z K( I Report No. 113 Impact of improved land drainage on river flows M. Robinson December 1990 Institute of Hydrology Crowmarsh Gifford Wallingford Oxon OX10 8BB UK @ Copyright Institute of Hydrology 1990 ISBN 0 948540 24 9 MI Report No. 113 published by the Institute of Hydrology December 1990 British Library Cataloguing in Publication Data Robinson, M. (Mark) Impact of improved land drainage on river flows. 1. Great Britain. Land. Drainage 1. Title II. Institute of Hydrology III. Series 627.540941 ISBN 0-048540-24-9 Contents Page 1. DRAINAGE - A CAUSE OF FLOODING? 1 1.1 Introduction I 1.2 Claims regarding drainage 2 1.3 Theoretical arguments 4 1.3.1 Increased drainage density 5 1.3.2 Soil water storage 5 1.3.3 Size and duration of storm 5 1.3.4 Type of drainage 6 1.3.5 Extent and location of drainage 6 1.4 Earlier reviews .7 1.4.1 Ministry of Agriculture and Fisheries (1951) 7 1.4.2 Howe et aL (1967) 7 1A.3 Rycroft and Massey (1975) 8 1.4.4 Bailey and Bree (1981) 10 1.4.5 Irwin and Whiteley (1983) 10 1.4.6 NERC Peat and Fen Committee 11 1.5 Summary and conclusions 11 2. THE EXTENT OF FIELD DRAINAGE 13 2.1 introduction 13 2.2 Purpose of agricultural drainage 13 2.3 History of drainage in Britain 14 2.3.1 Nineteenth-century drainage 15 2.3.2 Twentieth-century drainage 16 2.4 Drainage system types 19 2.4.1 Drainage layout 19 2.4.2 Secondary drainage 20 2.4.3 Pipe systems 22 2.5 Geographical distribution of field drainage 23 2.5.1 Data sources 24 2.5.2 England and Wales 26 2.5.3 Scotland 34 2.6 Hill drainage 38 2.7 Summary and conclusions 42 3. DRAINAGE PLOT STUDIES 43 3.1 Introduction 43 3.2 Available field data 44 3.2.1 Selection of study sites 44 3.2.2 Ballinamore 54 3.2.3 Grendon Underwood 56 3.2.4 Tylwch 59 3.2.5 Withernwick 62 3.2.6 Staylittle 65 3.2.7 Blacklaw Moss 68 Page 3.3 Data processing and analysis techniques 69 3.3.1 Data processing 69 3.3.2 Data analysis 70 3.4 Summary and conclusions 77 4. FIELD EXPERIMENTAL RESULTS 78 4.1 Introduction 78 4.2 Changes in flows at the study sites 78 4.2.1 Ballinamore 78 4.2.2 Grendon Underwood 84 4.2.3 Tylwch 87 4.2.4 Withemwick 89 4.2.5 Staylittle 91 4.2.6 Blacklaw Moss 93 4.3 Comparisons of experimental plot findings 96 4.3.1 Storm runoff timing 96 4.3.2 Storm runoff volumes 98 4.3.3 Soil water conditions 98 4.3.4 Dry weather flows 98 4.4 Comparisons with other field drainage studies 99 4.5 Special case of peat soils 105 4.6 Summary and conclusions 106 5. MODELLING FLOWS 107 5.1 Introduction 107 5.2 DRAINMOD - model description 107 5.2.1 Infiltration 109 5.2.2 Subsurface drain flows 111 5.2.3 Soil water content and actual evaporation 114 5.2.4 Surface ponding and overland flow 115 5.2.5 Modifications made to DRAINMOD 116 5.2.6 Additional input parameters 117 5.3 Input data requirements 117 5.3.1 Weather data 118 5.3.2 Soil properties 118 5.4 Validation of DRAINMOD with field data 123 5.4.1 Site input data 124 5.4.2 Grendon 126 5.4.3 Withernwick 130 5.4.4 Comparison of simulated peak flows 135 5.4.5 Comparison of simulated low flows 137 5.5 Effect of site properties 139 5.5.1 Drainage system 139 5.5.2 Effect of climate 141 5.6 Previous modelling studies 142 5.7 Summary and conclusions 145 Page 6. ARTERIAL CHANNELS 146 6.1 Introduction 146 6.2 Channel works in Britain 147 6.3 Previous studies 149 6.4 Study catchments 151 6.4.1 River Witham (Lincolnshire) 151 6.4.2 Barlings Eau (Lincolnshire) 159 6.4.3 River Ock (Oxfordshire) 160 6.4.4 River Ewenny (Mid-Glamorgan) 160 6.4.5 Low flows 162 6.5 Interaction with field drainage outflows 163 6.6 Summary and conclusions 164 7. CATCHMENT STUDIES 166 7.1 Introduction 166 7.2 Previous catchment studies 166 7.3 Analyses of catchment data 168 7.3.1 Changes in Ray flood response 170 7.3.2 Changes in Catchwater flood response 173 7.4 Uptake of drainage within a catchment 177 7.4.1 Rate of uptake 178 7.4.2 Effect of location 179 7.5 Summary and conclusions 182 8. SUMMARY AND CONCLUSIONS 184 8.1 Introduction 184 8.2 The debate 185 8.3 Site studies 186 8.3.1 Additional site factors 187 8.4 Individual catchment studies 189 8.5 Regional studies 190 8.5.1 Catchment drained area 191 8.5.2 Regressions with catchment characteristics 192 8.5.3 Regressions with flow parameters 197 8.6 Conclusions of this study 200 8.6.1 Field scale effects 200 8.6.2 Catchment scale effects 201 8.7 The future 202 ACKNOWLEDGEMENTS AND DATA SOURCES 204 REFERENCES 205 Summary The impact of land drainage upon the incidence of flooding downstream has long been a source of controversy. The main reason for the uncertainty has been the lack of suitable data. Consequently, a central part of this work has involved assembling a nationwide set of data from both published and unpublished field drainage experiments where flows were measured from both drained and undrained land. It was found that, in contrast to previously expressed opinions, the drainage of heavy clay soils (prone to prolonged surface saturation in their undrained state) generally results in a lowering of large and medium flow peaks. This is because their natural response is 'flashy' with limited soil water storage available, whereas when drained, surface saturation is largely eliminated. On more permeable soils, less prone to sudace saturation, the more usual effect of drainage is to improve the speed of subsurface discharges, tending to increase peak flows. This finding is also at variance with earlier theoretical opinions, which assumed that due to their higher porosity, the storage buffer created by drainage of these soils would always act to attenuate maximum flows. The results of this investigation emphasise the importance of the pre-drained response and indicate that the likely effect of artificial drainage (to worsen or reduce flood risk) at the field scale may be assessed from measurable site characteristics. These parameters include the soil water regime (if known) and the physical properties of the soil profile. Computer simulation modelling of soil water and drainage system outflows using measured weather data, soil properties and drain parameters confirmed these findings of the field experiments. It further indicated the importance of rainfall regime: drainage reduces maximum discharges from higher rainfall areas. Baseflows were higher from drained than undrained land, principally as a result of the greater depth of the drains than the former unimproved channels. At the river catchment scale, arterial channel improvements lead to larger flow peaks downstfeam, due to higher channel velocities and a reduction in overbank flooding and storage. This increase is greater for larger channel capacities and for bigger floods. Studies of flow records from individual catchments indicate that the combined effect of field drainage and arterial works is to increase streamflow peaks (and dry weather flows) whether or not maximum flows are increased or decreased at the field scale. At the regional scale artificial drainage was a statistically significant parameter shortening catchment response times. 1. Drainage - a cause of flooding? 1.1 INTRODUCTION A major change in recent years in Britain and elsewhere is the installation of field drains in farmland (Green, 1980a; Framji a aL, 1982). This report investigates the impact of artificial drainage on runoff, both as outflow from individual field-scale sites and at the river catchment scale. As Nicholson (1953) observed, 'The connection between field drainage and flooding in rivers has been a subject of debate for centuries'. And more recently in a review paper, Trafford (1978) stated 'The interaction of field drainage with arterial flooding ... is an area where more research is needed'. This study encompasses a range of site characteristics and a consideration of the changes to both high and low flows. Britain offers a unique area to study the impact of drainage on river flows because: a) Tile drainage originated in Britain (Van Der Beken, 1987), which is probably the most extensively drained country in the world (Green, 1979a); b) It has unique documentary evidence of farm drainage, due to government grant-aid records (MAFF, 1986). The land phase of the hydrological cycle is of profound importance, determining a wide range of processes, including the water supply to plants, recharge to aquifers and the movement of water and solutes to rivers. Artificial drainage of soils is a deliberate and direct intervention in this system, and an understanding of its impact is of importance for the practical and possibly legal consequences downstream of a drainage scheme. As noted by Thomasson (1975): 'Any attempt to change the soil water regime of large areas of land will affect the general environment and to some extent the pattern of flow in streams and rivers'. The hydrological effect of agricultural drainage was identified as a topic requiring further research at the ICE Conference 'Flood Studies Report - Five years on' (Johnson, 1981).