Measurement of suspended sand concentrations in the nearshore: field intercomparison of optical and acoustic backscatter sensors

Collocated field measurements of near-bed suspended sand concentrations were obtained using optical and acoustic sensors in order to assess the ability of such instruments to replicate sand concentration fluctuations over a range of time scales. Concentrations averaged over several minutes agree within ± 10%, indicating the instruments are probably both giving reasonable estimates of concentration. Deviations between the estimates of concentration increase as averaging time decreases. Deviations reflect both instrumental uncertainties and real spatial variability in concentration, over distance scales of less than 15 cm, related to vortex ejection from bedforms. Sediment transport rates computed from direct measurements are also sensitive to location and sampling, possibly due to instrument position relative to bedforms.