;_

9 L4n

TODD OROOK I CANONS 8AOOK LONGITUIINAL $ CA06S SECTIONS. "W.. . ý. r. L : - . .. ý. r-. _ý

" Figüre'6.4 Facsimile of a drawing from the Harlow Development Canon's Brook Corporation's survey of the channel of the in 1956.

S 318

ýi

tort

Harlow , Town Centre /

Netteswelll Pon

/ Watersheds ][ Gauging Station Channel Sections in this reach Built-up area O Miles 1 (from air photograph 1966) I

Figure 6.5 Location map. J 319 employed in the 1956 survey are not known to the present Engineers of the Harlow Development Corporation. There seem to have been relatively few points measured at each section and there are large numbers of right angles in the drawn up versions of the profiles.

The drawings may represent the true morphology of the channel at the time, but it seems more likely that they arc considerably simplified. Second, there may have been human interference with the channel in the form of engineering works during the inter- survey period. The culvert under Fourth Avenue and the channel on either side is certainly artificial, but in this case the 1956 plan depicts the realigned channel and shows the man-made shape of the channel. About half a dozen small (about 9 inch) surface water drains have outfalls on the righthand side of the Brook in the study reach. Each has been provided. with a small concrete surround but these do not appear to have influenced the channel significantly.

The large (about 36 inch) outfall on the left bank of the stream a little upstream of the gauging station is of greater importance.

This has been set into the bank about 10 feet and a concrete lined rectangular flume has been provided to discharge water from the pipe to the stream. This appears to have caused some bank slipping and erosion opposite the point of discharge. The engineers of both the

Marlow Development Corporation and the Leo Conservancy Catchuent

Board were approached concerning the possibility of enlargement of the channel since 1956. The former reported no work of this type whilst the latter reported some minor "channel cleaning and road

beginning cutting" at the of the 1960s. It seems, therefore, that human modification of the ch nnel since 1956 can be largely discounted found and any changes in the channel may be ascribed to 'natural causes'. 320

Third, and more importantly, the resurvey was hindered by

the fact that it was not easy to locate the exact positon of the

logged sections. In the original survey they were according to the the number of feet that they were upstream of the confluence of

Brook and the River Stort. It did not seem appropriate to attempt in fashion for there had. to relocate the sections i this probably if been some change in the channel plan and also it was not clear

the distance in feet was to be measured in detail along the thalweg

in of the channel or more generally by straight survey legs the middle of the channel. In the event, the sections were relocated

by three means. First, approximate measurement allong the channel

from fixed points such as bridges. Second, by reference to

morphological features, such as meanders, near to the initial sections

and thirdly from the evidence of features such as major old trees and

fences. There was error in relocating some of the` sections, but the

two surveys are best seen as having produced matched pairs of

observations of the channel morphology of the reach.

The fourth problem concerns the accuracy of the 1970 survey.

The students using the quickset levels were all familiar with the

instrument and were all undertaking courses in geomorphology so that e it ens likely that the instruments were used within tol, rable limits 'that and, the resulting sections truly represent the form of the

channel.

The final problem concerns the assessment of the channel morphology

when all the sections had been drawn up. The particular reach of

Canon's Brook under consideration is incised about 6 feet on average

into the sediments making up the floor of a wide gently sloping 01 321

valley. There are no levees and so the "banks" of the stream can be considered as the break in slope between the valley floor and the sides of-the channel. This sounds a straight forward decision in print but in the field the decision is, somewhat subjective.

Leopold, Wolman and Miller (1964) state "at time of low flow the determination bankfull is in itself of stage ... not a simple matter". Nixon (1959), in an examination of bankfull flows in

Britain, also states that "it is not easy to assess the value of the bankfull discharge without intimate knowledge of the river".

Two approaches were adopted towards the definition of meaningful parameters for the representation of channel morphology; the width, depth, cross sectional area, and perimeter of each section were assessed for both the subjectively detetmined bankful stage and also for the stage at which the width/depth ratio was a minimum. I The latter was chosen because it did not depend upon subjective decisions and it is a dimensionless index appropriate for the comparison of different sections; it was used by Wolman (1955) to determine the bankfull stage in Brandywine Creek.

Results

The pairs of sections for the 19 locations under consideration are shown in Figure 6.6 together with a super-imposition of the 1970 profile on that for 1956. The substantial preponderance of erosion over aggradation suggests that both channel deepng and widening have occurred between 1956 and 1970. Table 6.2 shows the numerical values of the parameters for each of the sections for each flow condition. It can be seen that for the bankfull state between 1956 1970, and the width of 15 sections increased, the depth of 11 increased, the 13 increased perimeter of and the cross sectional area of 13 increased. The impression sane of channel enlargement is given by 322

I II III Iv

ýý I

2 bb-Moom 2 3 ý_

ýý ý -4L... d99M

5 b

®8 J{ a

7 7

8 Irýý

a 10 11 Pi 12 9 ý.. ý 13

14 1?

13 15 _ý 14

1@ ! ice 1!

b I- ýý 17 ... 17

18 11 W ..'ate ýýýý 19 ® ý...... _ýý

Figure 6.6 The cross-sectional morphology of the channel of Canon's Brook in 1956 (I) and 1970 (II). Column III depicts the location of the sections and Column IV shows 1970 morphology (dotted line) superimposed on that for 1956 (solid line). M

L t O O f NNd AP .O pAAA Np "N A p 1. ý1 w0 0 n ~ o ö 0 P h 0 « A n w ^ O PN N . V ýO 0 0. OO w w ö ö p P . "0 - . r «1 R S o fG .. . i. i .e fý W% .4 i ti .4 ." .4 .4 .4 .r .4 C 1O O C d! M Mw N ! '! '0 h P A N 'O PN O A N- PS dN NO R N M % g v P A A OU w e4 O H . -ý 4 AN ýO A O"h N .O ww N YYII .4 w p% .O P Iý w O .ý w .I ýn Np 0 P P P O P .A ýO A N " N N .Oww r v. u N ý

I- 0 MO O O OO PO O O NON ON i ±N O O O ýn 0 0 00 0 0 0 0 0 0% 0 .4 W 'A w + .nO .n O O r O Q P d .1 A 10 pA .0 f N .0 ý N ö I. - NN A N .1 Of P N O r1 Pf NN N NNN NN Nn rº n N OA N 01 .0 w y M w .4 J .IN .O tl r 14 .4 AA .i hN A N N A H N N N C .4 NAA A A c c i .O OO N O ON OO N O NOO e4 O OO O O + O O h OO M O O W! O O O O A OOO j" - NN r1 O O P P dO .f O NCN N 1O dA N N . . . . AA wW N N N AA Nn N N N de i lN NN Pº N ý P A P P 0 0 A V 0 P. 0 N I4 .p N w .1 p. AAN .4w ti NA Wi A A N N N N A N .ý NAN N N l. a 09 t, O NP0 O N .. OO O 0- 0 NN .0 4% O O j S A 01 .t 'O .ON NV .O N '0 r1 h h 41 N N O ýn A O ON ýn O O O w 'n O N A Zý '1 w - 'e w% O O N A + .Oh .ý V V .O N A h h + Vf %N h .y Yi 1$ Q e o r. 0 0 '. N v-. r1 .« 41 O + 0- 0 0 .+ P um 0 0 N . . " .. A h Y1 NN Li h .T h .f .Ot h Cl OAd d rf n w A V N ä w O O .on v4 A + O Lli ýn .r w O O .rPA O O r\ Ä A L 0 .4M .I V r1 A I N h+ I. M A h O NN Np .en N .. 0 .0 tn c> O nO .+0 .n O » OO A N p . W% 'n N 00 0 0 O 'O 4O f1 M f .s Pr O q .f d i .e ýn O O A O .ýO .n O N A N mA n ýf Cl A .r- NN Cl N Cl Cl N A O i aD w .O Wi !O A 0 W-b tl V + N O y P f4 N 'f N N N A .4 .4 N N N NNA A ý'1 ý r. s 0 0. 00 O O AN c 'iz. y .e O ONO r. O N .n O O Z ýo 0 oý 10 4» p In .4NP N PN 0 N N Co N N NN N .r ß`1 1N NN A O 10 10 .ý .4.aN i O 9p p O "e O O O vn O w p wn O p P s0 b O .O ýO A .O P O b O P. P + NPf O A N N N .1 NA 'V N .r fV .y .4 N .4 .r NNN N N

NC H 4% N N O e0 O .O /- r !1 aD N n .O d 'O 0 O P1 'O N NN N N N p NN . -. r% r% O .4 n dN .D CD n d d1 OÄ 10 A M O N N 'p 'p O ýO w N A 'p N . 0O N N N w n b+ .O p PP O O OO OO - .ý.. .r .. .d NN N N p p n .5 NA .4 N r4 .4 AV .. r. .1A .4 .4 r1 ANA.. N .4 .. .y Y1 .O O A .I dt . D O PO N n 2 P P O OO O O O .4 - .4 .4 .r - .r er N NZ N .. 324 the medians (Table 6.3). There is no clear trend in the dispersion of the individual values as evidenced by the standard deviation and range. With width/depth ratio minimised; the dispersion of values for channel size appears to have increased in general, depth being the only parameter which shows decreased variability. This basic difference-between the two methods of determining the parameters is further emphasised upon examination of the skew in the frequency distributions, as represented by the relative sizes of the means and medians. It cah be seen that in the case of the subjectively determined parameters, the 1956 distributions are all negative, whilst for 1970 they are all positive. The width/depth ratio minimised parameters exhibit the same skew at both dates. There would seem to be only one conclusion that can be drawn from this analysis, namely, that there has been an increase in size of the channel from 1956 to 1970.

These ideas can be tested statistically. The mull hypothesis to be tested is that there is no significant difference between the mean values of each of the parameters measured in the samples for

1956 and 1970. The 0.01 probability level is used so that there is little chance of error in interpretation. A version of student's t test, suitable for dependent samples made up of matched pairs, was employed (Blalock 1960). A normal t test would not be valid because of the nature of the sampling. Table 6.4 shows that in every case the test statistics were less than the critical value and therefore in all cases the null hypothesis cannot be rejected. The conclusion must be that 'there was no statistically significant change in the morphology of the channel between 1956 and 1970. 325

for Channel Morphology Varinblcs for 1956_An4 Table 6.3 Descriptive Statistics _1970

STANDARD "BANKFULL MEAN MEDIAN DEVIATION MAXUNM IN IMÜM RANGE / DIMANSIONs 1 956 1970 1956 1970 193k 1970 1956 1970 19,6 1970 1956 1970

WIDTH (feet) 25.0 29.0 26.0 28.1 7.7 6.9 41.0 46.0 10.0 16.6 31.0 29.4

DEPTH(feet) 4.8 5.1 5.0 5.0 1.3 1.0 7.0 6.8 2.0 2.7 3.0 4.1

PERIMETER(feet) 28.5 32.1 29.5 31.5 8.4 7.0 45.5 50.0 12.0 18.0 33.3 32.0

CROSS-SECTIONAL 30.0 102.0 135.0 AREA (square feet 76.2 93.2 81.0 90.0 28.5 34.5 122.0 165.0 20.0

WIDTH/DEPTH STANDARD MEAN MINIML?t RANCH MINIMISED MEDIAN DEVIATION MAXIMUM DIMENSIONS 1956 1970 1956 1970 1956 1970 1956 1970 1956 1970 1956 1970

WIDTH (feet) 22.8 26.6 24.5 27.2 5.6 7.6 30.0 46.0 10.0 13.5 20.0 32.5

DEPTH(feet) 4.5 4.8 4.9 5.0 1.1 1.0 6.3 6.8 1.0 2.7 4.3 4.1

PERIMETER(feet) 70.1 85.4 70.0 74.0 28.2 36.7 113.0 163.0 20.0 22.0 93.0 143.0

CROSS-SECTIONAL 26.1 29.6 81.0 15.5 22.0 34.5 AREA (square feet) 90.0 6.4 7.7 34.0 50.0 12.0

I. 326

Table 6.4 Results of "t" test for matched pairs' of channel sections

BANKFULL DIMENSIONS

WIDTH -t-1.882

DEPTH -ts1.66

PERIMETER -t-1.67

CROSS-SECTIONAL AREA -'t-2.10

WIDTH/DEPTH RATIO MINIMISED

N WIDTH -t-2.09

DEPTH -t-1.28

PERIMETER -t-1.82

CROSS-SECTIONAL AREA -t-1.77

1 See Blalock (1960)

2 Critical Value of t for a two tailed test with p-0.05 and df - 18 is 2.101.1 arf

The results of the compass traverse were plotted at a scale

of 1/500 so as to be directly comparable with the Engineer's plan

of 1956. Superimposition of the 1970 plan upon the earlier one,

revealed no detectable change in the banks of the stream except in

the immediate environs of the surface water drain from the Pinnacles

Industrial Estate. Details of terraces,. point bar deposits and meander cliffs were not given on the original drawing which depicted

only the banks of the stream; but, in the area immediately around

the Fourth Avenue culvert the 1956 condition of the stream was known

for it was here that the stream was redirected along an excavated

channel. The plan (Figure 6.7) shows clearly the development of a substantial point bar deposit in the form of a terrace whilst sections A to D. in Figure 6.7, indicate the extent of bank cutting on the outside of the meander and associated aggradation on the point bar.

It is impossible to draw general conclusions from this particular reach of the stream because the changes noted were from a wholly man-made to a more natural state and not from a natural rural stream to an urban stream situation.

In conclusion it seems that the 220% increase in mean maximum monthly floods, the three fold increase in the frequency of summer floods of between 40 and 100 cusecs, and other increased flood

flows magnitudes up to with a 20 year return period do not appear

have to wrought any major changes in the form of the channel of

Canon's Brook. There are five possible reasons for this. First

is and most unlikely the possibility that the ideas expressed in the review were wrong and that channel form is not a product of discharge load but and sediment perhaps the product of irreversablo sequential 328

1970 1956 1970 1956 A o =O

-7777 t.

, ý» .f ß ý° ftH

Y4; 0 77 L-H

. rýv c ,ý

Figure 6.7 The excavated channel of Canon's Brook at Fourth Avenue in 1956' and 1970. 329'

change over "geological" periods of time. Second and again unlikely,

is the fact that there may have been significant changes-in the

channel form which were not detected by this analysis. Third, it

is conceivable that 14 years is insufficient time for change to take

place. This point is emphasised when one recalls that 4.2% of

the basin was urbanised in 1956 and that this increased only

gradually to 16.6% in 1968. Fourth, the other two dependent

channel variables slope and bed roughness may have been influenced

at the expense of channel shape. This'is possible but seems unlikely

in the light of the insignificant changes in channel shape. The

A -last and most likely explanation, i, the light of increased flood

flows, is that an increase in sediment concentrations, which goes

with building activity, has more than compensated for the observed

change in flow. It seems probable, therefore, that the completion

of the new town will see marked channel enlargement as flows increase

even further and sediment concentrations fall to a level below even

those occurring in the rural Canon's Brook.

Rates of Erosion in Canon's Brook: The Reservoir Study.

The foregoing has highlighted the importance of urbanisation in

the modification of the natural hydrological and sedinentological systems

in drainage basins and has pointed to the paucity of work on erosion

in Britain. This section continues the examination of erosion and

sedimentation in Canon's Brook with an analysis of the rate of

sedimentation in the Nettleswell Regulating Reservoir (Figure 6.5).

The morphology of the basin before completion of the dam was determined

from the Engineer's plans of the early 1950's and the shape of the floor of the lake on 15.9.70 was determined by soundings in the field.

The site of the reservoýr, before development, was a part of the the Todd Brook. valley of It appears to have had no major ý- -- --

" Pigure'6.8 Plate of St. Andrews church and the site of the Netteswell flood regulating reservoir. (fron an old photograph reproduced in: History of Harlow, edited by Bateman, L. H. Harlow Development Corporation, 166pp. ) 331

distinguishing features except that there was a vary small pond associated with the nearby St. Andrew's Church. Figure 6.8 gives a general view over the area long before development began. The

Engineer's Drawing, dated 11.8.52 shown in fasinil'e in Figure 6.9 forms the historical datum for this section. It shows the floor of the proposed lake by means of contours with an interval of

1 foot, the site of the inlet concrete culvert, the earth dam and concrete spillway. The earth for the dam was derived from the excavation of the temporary watercourse to the north of the basin and from the area immediately downstream of the dam itself. too major excavation of the floor of the lake was planned or executed but it'seems inevitable that there was some disturbance there.

Regretably there is no record of this, nor is there any record of the exact floor contours of the lake after completion of the structures. A map showing the lake levels at different stages as it filled would have been the ideal. The functions of the scheme were two fold; first, to improve the amenity of the area

through the provision of an open water area with wild life and an associated area of open space. Second, to attenuate the flood peaks in the Todd Brook by storage of peak runoff upstream of the restricted weir. An analysis of the effect of this basin upon flood flows at the gauging station was reported in Chapter S.

The regulating basin began operation on 7th Novenbor 1954.

It collects the runoff fron about 1.97 square miles, at the topmost

end of the Todd Brook Catchment. This area contains the neighbourhoods of Potter Street, Brays Grove, Hark Hall and Tye Green. It may be seen from Figure 3.3 that this area was one of the first to be built over. The rate of dwelling completions in the catchment 332

Image removed due to third party copyright

Figure 6.9 Facsimile of the engineering drawing for the reservoir at Netteswell. 333

area is given in Figure 6.10 which was drawn from data contained in Chapter 3. It is clear that although the construction of dwellings paralleled the building of the balancing pond, some 3,700 dwellings were completed after the reservoir became operational.

There has been a minimum of management of the basin during its

16 year life (up to 15.9.70). A small wooden raft was erected in the centre of the pond during the early 60s to provide a refuge for the wildlife. Reeds were encouraged in certain of the fore- shore areas, again to help wildlife and finally a wooden construction was added to the notched weir to raise the pond level and so enlarge the permanent water area. There has been no systematic dredging of the pond except for the removal of noxious debris, such as prams and cycles, from the area around the inlet culvert and weir. A small amount of sediment was removed from the inlet area of the pond at one stage and it was subsequently dumped at the eastern end of the lake. The heaps had by 15.9.70 been flattened and do not appear to have constituted major excavations.

The resurvey of the floor of the lake was undertaken by a group of staff and students from the Department of Geography, University

College London. The lake floor was sounded from an inflatable rubber boat by means of a weighted tape measure, the depth of water being read when the tape became slack. Soundings were made every ten feet along the thirteen transect lines shown on Figure 6.11. Each

line in transect was marked the field by two poles and a taut string line with markers at ten foot intervals. The position of the ends of the transect were surveyed and marked on a plan of the pond. The altitude of the water level in the lake was measured by reference 334

5000

J 4000 0

M ÖE3000

2000 0

rn 1000 C)

0 1950 -- .... v. vp v-9 vJ VO O/ tau

Figure 6.10 The rate of completion of dwellings in the catchment of the Netteswell reservoir. 335

koRl Culvert UO 'Nr, \

r tg3ýt ý f98

Sp4iway weir

Surface Wot« Sewer

Netteswell Regulating Reservoir October 1954 -IQ6 - Contours of reservoir floor in feet above O. O. Source : Harlow Development Corporation Drawing no2519

0 feet 200

Wet culvert

-ITS `/

" `, "":. /```'Jýý`(/" " ý- --r---196 .

"' J' ". ýs ý.. 93 -119 .'jý 5pillwayWeir """', " 19ý ', '. Surfacimi Water Sewer 1,93 ,: ". " "iqA Ir ý. ý' NetteswNi Regulating Reservoir "ý September 15th 1970 r9s '-196x- Contours of reservoir floor in feet above O.Q """"" Position of soundings

Inlet Culvert :.

SPIIIW. yWe1P.

Swtxe : ýYaler Sewer .ý ti ýý Deposition floor ` ýý' on of reservoir 00- 2-3 feet 1-2 feet 0-1 feet No change Erosion 0-1 feet

from Figure 6.11 Sediment accumulation in the Netteswell reservoir 1954 to 1970. 336

to the weir and inlet culvert both before and after the sounding

work. The level was found to be the same in all four cages. From

th "s data, it was a relatively simple matter to prepare (the map of

the top of the sediment surface. This map is presented in Figure

6.11 along with the original map of 1952 and a composite showing the

estimated rate of accretion in the reservoir. At the time of the

sounding work, it was not possible to take samples of the bottom

sediments but an opportunity did arise later.

The Engineers of the Harlow Development Corporation drained

the Netteswell Regulating Reservoir during the summer of 1971 so that

they might remove the accumulated sediment, install devices at the

upper end of the lake to restrict sediment accumulation to that area

and to improve the quality of the shoreline. At this time, they

also took samples of the sediments along a line linking the inlet

culvert to the weir via the deepest part of the lake. Their analyses

were unofficially reported to show no longitudinal sorting of the

sediment along the lake and a very great preponderance of silt grade

material in the bottom sediments. Samples provided for analysis

at University College were numbered, but it proved impossible to

secure a map showing the location of the sampling points. Analysis

of these samples would, therefore, have been valueless.

In the absence of detailed sedimentological analysis, the

simplest method of calculating the net rate of erosion from the basin was adopted; the volume of sediment being taken as the difference

in volume of the lake below 196 feet O. D. in 1954 and 1970. This volume, when expressed as inches of erosion over the whole catchment, yields a figure of 0.088 inches per century; the capacity of the

lake having fallen from 238,800 cubic feet below 196 feet O. D. to 337 only 173,940 cu.ft.

There are three problems that need to be discussad before conclusions can be drawn; they are (i) human 'interference with the sedimentation, (ii) the trap efficiency of the lake and (iii) the accurlcy of the resurvey. The activities of the barlos Development

Corporation with regard to the deposits have been considered above.

Their minimal dredging of the pond must undoubtedly have caused a slight under-estimation of the rate of erosion. The anplysis of esediments I th by the Harlow Development Corporation found then to be' largely silt grade. Since the catchment is in London clay overlain by boulder clay, the clastic load of the river should have a substantial clay content. The apparent absence of this fraction suggests that almost all of the fine grades of sediment have been carried over the weir and so have been lost to the resurvey. There is no way of determining the precise trap efficiency of the lake, without laborious contemporary measurements of river sediment loads, but with the limited data available it would appear to have boon substantially less than 100%. This fact will also have resulted in the derivation of an underestimate of the rate of erosion. The data collection method of the 1970 resurvey also probably resulted in an underestimate of the volume of sediment because, although only a 100gm weight was used, it seems inevitable that at least the upper fluid layers of mud would have allowed the sinker to settle into the lake bed. This supposition was confirmed weeks after the draining of the pond in 1971 when small stones thrown into the mud caused a splash rather than a solid impact. The problem of water entrained in the sediment, which so occupied Cu=ins and Potter (1967), certainly in existed the case of Netteswell for lorries taking away the excavated 338 material left a trail of muddy water for miles. In ignoring the water content of the sediment, an overestimate of sediment volumes may have been made. The instrumental accuracy of the resurvey does not seem to be a major source of errors; the density of readings was great, all the figures were consistent, the crew of the boat identical was changed frequently and the two transverse traverses gave results to the earlier direct transects.

The net rate of'erosion calculated for the upper part of the

Todd Brook catchment for the period 1954-1970 is rather low at 0.088 inches per century. Only the very conservative work of Cummins and

Potter (1967) on Cropston Res., one of Fleming's (1970) calculations and two of the catchments investigated by Imeson (1970) have shown lower rates of erosion. It as been said that on several counts the Netteswell data may be an underestimate, but the neglect of water contained in the sediments probably offsets most of this error.

It is probably very dangerous to attempt a general comparison of the for Netteswell study with the studies su=ariscd in Table 6.1, as the review of literature showed, sediment yields can vary vary greatly from locality to locality and from climatic regime to regime.

It is therefore difficult, to say that building activity in the

Canon's Brook has increased sediment yield by 'so many times', but when compared to the only catchment in Table 6.1 which bears any hydrological semblance to the Canon's Brook, i. e. the Catchwater

Drain (Imeson 1970), one finds that the rate of erosion is 2.7 tim os higher. I

Conclusion

It is clear that building activity and construction work can substantially increase sediment concentrations in streams and that 339 aggradation of river beds is likely during the early phases of urbanisation. Once urbanisation is complete and the paved surfaces are draining through a outface water sewer network, it is likely that sediment concentrations will fall to below even the levels existing in wholly natural catchments of the same type. Moreover, the increase in flgöd peaks and increased frequency of, at least, bankfull and slightly over bankfull flows is likely to scour the building induced sediment from the streambed as well as enlarging the channel to accommodate these increased discharges. These effects rf urban- Ition is on the catchment sediment system are deleterious to the quality of the environment in that excessive sediment clouds rivers and blankets aquatic habitats. The post-urbanisation scouring and bank caving is likely to produce unsightly streambanks in urban areas and excessive sedimentation in flood balancing ponds, canals and water supply reservoirs. The investigation of the channel morphology of the Canon's Brook produced some evidence of channel enlargement, but the findings were not statistically significant.

The rate of erosion from an upper part of the catchment during a period of construction activity appears to have been around 0.088 inches per century, and this may be up to 2.7 times the natural rate for the area. .

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Dury, C. H. 1958 Teets of a genoral theory of misfit streams. Trann. Innt. Irit. Cnog. p, 105 1? _25, Dury, G. K. 1968 hankfull discharge and the magnitude frequency series. Australian Journal of Science, 30(9), p. 371.

Felton, P. M. and 1963 Suburban hydrology can improve watershed Lull, ü. W. conditions. Public Works, 94(1), pp. 93-4.

Fleming, G. 1970 . Sediment Balance of the Clyde Estuary. Proc. An. Soc. Civil Eng. hydraulics Division ILYll., Vol. 96, pp. 2219-2230.

Fredricksen, R. L. 1963 Sedimentation after logging road construction in a small western Oregan Watershed, " U. S. Department of Agriculture Miscell. Pub., 7, pp. 56-59.

Glymph, L. 1954 Studies of sediment yields fron watersheds. I. A. S. ii. Assembly, Roie, Vol. 1, pp. 178-192.

Goodwin, C. R. and 1968 Changes in channal morphology. U. S. Geological Crippen, J. R. Survey Prof. Paper, 600-A, p. 121.

Guy, H. P. 1963 Residential construction and sedimentation at Kensington, Md., U. S. Dept. of Agriculture Miscell. Publication, 970 pp. 30-37.

Guy, H. P. 1970 Sediment Problems in Urban Areas. U. S. Ceolo, ical Survey Circular 601-E, 8pp.

Guy, H. P. and 1962 Sediment in shall reservoirs due to Ferguson, G. E. urbanisation. Proc. A. S. C. E. }iy. Division HY2, Vol. 88, pp. 27-37.

Hack, J. T. 1957 Studies of longitudinal stream profiles in Virginia and Maryland, U. S. Geological Survey Prof. Pap r, 294-n. 97pp.

Hack, J. T. and 1960 Geomorphology and forest ecology of a Goodett, J. C. mountain region in the Central Appalachians. U. S. Geological Survey Prof. Paper 347.

Hadley, R. F. and 1961 Sediment sources and drainage basin Schumm, S. A. characteristics in Upper Cheyenne River basin. U. S. Geological Survey Water Supply Paper 1531-B, pp. 137-197.

Mains, C. F., 1952 Sedimentation rates in small reservoirs in Van Sickle, D. M. and the Little Colorado River Basin. U., Peterson, H. V. Water Supply Paper - I1110-D. Ceological Survcy , 26pp.

Hammer, T. R. 1972 Stream channel enlargement duo to urbanisation. Water Ree. Ree., 8(6), pp. 1530-1540.

Harvey, A. M. 1966 Channel capacity and adjustment of stroams to hydrologic regime. Journal of 1lydrolog}!, 8, pp. 82-98. 342

Hlenshaw, R. S. 1971 Sedimentation in a Wunlden Hammer pond. Bloomsbury Geographer 4, pp. 47-52.

Holeman, J. N. 1965 Sedimentation of Loch Raven and Prettyboy reservoirs, Baltimore County, Maryland. USDA SCS. SCS-TP-145 l8pp. (Abstracted in Ceo. Abstracts Hydrology 19688/1528).

Imeson, A. C. 1970 Variations in sediment production from three East Yorkshire catchments. In: The role of water In agriculture: University -_ of Wales Aberystwyth- MemorandumNo. 12 Edited by Taylor, J. A., Pargamon. _(1969)

Jones, B. L. 1966 Effects of agricultural conservation Crack " practices on the hydrology of Corvay Basin, Penn., 1954-1960. U. S. Geological Survey Water Supply Paper, 1532-C S5pp.

Keller, F. J. 1962 Effect of urban growth on sediment discharge, north west branch Anacostia in River Basin, Md., Geological Survey Prof. Paper, 450-C1 pp. 129-131.

Langbein, W. B. and 1958 Yield of sediment in relation to moan Schumm, S. A. annual precipitation. Trnns. Am. Ceophys. Union, 39, No. 6, pp. 1076-1084.

Leopold, L. B. 1956 Land use and sediment yield. In: Man's Role in Changing the Face of the Earth, Edited by Thomas, W. L., Chicago. pp. 639-647.

Leopold, L. B. and 1957 River channel patterns: braided, meandering Wolnan, M. G. and straight. U. S. Geological Survey Prof. Pap 282-B, 85pp. , Leopold, L. B., 1964 Fluvial processes in geomorphology. Wolman, M. G. and Freeman. 522pp. Miller, J. P.

Lustig, L. K. 1965 Sediment yield of the Castaic Watershed, W. Los Angeles County, California, -A Quantitative, Geomorphic Approach. U_S. Geological Survey Prof. Paper. 422P, 23pp.

Meade, R. H. 1969 Errors in using modern stream-load data to estimate natural rates of denudation. Ceol. Soc. Am.Bu11., 80, pp. 1265-74.

Melton, M. A. 1958 Correlation structure and morphometric properties of drainage systems and their controlling agents. Journal of Ceology, 66, pp. 442-460.

Musgrave, C. W. 1947 The quantitative evaluation of factors in water erosion -a first approximation. Journal of Soil and Water Conservation, 2(3), pp. 133-138. E 343

N. R. R. C. 1970 Hydrological Research in the United Kingdom (1965-1970). N. E. R. C. 140pp. . Nixon, M. 1959 A study of the bankfull discharges of rivers in England and Wales. Proc. Inst. Civ. Fn,., 12, p. 157.

Piest, R. F. 1963 The role of the large storm as a sediment contributor. U. S. Dept. of Agriculture Miscoll. Pub. 970, pp. 98-108.

Reed, L. A. 1971 Hydrology and Sedimentation of Corey Creek and Elk Run Basins, North Central Penn. U. S. Geological Survey Water Supply Papery1532-E. 27ppe _

Schumm, S. A. " 1955 The relation of drainage basin relief to sediment loss. Publication 36 I. A. S. 11. Rome Assembly Voll, pp. 216-219.

Schumm, S. A. 1960 The shape of alluvial channels in relation to sediment typo. U. S. Ceological Survey Prof. Paper 252-B.

Schumm, S. A. 1961 Effect of sediment characteristics on erosion and deposition in ephemeral stream channels. U. S. Geological Survey Prof. Paper 352-C.

Shue, Tück Wong 1963 A multivariate statistical model for predicting mean annual flood in New England. A. A. A. G., 53, pp. 298-311.

Striffler, W. D. 1964 Sediment, atrcaciflow and land use relationships in northern Lower Michigan. U. S. Forest Service Research Paper LS-16, l2pp. (Abstracted in Goo. Abstracts Hydrology 1967A/1347).

Swenson, 1I. A. 1964 Sediment in streams. Journ. Soil and Water Conservation, 19(6), pp. 223-226.

Vice, R. B., 1969 Sediment movement in an area of suburban Guy, H. P. and highway construction, Scott Run Basin, Ferguson, C. E. Fairfax County, Virginia, 1961-64. U. S. Geological Survey Water Supply Paper 1591-E, 41pp.

Walling, D. E. and 1970 The measurement of the effects of building Gregory, K. J. construction on drainage basin dynamics. Journ. of llydrolo£y, 11, pp. 129-144.

Wolman, M. G. 1955 The natural channel of Brandywine Creek, Pennsylvania. U. S. Geological Survey Prof-Paper 271,56pp.

Wolman, M. G. 1967 A cycle of sedimentation and erosion in urban river channels. Geografiska Annalen, 49A, pp. 385-395. 344

Wolman, M. G. and 1960 Magnitude and frequency of forces in Miller, J. P. geomorphic processes. Journ. of Geol. 68(1), pp. 54-74.

Wolman, M.G. and 1967 Effects of construction of fluvial Schick, A. P. sediment. urban and suburban areas of Maryland. Water Res. Res., 3(3), pp. 451-464.

Yamamoto, T. and 1967 Erodibility indices for wildland soils Anderson, H. W. of Oahu, Hawaii as related to soil forming factors. Water Rea. Res., 3(3), p. 785.

Yorke, T. H. and 1971 Effects of urbanisation on sediment Davis, W. J. transport in Bel Pro Creek Basin, Maryland. U. S. Geological_ Survey Prof. Papar___750-B, pp"218-223.

0 345

CHAPTER 7

CONCLUSIONS AND IMPLICATIONS

There is one branch of research which is importance but of the utmost ..., which, from the difficulty of direct observation upon it, has been less successfully studied than almost any other... I refer to the proportions between precipitation, superficial drainage,. absorption and evaporation ... when, therefore, we attempt to use the phenomena few observed on a square ... yards of earth, as a basis of reasoning upon the (hydrology) of a province, it is evident that our data must be insufficient to warrant positive general conclusions. George Perkins Marsh

The initial aim of this wark was the quantification of the

hydrological effects of urban cation, but because of the vast scope of s this apparently rather narrow specialist field, which ranges from flood

plain management through water supply, sewage disposal and water quality

studies to sewer design and river flooding investigations, a fluch more

specific project emerged; namely, an investigation of the effects of the

construction of Harlow New Town on the hydrology and regime of the Canon's

Brook for the period 1950 to 1968. P

Research on urban hydrology has blossomed after initial work in the early 1960s (e. g. Carter 1961), and repeated calla for further studies by,

for instance, Rodda (1971) who implied a need for more work in his report "there that were only two studies being undertaken in lowland England on

the consequences of urbanisation - one of the most radical of all land use changes". American research has been abundant enough in the fields of flooding urban runoff and to merit substantial reviews by Moore and Morgan (1969) Leopold (1968) and with a major bibliographical contribution by the American Society Civil Engineers of Task Force on Urban Hydrology (1969)" 346

British work on this topic is proceeding in several parts of the country

now, but published results are few, since most projects have only published

outlines of their approach and hydrometric networks (e. g. Waller & Shaw,

1970). This study of the Canon's Brook appears to be one of the earliest

reports of completed work on the impact of urbanisation on thu physical

hydrology of a small catchment in the British Isles. Soil erosion induced

by building activity and consequent river channel changes and sedimentation

problems have been wifely studied in North America and reviewed by Wolman

and Schick (1967) and Guy (1970), whilst Walling and Gregory (1970) have made

the only British contribution to this field in respect of their work on a

small suburban area of Exeter.

Other aspects of urban hydrology, which have had to be excluded from

this study, include damage stIdies on flood plains (e. g. White, 1964;

Harding and Porter, 1969), water quality investigations associated with

urban surface water drainage, sewage effluent and trade wastes (e. g. Douglas,

1972, Crava 1969) and surface water sewer design. This latter area is

closely associated with physical hydrological investigations, but the aims,

methods and approach of civil engineers concerned with design tend to differ

from those of the river hydrologist. This divergence is well exemplified

in the 1973 CIRIA Colloquium of Rainfall, Runoff and Surface Water Drainage

of Urban Catchments, (CIRIA, 1973).

Canon's Brook was selected for study from a countrywide review of

possible sources of data; the instrumentation of an experimental catchment

having been rejected on grounds of time and resources. The eighteen water years of streamflow and autographic rainfall record fron October 1950 to

September 1968, covered three years of relatively undisturbed rural conditions

and fifteen years of continuous urbanisation, until by the and of the period ýcwors the 8.25 sq. mile catchment was 16.6% paved. The were constructed on a separate system, the original watersheds being retained, and outfalls 347 were into the semi-natural channel of the Brook. The London Clay bedrock coupled with the overlaying boulder clay and glacial gravel strata, made the catchment effectively watertight and only one relatively small balancing pond was constructed upstream of the gauga.

A full and wide ranging review of "fundamental problems of watershed experimentation" has been made by Ward (1971) and it is appropriate to use his framework for a concluding review of the problems which beset the

Harlow study. First, lack of control, described by Ward as "probably the fundamental weakness of the experimental watershed technique", was never a real problem since the record included a three year rural calibration period, the work would have benefitted from a longer pre-urban record, but

36 months has sufficed. Moreover, for some pieces of analysis it has proved possible to employ the record of the River Ash at Mardock Mill as a control.

Second, experimental catchment must be representative of larger basins if

"be for they are to used as a basis general statements about wider areas ... but in few is likely be cases ... it even that a small watershed will geographically representative of the larger watershed" (Ward 1971). Since this work was based upon pro-recorded secondary data, representativeness was never a real consideration; and certainly there was no possibility of employing the scientific catchment selection procedures used in Australia and New Zealand and reported by Rodda (1971). However, the form and geology of the Canon's Brook catchment is similar to many areas in lowland

Britain and urban development at Harlow has. followed the normal style of

British town building except in one or two neighbourhoods where housing densities are higher than normal. The third fundamental problem is that of accuracy of data and W7ard,(1971) asserts that "predictions based on water-

be data shed experiments can no more accurate than the original ... and may be considerably less accurate than the measurements of any single paramator". The design, lack network or of design, in the capo of the Canon's Brook was accepted at the outset of the study. The single st'oaß gauge took in much 348 of the development in the basin and was not unduly affected by any one sewer. The three autographic rain recorders dwindled to one in 1956 and this was subsequently moved on two occasions. This is clearly not an ideal situation, nor is it even a good one, but there ware no blatant errors in the record and such inconsistencies seem-to be the norm for hydrological reco7 of any length. The accuracy of the stream gauge as carefully evaluated in Chapter 3; some substantial errors were found for very low flow conditions but these were offset to some extent by the gauge's reliability and its capacity. to take all but one of the flood peaks. The final problem of watershed research, according to Ward (1971) is its cost. Ito quoted

Rodda who gives a figure of £200,000 for the capital costs and 00,000 per annum for the running costs of recent projects. The Canon's Brook work sidestepped this difficulty almost entirely since pre-recorded data was available for the cost of the collection and return of the charts. Certainly, the data base was not ideal since it lacked any information on soil moisture and groundwater levels, there was only one rain gauge and no data for sub- catchments or individual plots. However, it has proved possible to use the available information to fairly good effect.

The results obtained in the analytical chapters, 4,5 and 6, accord in general with the results of previous work which was reviewed in chapter

2. The investigation of the total water yield of the catchment showed that it had undoubtedly increased, by a factor of 2 in a cry year and almost

0.5 in a wet year. A similar dichotomy was apparent in the effect of urbanisation on summer and winter water yields. The former are usually increased substantially while the latter show a vary small increase or no change at all if the winter was particularly moist. These changes arc almost certainly the result of reduced evaporative losses from the paved surfaces and consequent increases in the volume of water available for runoff, as well as the increased rapidity of the translation of rainfall into riverflow in the urban situation. These results parallel exactly American the results of work by James (1965), Harris and Rnntz (1964). 349

Crippen & Waananen (1969). Seaburn (1969) and others, for all of these investigators found increased yields after urbanisation. 8ut., as was shown in chapter 2, as one moves to more arid environments or when one examines dry years at a given sites the proportional effect of urbanisation on water yield increases. It was shown that this increased water yield had occurred, dry at least in part, by means of an increase in the magnitude of the weather flows. The modal flow in the Brook during the rural years was 1 to 2 cusecs with the median flow being 2 to 3 cusocs; when 15 to 16% of the basin was paved, these flows increased to 4 cusecs and 5 cusecs respectively. This finding is at odds with the expected result of a decline in basoflows associated with reduced percolation to groundwater and diminished importance of soil moisture storage (Leopold 1968). However, in the only other study to examine the topic in detail (Crippen & Weananen 1969), low flows were found to increase markedly, with the inclusion of imported water inflating them even more. Miller (1966) has also reported increased low flows after urbanisation, but in all cases imported or irrigation water was the cause.

flows In the case of the Canon's Brook the reason for tho. increaso in low lay in the response of the catchment to virtually all rainfall events after a substantial area had been paved, compared to the rural situation whore runoff only occurred after fairly heavy rainfall.

The analysis of flood frequency and magnitude in the Canon's Brook furnished useable quantified results which complement published results.

The frequency of flood peaks was shown to increase markedly in the suer, e. g. 7 peaks of over 40 cusecs during the summers of 1951,52,53 and 59 during the summers of 1966,1967 and 1968, whilst there was little change, if any, during the winter-flood frequency. The peaks of the unit hydrograph increased 4.68 times, a figure wall within the range of published increases, e. g. Seaburn (1969) 2.5; Espey, Winslow & Morgan (196c, )3.0;

Crippen & Waananen (1969) 1.4; Wiitala (1961) 3.0; Anderson (1967) up to

8.0. Reductions in the width of the hydrograph and lag time for the 350

Canon's Brook are also comparable with published figurea. Because of reservations about the unit hydrograph method as applied to the study, a regression analysis was undertaken using 192 hydrographa. Winter flood hydrographs were found to be little affected by urbanisation whilst their summer counterparts had peaks increased by up to 11.5 times, on average, and equally dramatic changes in other parameters. But when attention was concentrated on floods of 100 cusecs and greater; it was found that floods equivalent to a five year rainstorm are unaffected by urban development, and it was suggested, therefore, that floods with a return period of

20 years or more arc-independent of urbanisation. This finding lands support to the ideas of American writers that very large floods are unaffected by catchment land use even if that is urban. It can also be set alongside the conclusion of Hatens (1968) that 50 year floods are unaffected by urbanisation, and of Curtis, Lee and Thomas who found that it was the

100 year flood which was independent of urbanisation.

The geomorphological investigation of the Netteswall Reservoir discovered that the sedimentation rate was lower than almost all published erosion sites for the U. K. and the channel cross-sections of the Canon's Brook showed no significant change between 1954 and 1970. The former may have resulted from the fact that the catchment is geographically very remote from other sites with published results, -but it is more likely to have been caused by the low trap efficiency of the lake. The relative stability of the channel could easily have been a measurement error or "over-reliance" on statistical methods, because the levelling figures and field evidence of bank caving pointed to an increase in channel width and depth.

These findings have several implications for planning, river management and water resource exploitation, although, as with no many research reports, the major finding is the need for more research. The increase in total is important water yield to water resources planning, but of even greater 161 increase during significance is the fact that most of the comes the sutunor months. It would seem that urbanisation increases surface water resources at the time of maximum demand and minimum supply and is instrumental in maintaining relatively high dry weather flows which must be beneficial for river life. This beneficial e)fect is tempered by the fact that although quantity has increased, quality,, which was not investigated here, has declined. The 1970 river pollution survey (Dept. of Environment 1971) gave the classification "poor quality requiring improvement as a matter of some urgency" to the Canon's Brook whilst other non-urbanised rivers in the area such as the Pincey Brook, R. Ash, R. Rib and the Ct. lLollingbury

Brook were all classified as unpolluted. The flood investigation, in showing that floods up to the 20 year flood have been exacerbated by the growth of the town, reveals! that, had the channel remained entirely natural downstream of the gauge, there would have been an increase in the number and severity of overbank flows. This follows from the fact that bankfull discharge approximates the 1 to 2.33 year flood (Nixon 1966, Leopold,

Wolman & Miller 1964). The modification of the channel and construction of a substantial flood storage area near the confluerco With the R. Stort, should have ameliorated this situation. The finding that a recurrence interval of 20 years marks the upper limit of the effect of urbanisation, is most significant for urban planning and flood plain zoning because, althought there is no national standard for urban fldod prevention works, most towns are protected against the 50 or 100 year flood. Consequently, it would appear that no additional urban flood defence works need follow further development upstream. The review of literature on aroaion and sedimentation in developing urban areas, if not the actual results of the

Harlow work, suggests that cognizance must be taken of high rates of erosion on slopes and sedimentation in streams during building activity and subsequent having bank and erosion as a result of increased flood flo)a after develop- bent is complete. At Nottcswoll Pond a relatively expensive excavation was deemed necessary in 1971, but bank caving does not appear to be as 3ti z

significant a problem w the deposition of old prams, cycles, ironmongery and garbage into the river.

This thesis has not provided all the answers to questions arising out of the impact of urbanisation on the physical hydrology of a catchment.

It has shown the way ahead and has provided some quantitative empirical results for one small clay catchment in South East England. There appears

be to a need for not only investigations of new sewer design methods but also methods of predicting the effects of urbanisation from easily measured catchment parameters and further investigation of the functioning of the hydrological cycle within small urbanised catchments. The recent marketing by N. A. S. A. of an instrument to measure water depths on impervious surfaces seems to be a promising development in this field. The most pressing problem, requiring a general solution, is the evaluation of the effects of urbanisation on flood hydrographs and the determination, for a wide range of catchments, of the extent of increase to be expected in small floods and the magnitude of floods which are independent of the land use of the catchment. 3; 3 Bibliographyaphy

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Espey, W. 1I., Winslow,; ºD. E. 1969 Urban effects on the unit bydrograph. In: and Morgan, C. W. Effects of Watershed Changes on Streamflow. Edited by Moore, W. L. and Morgan, C. N., Texas Univ. Press. pp. 215-228.

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