Determining suspended sediment concentration and settling velocity from PC-ADP measurements in the Beibu Gulf, China

Modeling sediment transport depends on several parameters, such as suspended sediment concentration (SSC), shear stress, and settling velocity. To assess the ability of Pulse-Coherent Acoustic Doppler Profiling (PC-ADP) to non-intrusively quantify spatial and temporal SSC and settling velocity at seabed, a field experiment was conducted in the Beibu Gulf (Tonkin Gulf), in the South China Sea. The spatial profiles and temporal variations in SSC at 1 m above bottom were derived from PC-ADP acoustic backscatter intensity determinations after being calibrated with the optical backscatter sensor (OBS) measurements at the same elevation. The PC-ADP and OBS results agreed well. The temporal settling velocity obtained from Rouse profiles agreed well with the Soulsby formula based on size information by LISST (laser in situ scattering and transmissometry). Tides and tidal currents are diurnal in the gulf. SSC increased with increasing ebb and flood flow, and it rapidly decreased with the increase of distance from the seabed. The maximum SSC at 0.16 m and 1.3 m above bottom reached 816 mg/L and 490 mg/L during spring tides, respectively. The sediments consisted of mineral particles 23–162 μm in diameter and 0.05–2.04 cm/s in settling velocity. Generally, both the SSC and settling velocity followed variations in the bottom friction. Results suggest that PC-ADP is able to provide reasonable SSC and settling velocity measurements of both profiles and time series for a long study period.