Analysis and numerical modeling of the flow and sand dynamics in the lower Song Hau channel, Mekong Delta

摘要

Two- and three-dimensional Delft3D Flow and Morphology models were constructed for the lower Song Hau distributary channel of the Mekong River in Vietnam to provide insights into the hydrodynamics and sand transport of the channel system. The models were calibrated and validated with data for observed water level, water discharge, velocity, and suspended-sand concentration during the high- and low-flow seasons of similar to 2014 and 2015. The water and sand budgets of the Dinh An and Tran De channels, the two sub-distributaries of the lower Song Hau channel, were calculated, showing that similar to 73% of the fluvial water discharge and similar to 90% of suspended sand were transported through the Dinh An channel, while the rest was transported through the Tran De channel in the high-flow season. In the low-flow season, the total fluvial water discharge was < 20% of that in the high flow season; and similar to 96% of it was transported through the Dinh An channel and the rest was transported through the Tran De channel. Sand was transported from channels into the ocean in both the Dinh An and Tran De channels during the high-flow season, but from the ocean into the two channels during the low-flow season. The reversed sand flux from ocean into river channels during low-flow season was not observed in field studies. The different behaviors between models and field observations might be caused by the absence of baroclinic effects in the model set up. The Dinh An channel was found to be doininated by ebb tide, in favor of transporting water and sediment to the ocean, and the Tran De channel was dominated by flood tide. In the high-flow season, the residual currents were directed toward the ocean for both the Dinh An and Tran De channels. In the low-flow season, the magnitude of the residual currents decreased in the river channels. Under these conditions, the residual-current direction changed to be ambiguous in the Tran De channel, but remained seaward in the Dinh An channel. The low fluvial discharge combined with strong tides under very low-flow conditions can probably generate a net clockwise water circulation between the Dinh An and Tran De channels, which transports water from the ocean to the river through the Tran De channel, then back into the ocean through the Dinh An channel. The clockwise circulation pattern is likely to influence the channel evolution and impact saltwater intrusion issue in the lower Song Hau channel.