Suspended Sediment Concentration And Discharge In A West London River.

J.G.Linwood MSc FGS MCIWEM C.WEM CEnv AssocRICS

INTRODUCTION

Stormwater and dry weather flows from a 1.95 km2 separately sewered urban catchment in Whitton, West London were monitored to determine suspended sediment and water quality characteristics.  Measurements showed that water quality parameters varied considerably with discharge and that high concentrations of pollutants are transmitted in pulses during storm-flows.  Suspended sediment concentrations varied between 2 mg/l - 968 mg/l with concentration levels exceeding that of raw sanitary sewage being recorded during storms of light precipitation intensity.  The power-functional relationship between sediment concentration and discharge for the catchment was determined to be Conc. = 0.4183 Q^1.0492 the relationship being complicated by hysteresis and sediment depletion.  

THE STUDY AREA

The study drainage basin containing the Whitton Brook extends over an area of 1.95 km2 with a maximum elevation of 19m (AOD) and a minimum of 4m (AOD) giving an absolute relief of 15m. The catchment has a length of 3.1 km and a mean width of 0.6 km. The geology of the Whitton catchment consists of Quarternary Thames river gravel deposits over the Eocene London Clay. The source of the brook was originally Hounslow Heath but, due to urbanization and sewerage of the catchment, only the lower 1.3 km between Mogden Lane and Moor Mead Bridge remains of the original channel of the brook. The brook is primarily fed by several surface water outfalls serving the area. The Whitton Brook catchment was chosen because it is typical of most separately sewered outer London suburban areas.

INSTRUMENTATION AND METHODS

To enable representative sampling during high and low flows the author constructed a purpose-built depth-integrated hand sampler from a plastic container and two lengths of plastic tubing. In all samples the sampler was lowered at a constant rate through the brook at mid-width until the bottom was reached and then raised to the surface until the sampler was filled. Care was taken to avoid any bed material or floating matter being drawn into the sampler. Samples were taken within minutes of each other to allow for concentration differences and bulked into a larger container from which a well-shaken sample between 250ml and 500ml was decanted into the final sample bottle. The result of each sampling being an integrated sample with the relative quantity collected at any depth being proportional to the discharge at that depth. The samples were refrigerated at 10 C prior to analysis to retard bacterial action and prevent the formation of green algae.

Laboratory determination of total suspended solids concentrations expressed as mg/l was undertaken by vacuum filtration through Whatman GF/C grade 110mm diameter glass fibre papers following the recommended method given by the Department of the Environment (1972). Observations of air and water temperatures were made and pH determined in-situ using a field pH meter. Measurement of flow velocity was undertaken using the "Braystoke" miniature current flow meter and discharge calculated using the velocity-area method (Lewin, 1981).

A sampling point at Ordnance Survey Grid. Ref. TQ 1629 7441 was selected 50m upstream of the confluence of the brook with the River Crane. Within the constraints posed by other commitments and the short period of data collection (three months) a systematic sampling programme of 4-5 samples a week during dry weather flows was adopted and frequent sampling undertaken during storm events. Storm observations ideally consisted of samples and discharge readings taken at 15 minute intervals to produce discharge concentration hydrographs and rating loops. Precipitation and sewage analytic data was obtained from the Thames Water Authority's sewage treatment works at Mogden to the north of the catchment.

RESULTS AND DISCUSSION

During the study period (June-August, 1982) 70 measurements of discharge and suspended sediment concentration were made together with associated temperature and pH values; 50 of which were stormflows or the resultant increased baseflows.

Parameter

Mean

Variation

Range

Coefficicient of Variability

Discharge (l/s) 80.0 132.1 9-686 1.652
Suspended solids (mg/l) 57.5 137.2 2-968 2.385
pH 7.74 0.4 6.6-8.2 0.055
Air Temperature C 17.0 2.4 14-24 0.141
Water Temperature C 15.5 1.4 13-20 0.090

Throughout the study period the total precipitation recorded at the Mogden S.T.W. was 162.3mm being 99.8% of the 1916-1950 average for June, July and August recorded at Kew Observatory (Brazell, 1968), 2km east of Mogden. The June rainfall represented 177.6% of the monthly average, July 37.8% and August 107.1%. The majority of storm events were localized convective thunderstorms of up to three hours duration producing "light" (0.01-12mm) to heavy (12-19.9mm) intensity rainfalls per outbreak (Atkinson, 1979). Unfortunately the instrumentation at Mogden did not measure rainfall intensity; however, the author noted the approximate times of concentration and duration of each storm event for which the hydrographs were produced. The rainfall-runoff lag times of 60-80 minutes within the catchment are relatively short for a low-relief catchment. Typical hydrographs have leptokurtic forms with sharp rising limbs with a skew on the falling limb (Ellis, 1976) typical of sewered catchments with impervious surfaces in comparison to the flatter, more symmetrical, hydrographs in natural undeveloped watersheds.

The average pH value of 7.74 falls within the generalized nationwide values of 1-8 (Walling and Webb, 1981) being slightly alkaline and typical of most British rivers. The average air temperature was 1.5 C greater than that of the stream water; however, during storm events this difference decreased and during extreme flows water temperature as much as 2 C greater than the ambient air were recorded.

Values for discharge and suspended sediment concentration during the study period ranged from 9-686 l/s and 2-968 mg/l respectively with the highest values being recorded during storm runoff events. The power-functional relationship between concentration (mg/l) and discharge (l/s) in the form C=aQ^b was found to be Conc. = 0.4183 Q^1.0492 using linear least-squares regression of logarithmically transformed data; a rating curve was produced through the scatter of points using conventional logarithmic co-ordinates. 

Log Conc. =   1.0429 Log Q - 0.3786
or Conc.   =   0.4183 Q^1.0429
N             =   70
r              =   0.864
r^2          =   0.747
Conc.       =   Instantaneous suspended sediment concentration (mg/l)
Q             =   Instantaneous discharge (l/s)
N             =   Number of Observations
r              =   Correlation coefficient
r^2          =   Coefficient of determination

All Data (June-August 1982)

This method has been adopted by other researchers in the field of suspended sediment studies and can be justified on statistical grounds in terms of normality of data, the linearity of the relationship and an assumption of homoscedasticity (Walling, 1977). Walling (1974) considers that the suspended sediment relationship with discharge should be considered as rating loops rather than in linear form, with the form of the loop being influenced by exhaustion effects. Rating loops for the storms were produced  by taking a curve through the recorded discharge/concentration points in chronological order with time continuing around the' loop in the direction indicated.

The effects of sediment depletion by storms occurring in rapid succession are evident in the hydrograph of the two storm events of 26 June 1982 . The first storm peak of 90 l/s resulting from a light shower is followed by a second storm peak of 205 l/s caused by a thunderstorm. Although the second storm's discharge has increased by a factor of 2.3, the associated peak sediment concentrations of 54 mg/l and 68 mg/l have only increased by a factor of 1.3; the time difference between peak discharge and peak concentration for the two events was reduced by 15 minutes.

CONCLUSIONS

Measurements taken during the study period showed that suspended sediment concentrations varied considerably with discharge with concentration levels exceeding those for average raw sewage (299 mg/l recorded at Mogden during the study period) being reached during storms of light precipitation intensity. The power functional relationship between concentration (mg/l) and discharge (l/s) for the Whitton Brook was found to be Conc. = 0.4183 Q^1.0492  which is more typical of upland catchments on resistant rocks than lowland gravels-on-clay catchments (Walling and Webb, 1981) and shows the influence of the built environment on sediment supply. However, only 15% of the variation in suspended sediment concentration is explained by the variation in discharge, the relationship being complicated by the effects of hysteresis associated with rising and falling stages, a progressive decrease in sediment availibility caused by multi-peaked or closely-spaced storms and the tendancy for the sediment concentration peak to precede the peak discharge. The sediment transport patterns presented here are from the complex hydrological environment of a mostly sewered catchment in which the availability of sediment is dependent on the presence of temporary or long-term sediment storage systems in the form of road-gullies, inadequate pipes and culverts and varying rates of delivery from a wide range of land surface types.

This technical report is a brief summary of part of the author's MSc thesis Suspended Sediment Patterns and Water Quality in an Urban Watercourse. Kingston University. (1982).

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