Flow measurement and suspended solids sampling on small watercourses

Abstract

Two complementary methods for estimating Total Suspended Solids (TSS) fluxes in small rural watersheds have been tested. The first method consists in correlating automatically measured turbidity with TSS concentrations. TSS fluxes can then be calculated using flow and turbidity data recorded on short intervals of several minutes. Wide and sudden variations in flow rates and TSS concentrations can thus be taken into account for flux estimations. The second method uses an in-stream sediment sampler made out of plastic pipe and placed horizontally to filtrate part of the water column. It was left in place for two weeks time. Both methods were tested during a whole hydrological year at the outlet of two small (2.2 and 4.5 km²) rural watersheds in Normandy, France. Turbidity data require relatively long and meticulous processing to correct for long and short-term drifts, as well as for diurnal variations. The value of the method lies on the goodness of fit between turbidity values and TSS concentrations, particularly for large values. Indeed, these values, although relatively rare (<1% of the time), can account for more than 50% of the cumulated TSS fluxes over a year. Amounts of sediments collected by the in-stream samplers could be correlated with TSS fluxes estimated using the first method. Although this represents a rather crude method, these samplers could be installed, after calibration, at several locations within watersheds for flux estimations in addition to information provided on the TSS composition. The two methods seem to be an interesting approach for studying TSS fluxes in small rural watersheds for which obtaining detailed information on spatial and temporal scales remains a true issue.