MEASUREMENT OF SUSPENDED SAND CONCENTRATIONS IN THE NEARSHORE - FIELDINTERCOMPARISON OF OPTICAL AND ACOUSTIC BACKSCATTER SENSORS

Collocated field measurements of near-bed suspended sand concentration s were obtained using optical and acoustic sensors in order to assess the ability of such instruments to replicate sand concentration fluctu ations over a range of time scales. Concentrations averaged over sever al minutes agree within +/-10%, indicating the instruments are probabl y both giving reasonable estimates of concentration. Deviations betwee n the estimates of concentration increase as averaging time decreases. Deviations reflect both instrumental uncertainties and real spatial v ariability in concentration, over distance scales of less than 15 cm, related to vortex ejection from bedforms. Sediment transport rates com puted from direct measurements are also sensitive to location and samp ling, possibly due to instrument position relative to bedforms.