A field and numerical investigation of wave influences on river-mouth depositional process

Understanding hydrodynamics and sediment transport at river mouths and coasts is critical but challenging owing to the highly nonlinear interaction among tides, waves, and bathymetry. Locally intense circulations can be generated under the interaction of tidal currents wave-induced currents and complex bathymetry. In the past decade, evidences of seabed erosion near the Zengwen river mouth has raised concerns regarding coastal resilience. A diagnostic study combing field and numerical methods is carried out in order to understand the dominant mechanisms causing the resulting complex flow pattern and sediment transport. An array of co-located wave gauges, ADCPs, and turbidity meters were deployed throughout the channels and ebb tidal shoals to obtain the time series and spatial distribution of hydrodynamic and sediment transport conditions at the river mouth. A quasi-3D nearshore community model is applied for the integrated observational and modeling study. NearCoM-TVD, couples SWAN and SHORECIRC, reproduces water levels, waves, currents observed at the river mouth reasonably well. Model results are used to provide insights into the patterns of flow residual for a range of spring-neap tidal forcing and wave conditions over the complex bathymetry. Both observed and simulated residual sediment transport patterns demonstrate that the transport process is dominated by interaction of ebb tidal jet and wave induced longshore current during low to moderate flow conditions. The southward residual transport causes erosion at the northern part of river mouth and accretion in the ebb tidal shoals around the center of the river mouth. During the extreme condition, because the artificial structure restricts the flood spreading and enhances the flow velocity, it is conjectured that the riverine sediment could be transported to the deep-sea instead of depositing in the nearshore region.