Estimating suspended sediment concentration in streams by diffuse light attenuation

A light attenuation sensor system (LASS) for measurements in waters is described in this paper. The LASS records irradiance at multiple levels in the water column to provide a measure of the diffuse light attenuation coefficient which is strongly affected by suspended sediment. Dimensional analysis and geometric optical theory are used to relate the irradiance attenuation to sediment properties through a dimensionless product. The latter is termed as a light attenuation number for suspended sediment in waters. The LASS and dimensional analysis results are validated in the laboratory using fluvial sediments collected from a third-order stream and monodisperse quartz sediment. The attenuation coefficient estimated with LASS data varied nonlinearly with total suspended sediment concentration due to particle shadowing and multiple scattering at large optical depths. The light attenuation coefficient for each sediment type is well described as a function of total suspended sediment concentration by empirical power law relationships, which provides confidence in the functioning of LASS. Light attenuation curves for different sediment types collapsed onto a single curve when replotted according to dimensionless numbers arising from the dimensional analysis, which confers confidence in the analysis for future research and application. A successful field demonstration of LASS in a small stream highlights its potential application in hydraulic and ecological research as well as future avenues of research to improve the sensor.