On the application of horizontal ADCPs to suspended sediment transport surveys in rivers

In the last five years, horizontal acoustic Doppler current profilers have come onto the market as an instrument designed to measure real-time horizontal current profiles from a permanent fixed mount. The focus of this paper is on the feasibility of using these commercial instruments to measure suspended sediment fluxes in rivers. Measurements are presented from a study site that was equipped with an optical turbidity meter and three horizontal ADCPs operating at 300 kHz, 600 kHz and 1200 kHz between November 2009 and June 2010. Laser grain size measurements showed the primary particles of the suspended sediment to be predominantly silts with some clay and occasionally some fine sand. Measurements of sediment attenuation by the three horizontal ADCPs are presented for two high concentration events during which concentrations reached 2.5 and 8 kg/m3, respectively. Very clear linear relationships are seen between sediment attenuation and particle concentration and the sediment attenuation is consistent with the theory for viscous absorption by fine sediments. Using the relationship between attenuation and concentration, we are able to reconstruct the concentration time series for events during which the turbidity data are unavailable. Our findings show that, when properly positioned, horizontal ADCPs can provide suitable measurements of sediment transport during extreme river flow conditions such as floods, at least for predominantly silt-sized particles. This ability is of primary interest for river survey applications since the long term sediment transfer is believed to be driven by these intense events.