Downdrift Erosion adjacent to a River Mouth: Mechanisms and Effects of Episodic Riverine Sediments and Littoral Sediment transport

Abstract:

The mechanisms controlling sediment transport at river mouths and estuaries are complicated due to the interaction among waves, tidal currents, river flow over complex bathymetry. During extreme conditions, episodic river discharge triggered by large rainfall due to tropical cyclones may contribute a significant amount of riverine sediment into the ocean. Bottle samples collected from bridge stations in the Choushui River during the passage of tropical cyclones suggest sediment concentration can exceed 40 g/l for the major duration of the storm (Milliman et al., 2007). The goal of this study is to clarify the contribution of exposed riverine sediment to the observed morphological evolution in the adjacent area. EOF (Empirical Orthogonal Function) decomposition separates the observed variability of bathymetry into independent spatial and temporal modes: the 1st mode (63%) is relevant to littoral drift and groyne influence; the temporal variation of the 2nd mode (11%) is proportional to the observed riverine discharge. To understand the transport process of exposed riverine sediments, model results were used to provide insights into the patterns of flow residual for a range of riverine discharge, spring-neap tidal forcing, and wave conditions. NearCoM-TVD, couples SWAN and SHORECIRC, reproduced water levels, waves, currents observed along the sandy coasts reasonably well. Simulation results reveal that the sediment-laden river flow was first deposited on the ebb-tidal shoal adjacent to the river mouth, subsequently and transported via nearshore circulations, which is modulated by the interaction of waves and the shallower shoal at the river mouth. Waves and wave-driven currents transported a significant amount of sediment from the river mouth to the adjacent area during the northeast monsoon season.