Assessment of suspended sediment concentration measurement error in relation to particle size, using continuous sensors in a small mountain stream (Vallcebre catchments, Eastern Pyrenees)

Abstract

Sediment concentration in the gauging stations of the Vallcebre research basins has been estimated for over ten years from stream water samples obtained by automatic samplers and from continuous measurements made by optical infra-red backscattering turbidimeters (OBS) and ultrasonic beam attenuation sediment sensors (US). The relationship between sensor output and suspended sediment concentration (sediment weight per mixture volume) is critical, as it depends on characteristics of sediment particles such as size, density and colour. While most sediment properties are relatively stable in such small basins, particle size is known to vary signifi cantly with fl ow power. After an experimental calibration, for the same value of sediment concentration, the OBS sensor gave a much higher signal for fi ne material (silt) than for coarse material (fi ne sand), whereas the US sensor behaved in the opposite way. For operational use for most events, the customary calibration of the OBS sensor for fi ne sediments could be used, but adequate estimation of heavy suspended loads during large fl oods needed a calibration of the US sensor close to the calibration for sands, much coarser than the grain size determined in the samples. This, along with fi eld observations, suggests that large heavily charged fl oods transported in suspension signifi cant proportions of sand-sized pieces of poorly weathered clayey bedrock, which dispersed before the grain size was determined. Continuous sensing devices were very useful at the Vallcebre basins for estimating suspended sediment loads, but these instruments needed to be coupled with automatic samplers to verify the functioning of the sensors and to allow adequate calibration of the sensor outputs to the real sediment properties.