High-resolution resonance signature of suspended materials derived from ADCP backscatter strength

In a shallow estuarine system, sediment dynamics is controlled by feedbacks of interaction between various hydrodynamics forcings including tides, currents, and winds. As a representative feedback, sediment resuspension plays an essential role in determining the fate of sediment transport and health of estuarine ecosystem. Understanding the principal physical processes that govern the sediment resuspension enables to predict a main factor causing the erosion and sedimentation. With a high-resolution sampling scheme, in this study, an upward-looking acoustic Doppler current profiler (ADCP, 1000 kHz) was moored in Masan Bay, Korea for about 3 weeks (September 20 to October 13, 2017). The study area is a shallow (depth: 16 m), semi-enclosed estuary. The mean volume backscattering strength derived from ADCP, which is a proxy of total suspended materials, exhibited a distinct repetition of resuspension and sedimentation of near-bed sediment over tidal cycles. The analysis results suggest that the water column exhibited a two-layer structure between discharged freshwater and intruded seawater.The upper boundary of sediment resuspension from bottom was primarily limited by the thermohaline-induced stratification at about 7 m above bed. Once the sediment was resuspended, the upper boundary was fluctuated by local resonance (about 1-hr period) that moved back and forth along the main navigation channel. Our study gives insight into the complexity of hydrodynamics and associated sediment transport processes in micro-tidal estuarine systems with an emphasis on episodic events.