THE PATTERN OF SEDIMENT MOVEMENT IN THE

David G. HALL B.Sc, PhD., A-M.I.C.E., A.M.I.W.E., Assistant Chief Engineer (Water Resources), Devon River Authority.

1. INTRODUCTION

The catchment of the River Tyne is located in the North-East of , ft drains the eastern slopes of the northern Pennine Chain, the range of hills commonly known as the backbone of England. The catchment area is shown in figure 1. It is situated within the influence of both oceanic and continental type climates and as a result the climate pattern is of a continual series of depressions and anticyclones. Orientation of the catchment is in a general easterly direction. After draining some 1,142 square miles (2,957 square kilometres), the river discharges into the at . The tidal estuary of the Tyne is some 19 miles (31 kilometres) long, 12 miles (19 kilo­ metres) being navigable to ocean-going cargo vessels, as a result of an extensive pro­ gramme of channel dredging. The purpose of the study described has been twofold: (i) To understand in a quantive manner the magnitude of catchment erosion and its consequent sediment transporc in the rivers and streams through to its deposition in the tidal estuary; (ii) To understand the manner in which the estuarine sediments are moved around under tidal action with the object of more fully understanding the problem of pol­ lution in the estuary of the Tyne. Survey work commenced in 1958 and extended into the summer of 1961. Throughout this time frequent samples of both fresh and tidal water were undertaken in order that the most comprehensive picture possible could be obtained of sediment movements throughout the river system. The sediment survey of tidal waters was undertaken in conjunction with a survey of the tidal hydraulics of the estuary. The work was financed by a committee of Local Authorities in the area.

II. THE RIVER TYNE CATCHMENT

II. 1. Topography The catchment area comprising parts of counties , Durham, Cum­ berland and , is roughly triangular-shaped. The River North Tyne drains in a South and Easterly direction from Hills (1,975 feet) (602 metres), whilst the South Tyne drains Northerly and Easterly from the highest point in the catchment— (2,930 feet) (893 metres). Two miles (3 kilometres) West of , the two rivers join and flow in an Easterly direction along the Tyne valley to the head of the estuary in the area of Ryton. Between Hexham and Ryton the river is gauged at By well whose catchment area is 834 square miles (2159 square kilometres). There is a second major tributary to the estuary; that of the River Derwent, which drains some 104 square miles (269 square kilometres) of varied landscape in . This river has only been gauged in its lower reaches since 1963 and hence no records relevant to that river were available during the survey. Gauging commenced at in 1956 and some six years of records were utilized in analysis.

117 Fig. I — The River Tyne catchment.

II. 2. Vegetation The upper courses of both principal tributaries are desolate and rocky and the streams tumble over their stoney beds. The tributary courses of the south Tyne tribu­ taries are more steep than those of the North Tyne. Many of the steep valley sides are scarred by the remains of mineral workings of the last ceraury. The catchment of the North Tyne is more open fell and moorland. About 30% of it is laid out in coniferous forest. The headwaters of the , a tributary of the North Tyne, feed into Catcleugh Reservoir, the only sizeable impoundment in the whole catchment.

118 The lower courses of the rivers have been rejuvenated. They flow through wide flat-bottomed valleys which are devoted to agricultural pursuits. The Tyneside conur­ bation lies along both sides of the estuary of the river. Housing nearly one million people, it is an area of intense industry varying from shipbuilding, and heavy engineer­ ing through a wide range of trades to the pursuits of a highly successful shipping port.

II. 3. Geology Broadly speaking, the geology of the catchment area of the Tyne can be described as a tilted mass of Devonian, Carboniferous and Glacial Tertiary deposits, dipping gradually to the South-East. Superimposed upon this structure is a shallow syncline which spans between the volcanic Cheviot dome and the uplifted Bewcastle dome. The movement of the area against the more rigid Alston Block centred around Cross Fell has given rise to faults which considerably dislocate the simple succession of the strata. The area is criss-crossed by a series of igneous dykes. The oldest strata are found in the North Tyne area and the extreme West of South Tyne and in general the strata become progressively younger in the direction of river flow. The principal rocks outcropping in the North Tyne are shales, carboniferous lime­ stones and sandstones. Some of the thicker carboniferous bands have been worked in times past. The millstone grits form the bulk of outcropping rocks in the South T^ne and Derwent areas whilst the Permian series of dolomites and magnesian limestones form a narrow belt along the coast. Glacial cover was widespread over the whole of and it is of extreme interest in this study since much of the material eroded by the river in its present state of development originates from the coccurrences during glacial periods. The principal glacial movements have resulted in drift cover up to 2,000 feet (610 metres) in the Cheviots and 1,250 feet (380 metres) in the lower eastern areas. Glacial flow has brought erratics from the Lake District and from South­ ern . Considerable gravel deposits lie in the valleys of the Rivers Allen and the South Tyne and Lower Tyne itself. The most recent deposits are those of peat. It has formed mainly on the high fells in thicknesses varying from eighteen inches (40 cm) up to six feet (180 cm). The activities of the forestry commission have done much to disturb the formations of peat over the wide areas of the North Tyne Fells.

II. 4. Precipitation

The Tyne catchment lies in the path of the prevailing south-westerly winds which blow from warmer latitudes gathering moisture as they cross the expanse of the Atlantic Ocean. The Pennine Chain of hills gives rise to a very pronounced orographic influence in the resulting rainfall pattern. Figure 2 illustrates this point. Only a small proportion of the total rainfall is of a convectional nature, but thunderstorms do occur mostly in the summer months. Many can be violent and result in considerable erosion from more unprotected land surfaces. The annual distribution of rain in the Tyne catchment is as follows: Jan. Feb. Mar. Apr. May June July Aug. Sep. Oct. Nov. Dec. 4.2 3.0 2.7 2.6 2.7 2.6 3.8 4.1 3.7 4.2 3.9 3.6 (inches) 107 76 69 66 69 66 97 104 94 107 99 91 (mm)

Average for catchment = 41.10 inches (1,045 millimetres) per year. Much of the rainfall, originates in the west, and such falls are readily reflected in rapid rises in river level. The area is also subject to prolonged periods of easterly winds which are sometimes rain bearing. Rain is much lighter, but often very prolonged. In

119 Fig. 2 — River Tyne —• annual isohyets. the winter-time, easterly winds frequently bring snow and periods of very bitter weather. Rain days average 170 in the East and 220 per year in the West of the area. Mean annual evaporation is about 17 indies (430 mm) per annum and snow cover is limited to an annual duration of about 20 days at lower altitudes to an average of 80 days on the hills.

11. 5. Runoff Up to the time of the survey, the Tyne catchment was very sparsely gauged. Since 1960, there has been considerable improvement in the amount of flow data which has

120 become available due to the efforts of the ihen Northumberland and Tyneside River Board. The records of the gauging station situated at Bywell were extensively used in the survey. Bywell, the principal gauging station on the river Tyne at a height of 53 feet (16 metres) above sea level, was established in 1956, to record river flows from the. catchment of 834 square miles (2159 square kilometres) at a point eight miles (13 km) above the tidal limit of the river. The gauge measures discharge from 73% of the total river catchment west of Tynemouth. The flow distribution pattern is shown by the flow duration diagram in figure 3 indicating a comparatively even distribution of flows between the lowest recorded flow of 121 cusecs (3.4 cumecs) (1959) (0.144 cusecs/ square mile) (0.0016 cumecs/sq. km), return period rive to seven years, and the highest recorded flow of some 40,000 cusecs (1,150 cumecs) with a return period of roughly ten years. Mean discharge at By well during the years 1956 to 1964 was 1,452 cusecs (41 cumecs) (23.4 inches) (592 mm) with a modal discharge of 210 cusecs (5,9 cumecs). The sepa­ rate duration curves for summer and winter flows are given in figure 4.

Fig. 3 — Flow duration curve — River Tyne, Bywell.

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Fig. 4 — River Tyne, By well. Summer and winter flow duration curves.

The second principal tributary entering the estuary does so about sis miles (10 km) below the tidal limit. It has been gauged at the Eddysbridge site having a catchment area of 46 square miles (119 square kilometres) representing 44% of ihe total Derwent catch­ ment. The estimated discharge from the whole of the Der went catchment of 104 square miles <269 square kilometres) is 150 cusecs (4.2 cumecs).

II. 6. Tidal Circulation Within the estuary, the pattern of circulation very largely influences the pattern of sediment movement. Allied to the sediment survey a very comprehensive survey of the hydraulics of the estuary was undertaken {}). The estuary, some 19 miles (31 km) between sea and tidal limit, varies in depth between 40 feet (12 m) at the seaward end to some eight feet (2'/4 m) at the landward end. The estuary is dredged to maintain a navigable depth to ocean-going vessels and ash barges to a point twelve miles (19 km) from the sea. Apart from the principal discharges of the'Rivers Tyne and Derwent,

(*) ALLEN, J. H., Hydraulic Studies in the estuary of (he Tyne, Thesis presented for degree of PhD, Kings College, 1962.

122 a flow of some 27 million gallons per day (1.4 cumecs) of crude sewage discharges directly into the estuary. The hydraulic circulation pattern of the estuary is shown diagramatically in figure 5. An understanding of this was gained from a comprehensive series of measurements of current velocity, direction, together with salinity and temperature measurements, taken at a series of cross sections throughout the estuary. The configuration of the estuary together with sampling cross sections is shown in figure 6.

O O O y O O

V" 10 12 14 16 18

Distance from piers (miles)

MODIFIED CIRCULATION PATTERN From ALLENtJ-H. (HYDRAULIC) Hydraulic Studies in the ' Estuory of the Piver Tyne. Fig. No. 5. Fig. 5 — Modified circulation pattern (hydraulic).

It was found that the basic state of the estuary is that of partial mixture between the salt and fresh wafer. In the even: of a fresh water flood, there is a definite stra­ tification of flow where water can change from a fully saline state to completely fresh staie over a depth of only five to ten feet (2 to 3 metres> (see fig. 11). During periods of spring tides, the estuary exhibits a tendency towards complete mixing.

III. EROSION

One of the basic processes of (he geological cycle is that of the decay and erosion of natural rock and soils. The process is carried on by the natural agencies of temper­ ature, wind and tain together with the acceleration or otherwise by the agencies of man and animals. The rate of production of eroded material varies from place to place but has been found to be a function of the following factors: (i) Size of catchment area; (ii) Climatic characteristics; (iii) Surface runoff; (iv) Soil characteristics; (v| Hydraulic and topographical nature of the catchment area; (vi) Vegetational cover; (vii) River channel type.

Several forms of erosion were found to be active in the River Tyne catchment, namely sheet erosion, gully erosion and channel erosion. The measurement rates of progress of each of these individual facets of erosion over a large area was something of a superhuman task, so that a programme of sampling of individual areas was chosen:

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124 (i) A very careful survey of river channel erosion throughout the principal river feeders and tributaries; (ii) A very careful survey was carried out of the deposition which had occurred in a reservoir situated in the headwaters of one of the rivers; (iii) Tndirectiy through a sampling programme to determine the quantity of material transported from the catchment by the rivet at Bywell ; (iv) An assessment of bed load based on the feed of gravel to the commercial workings of sand and gravel companies extracting material from the river bed.

III. I. Reservoir Study Catcleugh reservoir, situated in the headwaters of the River Rede, a tributary of the River North Tyne, was chosen for the purposes of this study. It has a catchment area of 15.4 square miles (40 sq. km) which represents 3.3% of the total North Tyne catch­ ment area. It lies within an area which receives approximately 50 inches (1270 mml of rainfall per year. The reservoir catchment vegetation is typical of a large portion of the moorland North Tyne area. It consists of coarse heath grass, heather and at the time of the survey, a proportion of newly planted coniferous forest. The reservoir itself was completed in 1905 and since then has been supplying water to Newcastle. Fortunately, detailed contoured plans of the valley prior to construction were available from the Consulting Engineers. The aim of the study was to make a comparison of the present reservoir floor contours with those of the original valley. It was ascertained that no flushing of the reservoir had taken place throughout its period of use. A careful detailed survey was carried out of the present reservoir depths using a calibrated echo sounder and wherever possible taking spot levels on the reservoir floor during periods of heavy drawdown. The period of the survey coincided with the very severe drought of 19S9 and this enabled survey of large areas of the reservoir to be undertaken by conventional land survey techniques. Additional floor "contours" were obtained during refilling using the water surface level as indicator. After detailed comparison of reservoir profiles before construction and after the 55 years of life, the quantity of accumulated deposit was calculated. Deposition had occurred mainly in the deltaic areas around the inlet streams and mud thicknesses in these areas were up to twenty (6 m) or twenty-five feet (8 m.) In the deeper parts of the reservoir, accumulations have been very small and even adjacent to the dam itself very little material was found to be deposited. The average deposition over the whole of the floor area of the reservoir was found to be 0.845 feet (25.8 cm). On the assumption that the whole of this resulted from catchment erosion, the equivalent erosion was cal­ culated as 0.0227 feet (0.069 cmi in the fifty-five year period or 0.45 inches (11.4 mm) per hundred years. This rate compared very favourably with other rates from similar catchments in the north of England of 0.5 inches (12.7 mm) per hundred years.

III. 2. River Channel Erosion The study of river channel erosion presented problems since the actual erosion itself is virtually impossible to measure. The study was approached from two different aspects: (i) The short-term erosion of river banks; (ii) The long-term decomposition of river bed materials.

111. 2. 1. River Bank Erosion A survey of all known active river bank erosion was carried out. Approximately ten erodingstretches of river bank were selected and each was carefully measured using a

125 series of marked locating pegs, fixed into the river bank at a suitable distance from the edge. After a known time had elapsed, a further careful survey of the marked river bank sections was undertaken and the volumes in the bank before and after the test period were compared. Differences between these two measured volumes were then computed. A careful check on the river discharge characteristics during the test period was undertaken and it was found that the distribution of flows during the test approxi­ mated very closely to the long-term distribution pattern. By this means, measured rates of bank erosion were found to be of the order of 60,000 tons (60,500 tonnes) per annum. Clearly, not all bank erosion was accounted for in this survey, and it is considered that the total annual production for the whole catchment from this source is of the order of double this amount.

III. 2. 2. River Bed Erosion

In studying the manner in which erosion takes toll of river bed material a very indirect approach was necessitated. It is known that river bed particle sizes change by virtue of attrition and solution from the upper river reaches down to the lower river reaches, and that there is an apparent correlation between particle size, particle round­ ness and distance from the river source. In order to gain knowledge of the manner in which the change took place in the Tyne catchment, a comprehensive river bed particle size analysis was undertaken. Eighty-three sampling sites were chosen on the main rivers North Tyne, South Tyne, Tyne and Derwent. Locations were generally chosen where shoals had accumulated and which appeared to be moved by at least some floods. Individual sampling areas were arbitarily chosen and the principal X, Y and Z measurements at right angles to one another were made for each stone, using the par­ ticle size scale; in essence, two scales set at 90° to one another. During measurement, a subjective assessment was made visually of the particle roundness against a standard chart control. Consistency checks of visual reliability were made at various stages of sampling and these were found to give a good measure of agreement. A varying number of samples was taken throughout the river. In the upper reaches, forty stones were taken as the basic sample, whilst in the lower reaches of the Tyne itself, two hundred stones were taken in each sample. For each particle, the principal dimensions X, Y and Z were multiplied together to give an effective particle volume. The particle volumes were all added together and averaged to give a mean effective particle volume for each sampling site. A plot of particle volume against distance from the river source was made on a log-log basis. Despite the scatter obtained, it was found by drawing the line of best fit through the sample points, that the mean particle lost 85% of its volume between the source and the mouth. Values for the individual main rivers are given below together with the distances from their sources. % particle volume loss North Tyne 46 miles (74 kilometres) 85 South Tyne 47 miles (76 kilometres) 81 Derwent 32 miles (51 kilometres) 87 Rede 30 miles (48 kilometres) 83 Estimates of the gravel extraction companies (referred to later in the paper) show that about 20,000 tons (20,200 tonnes) of material are transported downstream in the River Tyne each year. It would seem reasonable to assume that this represented the remainder of the load after attrition (15% of original) had taken its toll and that the original load would have been in the region of 125,000 tons (126,000 tonnes) per year. Some of the loss of volume results in the production of suspended sediment and some

126 as solution load. No comprehensive measurements of this load have been made, but it is thought that it may be of the order of about 60,000 tons (60,500 tonnes) per annum.

IV. SEDIMENT TRANSPORT—FRESH WATER

The assessment of fresh water sediment transport occupied much of the experimental time in this study, and it was only possible to make measurements in the River Tyne itself. By kind permission of the then Northumberland and Tyneside River Board extensive use was made of the gauging station at By well. The actual gauged section is located at a point about two thirds of the way down a half-mile (one kilometre) reach on a very slight bend in the river. Flows are well contained between high banks. The bed is of gravel, roughly three inches (7.5 cm) in diameter and the section is not entirely uniform being somewhat deeper near the left bank than over the remainder of the sec­ tion. Flow distributions throughout the section are uniform. Point samples of sediment show a distribution typical of any normal river section.

IV. f. Suspended Sediments Spot sampling equipment was developed and manufactured specifically for use at this site. An electrically operated horizontal instantaneous type of sampler was adopted. It consisted of a three inch(;S= 8 cm) diameter perspex tube (volume—620 cu. cms) with cam operated spring loaded doors on each end ; closure was effected by a solenoid remotely operated on shore. The instrument was mounted beneath the normal type of streamlined gauging weight and was suspended at the desired location, from a normal river gauging cable which spans the river (plate 1). Spot samples were taken and inte­ gration of the sediment distributions were undertaken by sampling appropriately. Having obtained a number of sample integrations over a range of discharges, it was

Plate I

127 decided to take single spot samples at the river centre at a position 0.6 of the depth below the surface to represent the mean concentration. It was found that this was the only method physically possible when working single handed. Sample analysis was carried out on site using portable vacuum filtration apparatus and membrane filters of the cellulose ester type, having pore sizes between 0.5 and 1.0 microns and having the property that immersion oil renders it transparent, permitting microscopic examination of the material retained. Such a property gave rise to the possibility of particle size analysis. Accurate weighting of the deposited material under controlled conditions was carried out and the sediment content of the sampled water so determined. A correlation of sample sediment content and river flow was undertaken in the plot shown in figure 7, From this plot it will be noticed that the points fall into two distinct bands of samples taken during the winter nionths (line BC) and samples during the summer months (line DE.) Most of the points fail within the 95 % confidence limits of the two lines. The degree of scatter is less in the case of the winter flows than it is in the case of the summer flow, the correlation coefficients being 0.92 and 0.82 respectively. Line AB describes the samples taken during low flows in both seasons. Considerable

OISCH«BO£ tajsecs»

FIG. NO. 7.

Fig. 7 — Suspended solids and discharge. Bywell 1959-1961.

128 scatter is shown in this case because of the small amount of material being carried in suspension and the fact that concentrations approached the limit of accuracy of the analysis method. During very low summer flows prolific growths of algae took place and this falsified any true "sediment" readings. The basic data used for sediment discharge calculation is that given in figure 7. In order to calculate the annual sediment discharge at the gauging station the following basis has been used; Annual sediment discharge for any one given discharge (tons) (tonnes) = Discharge (Cusecst x p.p.m. x 5.3S2 x No. of days in one year given discharge occurred 2240 "~ ~ ' ' ~ (1 p.p.m. solids at discharge of one cusecs = 5.382 pounds per day) Discharge (Cumecs) x p.p.m. x 86.4 x No. of days in one year given discharge occurred 1000 (1 p.p.m. solids at discharge of one cumec = 86.4 kilograms per day) The integration of all such quantities for the whole range of discharges gives the total sediment discharge per annum. The above expression was applied separately to summer and winter flows (fig. 4\ The calculation so performed resulted in a sediment discharge of 132,000 tons per annum (133,000 tonnes per annum) based on five years of river flow records. With an average of 90,000 tons (91,000) tonnes being discharged in the summer period (April to October) and 40,000 tons (40,300 tonnes) discharged in the winter period (November to March). The relative sediment discharges between summer and winter are 2.4 : i .0 respectively, whilst the river flow discharge ratio for the same periods are 1 : 5.0 respectively. The basis of this proportionally very high summer sediment discharge is undoubtedly a function of the radical change of vegetation from summer to winter, and secondly of the characteristics of rainfall intensity. Rainfall intensities are highest during thunder­ storms which are almost entirely confined to the summer months. High intensity falls cause a high percentage of runoff with consequent heavy erosion of the ground surface. During the winter months, however, most precipitation is of a frontal nature and is less intense. During the autumn and winier, vegetational protection is reduced and the soil is more open to attack by rainfall. Prolonged snow cover, however, does offer a certain amount of resistance to direct precipitational erosion, though at low levels the duration of snow cover is not sufficient to produce a significant effect on sediment production. Elementary correlation of sediment discharge with other catchment parameters was carried out and it was found that, beyond the basic factors of discharge and season, a number of other variables influenced the sediment content of any given flow. These are: Ground wetness represented as Antecedent Precipitation Index (A.P.I.) Rainfall intensity Discharge ratio of North to South Tyne and Significant previous discharge. With the exception of rainfall intensity for which insufficient information was available, all three variables showed some degree of influence on the resultant sediment content. The computed curves shown in figures 8 and 9 show the results of these addi­ tional analyses.

IV. 3. Bed Load Transport The measurement of bed load transport is far from simple. The extent of the study precluded a detailed programme of bed load sampling. Since considerable abstractions are made throughout the river, the gravel companies were approached to determine

129 1 h

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Fig. 8 — Sediment rating ralated to A.P.I.

130 Fig. 9 — Sediment rating related to previous discharge. the actual rates of grave! replenishment at the sites being worked. Estimates of total replenishment rates of 20,000 tons per annum (20,200 tonnes per annum) were obtained and it was clear that material transport only occurred at the higher river stages. With no factua! information available a bed load discharge of 5,000 tons per annum (5,100 tonnes per annum) is suggested for the River Derwent.

IV. 4. Total Solids and Dissolved Load

Only the most meagre of sampling and analysis has been conducted on the solution loads of the rivers. From the few samples taken, it was clear that the proportion of dissolved material in the water gradually rose as the river advanced further from the source. To obtain good results a very comprehensive sampling and analysis programme would have to have been undertaken and time available once again precluded this work. From the very elementary series of samples taken the calculation showed that the mean annual increase of dissolved solids was 80,000 tons per annum (81,000 tonnes per annum). This figure is of the same order as the suggested solution load resulting from the size reduction of stones on the river bed. Clearly the aggradation of bed material is not the only source of solution load, for the suspended material itself must obviously lose some of its mass into solution during the passage down river.

131 IV. 5. Summary of fresh water sediment transport The following represents a summary of the total discharge of solids by the River Tyne at By well:

Tons per annum (Tonnes per annum) Suspended load 130,000 (131,000) Bed load 20,000 (20,200) Solution load 80,000 (81,000) Total 230,000 tons (232,000 tonnes per annum per annum)

Assuming an in-situ density of 100 pounds per cubic foot (1.59 gms/cucm), this is equivalent to a rate of erosion of 0.266 inches per 100 years (6.76 millimetres per 100 years*, a figure which is very reasonably comparable to that obtained for the upland catchment at Catcleugh reservoir of 0.45 inches per 100 years (11.4 millimetres per 100 years). Similarly for the River Derwent, the other principal tributary discharging into the estuary, solids discharges are estimated as follows; Tons per annum (Tonnes per annum) Suspended load 30,000 30,200 Bed load 5,000 5,100 Solution load 15,000 15,100

Say, 50,000 tons per annum (50,400 tonnes per annum) which is equivalent lo an erosion raie of 0.46 inches per 100 years (f 1.7 millimetres per 100 years).

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'w'dd sanos asaNSdsns V. SEDIMENT TRANSPORT—TIDAL WATER

V. I. Tranport of Suspended Material The basic hydraulic pattern of circulation within the estuary has been shown in Section II.6. Figure 6 shows a sketch map of the estuary. It indicates the positions of the twelve sampling cross sections used to determine the estuarine characteristics. In this section of the work the sampling was automated as far aspossible. A fully equipped 32-foot (9.8 metre) diesel launch was used as a survey laboratory and samples from known depths and positions in the estuary were pumped aboard, (plate 21 The samples were passed through a light absorption-type turbidity meter which was constantly and carefully calibrated using in-situ sampling and subsequent analysis by the filtration techniques described earlier. At each section in the river, three sampling verticals were used in order that an integration of flow could be obtained. Observations of temperature, suspended solids, current direction, current velocity and salinity were taken at frequent intervals throughout the full tidal cycle. A range of tides between springs and neaps were sampled to gain the most comprehensive picture possible, of conditions in the estuary. All observations were plotted in the form shown in figure 10. The distribution of transport throughout the tidal cycle is well exemplified. It shows the tendency throughout the ebb portion of the tide to a fairly uniform distribution both in depth and time. With the onset of the flood portion the solids contents layers near the bed rapidly increase resulting in the transport of material in an inland direction. Though

CORRELATION OF RIVER FLOW PARAMETERS -17H0UR5 AFTER FIRST RW.-HOWDON OCT 10th.1^60.

So. Unity

Fig No. 11.

30 Suspended solids Parts per million 30 Velocity Ft. per s ec. 10 Salinity Parts |per million << Temperature "centigrade

Fig. 11 —Correlation of river flow parameters. !7 hours after first high water. Howdon, 10th October, 1960.

134 the example quoted is the norm and shows the flood transport tendency in a very dra­ matic way, several of the river sections did not give any indication of this feature and solids contents could remain constant throughout the complete tidal cycle. In times where there was excessive discharge of fresh water, the whole picture was completely changed. Figure 11 shows typical profiles under such highly stratified conditions. The observed data was reduced from an actual time mode to that of a lunar basis. (there being twelve lunar hours in one iide cycle) and hourly vertical profiles of sus­ pended material contents were plotted and were integrated with similar plots of cor­ responding velocity measurements. With known cross sections of flow it was possible to integrate the transport amounts obtained from the vertical profiles to arrive at (he actual material transport past the river section at any one sampled time. The results of the analysis throughout the estuary are shown in figure 12. There are two noteworthy peaks to this curve, namely at a distance between five and six miles (8 and 10 kms re- apectively; from the sea and again at a distance of some fourteen miles from the sea. The significance of these points is indicated in figure 13 where the net transport, i.e.

Flood transport the % excess is plotted. ebb transport

At a point five miles from the sea ebb and flood transport are equal. Seaward of this point net transport is in a landward direction, whilst in general, landward of this point net transport is in a seaward direction. The exception to this occurs at the four­ teen mile (23 km) point, where excessive landward transport occurs, at the site sampled. Explanation of the significance of these facts will be shown in the section of the paper dealing with estuarine deposition.

.J- .. „ : ...J !.. . (..RELATIVE - TRA^k** -PER luNllj ÉRbSS.SEC TtOtJALi Aft ÎA. ! .--• .;

_^;:y:L.^:,.;,,i4,^-V m^Li/h-i t :un.m;r ît^&I^Hfe^-Tïï..p Fig. 12 — Relative sediment transport, River Tyne estuary, transport unit cross sectional area.

135 NETT TRANSPORT

% EXCESS FLOOD TRANSPORT EBB TRANSPORT. Fig No. 13.

Tidat Range in ipi«5& feet indicated.

Distance from Piers-Miles 4 6 6 10 12 14 16 la

Fig. 13 - Nett transport % «««"flood transport ebb transport

V. 2. Bed Load Transport

As in the case of fresh water bed load transport, the measurement of bed load in the estuary presented very considerable difficulties. Several sampling instruments were tried mostly with very little success. The bed of the estuary consists mainly on soft oozy biack mud into which sampling instruments sink very easily. The"Amhem" type instrument was tried with a special support plate mounted beneath but with little success, apart from the sections at the Harbour entrance, which were essentially sandy. A photo-electric device was also tried but without success. Sampling with the hori­ zontal instantaneous sampler used in fresh water work was the only method by which meaningful results were obtained, and these were incorporated within the section on suspended material transport.

VI. ESTUARINE DEPOSITION

The deposition of material in the tidal estuary results from the working of two basic factors, namely, the reduction in velocity and hence transporting power of the entrant rivers and streams, and the physico-chemical flocculation and settlement effects brought about by the presence of salt water. The deposition of material necessitates the dredging of the river to maintain navigation and to prevent undue flooding in the upper tidal

136 sections, when high tides and flood flows coincide. The detailed records of dredging of the river between 1861 and 1961 were made available by the Tyne Improvement Com­ mission, who dredge over one million tons (1010.000 tonnesi of material from the estuary each year. Figure 14 shows the distribution of the dredging effort in the river between 1952 and 1961. On this plot, distinction is made between total and mainte­ nance dredging. Once more it will be noticed that the plot has two distinct peaks— namely, that in the four to six mile (6.4 to 9.7 kilometres)river reach and that in the twelve to fifteen mile (19.3 to 24.1 kilometres) river reach.

Fig. 14 — Analysis of dredging data 1952-1961

The distribution of dredged material and hence deposited material throughout the estuary shows some most interesting comparisons with other estuarine parameters, such as estuarine sediment transport distribution, figure 12, together with theoretical veloc­ ity distribution, bed material particle size distribution, figure 15, and salinity distri­ bution. The basic pattern of sediment circulation within the estuary is shown in figure 16. The mechanism by which this operates is thought to be of the following form. Despite the fact that velocities are low, considerable quantities of sandy material are transported into the harbour entrance on flood tides, and in the main are deposited. Some of the sand from the harbour entrance is fed into the estuary proper, where it is transported upstream to the area of the Howdon— reach, the point where ebb and flood transport are equalised. This point also marks the limit of excursion of the bottom water from the river mouth during an average tide. Within this area, silt contents of the bottom muds reach a peak with correspondingly low contents of sand, whilst salinity contents near the bed even at low water remain above 50-60% sea water content.

137 ESTUARlNE BED MATERIAL PARTICLE SIZE ANALYSIS

Sana. 200-O.OSmm aid. Silt, 0.06-OOOEmm dia. Fig. No. 15. Clay. less thon 0.002 mm. dia.

ioo —

80-

Sand c 60-]

20-

-i | i r . |- 2 4 6 8 10 IE Distance from Pisrs Ï miles )

Fig. 15 — Estuarine bed material particle size analysis.

Distance from Piers (miles).

SEDIMENT CIRCULATION PATTERN RIVER TYNE ESTUARY. r,e.H.K.

Fig. 16 — Sediment circulation pattern — River Tyue estuary.

138 Between Hebburn [the 6 mile (9.7 kilometre} section] and Redheugh [at the II y2 mile (18.5 kilometre) section] dredging activity is low, whilst in this reach average veloc­ ities of flow increase as the mean bed width decreases. Between the 11 Y% and 12>,4 mile (18.5 and 20.1 kilometre) sections, the river widths once more increase, velocities decrease to their lowest values in the whole estuary, and are low enough for the silt fraction (75% of the bed material) to remain in considerable proportions. There is a notable absence of sand and clay sine panicles. Relative sediment again increases with a further peak at the 13V4 mile (21.7 kilometre) section. Particles of sediment sampled from the bed of the river in this reach have the appearance of floes which indicate the action of salinity on the very small particles In the fresh water flow. Salinity profiles indicate 30-40% sea water content at low water under normal condi­ tions. Above this reach, the bed becomes increasingly sandy. It is thought that as the fresh water discharges into the tidal estuary, the sand fractions of material are initially deposited and are only moved downstream when high flood flows occur. The finer material continues to be carried downstream in suspension to the 12-13 mile (19.3— 20.9 kilometre] reach where velocity, width and salinity conditions are conducive to settlement. The moisture content of the dredged material has been carefully examined, partly by actual sampling and consequent analysis of mud as it came aboard the dredgers and partly based on experience of the Supervising Engineer. The equivalent dry tonnage dredged on the average during the nine years, 1952-1961 was 606,000 tons (611,000 tonnes) per annum.

VII. SEDIMENT BALANCE

The major sources of sediment production have now been examined. Before an attempt is made to balance a complete list of all sources and volumes of sediment is given. Material enters the estuary from the following sources [expressed in dry tons (tonnes) per year].

tons/year tonnes/year (1) River Tyne (suspended and bed load) 150,000 151,000 (2) River Derwent (suspended and bed load) 35,000 35,300 (3) Sewage system (which includes discharges of several 32,000 32,300 small streams which enter the estuary but which are very highly polluted) (4) Spillage from the power stations and National 15,000 15,100 Board jetties (5) Material deposited in Harbour entrance from sea 120,000 121,000 (amounts obtained from dredging records) (6) An unknown quantity originating from the sea carried into the estuary proper

352,000 355,000 tons/year tonnes/year

This above quantity should balance with the amount of dredged material expressed in dry tons (tonnes) per year, i.e. 606,000 tons (611,000 tonnes) per year. The discrep­ ancy between these two figures of 250,000 tons (252,000 tonnes) is due to this unknown

139 quantity transported from the harbour entrance into the estuary proper. This annual quantity is equivalent to about 350 tons (353 tonnes) per tide cycle, i.e. a mean nett flow of 16.8 p.p.m. On the surface, this seems an astronomical figure, but a good pro­ portion of this quantity may well be deposited within the river in the space of a few effective easterly storms which occur about 7% of the time. There is very strong visual evidence to support this case and it has been found that very high concentration of suspended material are maintained within the estuary even two or three days after the storm has passed. Within this balance, no allowance has been made for the possibility of chemical action between sea water and the dissolved material in the incoming waters. No evi­ dence of this has been found in the estuary.

ACKNOWLEDGEMENT

The work described in this paper was undertaken at the University of Durham King's College, Newcastle under the departmental headship of Professor W.F. Cassie, to whom the Author is indebted for permission to publish.

DISCUSSION

Intervention of Jean BURZ Question: What kind of sediment production and sediment sources are in the drainage basin ?

Answer: All the normal forms of erosion are evident in the Tyne catchment. There are examples of splash erosion, sheet erosion, gully erosion and channel erosion. It is considered that the principal forms in the Tyne are sheet erosion and channel erosion. The investigation which was conducted did not reveal any distinct areas which contrib­ uted more sediment than others. The upper levels of the catchment produced more sediment than the lowland areas.

Question: The proportion between silt and sand of supended load and the grain size distribution ?

Answer. No measurements of grain size distribution was made. The technique used for filtration of the samples used cellulose ester type filters which become transparent when immersion in oil is applied. The samples on filter discs have been preserved. Lack of time has so far precluded analysis which would have been carried out using a microscope. No particle size distribution is therefore available.

Intervention of Dr. FRIEDRICH

Question: From what kind of maps (scale of maps) did you take your basic material ?

Answer: (Ï presume that bed material is referred to in the question). Bed material was sampled at regular intervals throughout the length of the river. Maps were not

140 used to locate the shoals; shoals were located by inspection and sampling from these shoals was carried out after random fixing of a small area within each shoal.

Intervention o/R.H. MEADE

Question: Of the sediment that is accumulating in the estuary, what proportion is ultimately derived from the river basin, and what proportions are derived from offshore and longshore sources ?

Ansiver.The answer to this question is given in the full text of the paper — Section VII describes the sediment balance of the river estuary. In brief 350,000 tons per year of material originates from fresh water inflow and 250,000 tons per year from marine origin.

Question: In comparing Allen's pattern of Water Circulation with your pattern of suspended sediment circulation, the water seems to be moving vertically upward at the point a few miles above the estuary mouth where the sediment is moving vertically downward at a point where the water is moving upward ?

Answer: The figures shown in the text represent the results of estuarial sampling at intervals of roughly 1,5 miles throughout the length of the estuary. The point referred to by Mr. Meade is located between seven and eight miles from the piers. Dr. Allen has shown a definive upward movement of water in figure 5 whilst in figure 6, 1 have indicated a rather indeterminate state of affairs. In view of the absence of deposited bed material at the eight-mile distance there is unlikely to be a downward component in sediment movement.

Intervention of Dr. SZESZTAY

Question: According to the experience obtained (in Hungary (Danube)) excavating (dredging) of gravel materials from the river is influencing radically bed load movements and the information may hardly characterise bed load movement and discharges under natural conditions.

Answer: The effect of gravel workings on the movement of bed load in the Tynt had been noted. An assessment of Tyne bed load movement had been made on a basis of the known volume of gravel removed by the companies. In my investigation I was not attempting to ascertain the conditions of the natural catchment but of the catchment as it exists.

Intervention of Dr. J. P. OUMA

Question: You said that in the tributaries and in the Tyne, size decreased by 80% from source to confluence. You base this conclusion on measuring individual particles. This is an erroneous conciusion, because lithology of individual particles affect their individual rate of comminution. What attention did you pay to lithology ?

Answer: The point made by Dr. Ouma has been appreciated. It should be remem­ bered that the figures collected represented the average decrease in volume of 80%. This 80% represented particles of all types which were sampled, and on this basis it

141 is considered that the conclusion reached was correct. No attention was paid in the experiment to the examination of lithology of the stone particles.

Question'. What did you find to be the significant determinants of the two points at which most deposition took place in the estuary ?

Answer. The factors which determined the deposition of material in the estuary in the River Tyne are principally as follows: (a) Reduction of the average velocity of flow, which is of course related directly to the cross-section of the esturary ; (6) The depositions which occurred some six to seven miles above the mouth of the river were located at the point of mean flood tide excursion in the estuary. The point upstream situated at a distance of about thirteen miles from the river mouth was located at a section where the salinity began to become effective in depositing fine colloidal particles; (c) The text of the paper shows that where deposition occurred, bed material was of small grain particle size. Where no deposition occurred, larger particle sizes pre­ dominated. It is probable that in fact this phenomenon could be highly correlated to the facts outlined in part (a) of this answer.

142