Combining echo and hydrodynamic measurements for estimating non uniform sand transport under waves and currents

Abstract

We present recently obtained measurements in the Atlantic coast of Uruguay, with a 1 MHz ADCP (Sentinel V20, Teledyne RDI, USA). The ADCP was deployed at 16 m depth for 4 months using a light weight structure deployed from a small boat. During the deployment, 3.6 m significant high, 12 s peak period waves, and 0.8 m/s currents were recorded. The vertical distribution of suspended sediment in this environment is controlled by the turbulence level induced by both currents and waves. Assuming a representative sediment size and adapting the Rouse-Vanoni profile showed to give results that did not agree with the concentration profiles inverted from the recorded echo profiles. Therefore, a method that combined variations of the sediment size distribution with the turbulence level was developed showing very good agreement with the information from the echo profile. In the developed method, the turbulent sediment diffusivity was computed from the mean current and wave measurements. However, this showed limitations for conditions with strong waves and weak currents. As an alternative, direct measurements of the turbulent fluctuations from the instantaneous beam velocity recordings were used to compute the turbulent diffusivity. In addition, different turbulent diffusivity distributions for combined current-wave conditions were evaluated.