A source-to-sink study of the Mekong River Delta: Hydrology, delta evolution, and sediment transport modeling.

摘要

The Mekong River is the third largest river in the Western Pacific. As the population and economy of the area booms, more and more dams are built in the Mekong basin. Concerns about negative impacts on downstream and the delta plain from upstream damming have been raised ever since the completion of the Manwan Dam, the first of the 13 major dams designed on the Upper Mekong, in 1993. The runoff of the Lower Mekong has a closer connection with the regional precipitation and El Nino Southern Oscillation during the post-dam period (1994-2005) than the pre-dam period (1950-1993). With ∼200 new dams to be added to the basin in the next couples of decades, changes are expected in both hydrological regime and delta dynamics.;The Mekong River delivers ∼160 million tons of sediment per year to the South China Sea (SCS). The Mekong River Delta (MRD) has the third largest delta plain in the world. High-resolution seismic profiling and coring during 2006 and 2007 cruises reveals a low gradient, subaqueous delta system, up to 20 m thick, surrounding the modern MRD in the west of the SCS. A late Holocene sediment budget for the MRD has been determined, based on the area and thickness of deltaic sediment. Approximately 80% of Mekong delivered sediment has been trapped within the delta area, which, together with a falling sea-level, resulted in a rapidly prograding MRD over the past 3000 yr.;The late Holocene evolution of the MRD has shown a morphological asymmetry indicated by a large down-drift area and a rapid progradation around Cape Camau, ∼200 km downstream from the river mouth. The coupled hydrodynamic-sediment transport modeling using the Regional Ocean Modeling System (ROMS) and Community Sediment Transport Model System (CSTMS) showed that wind is a most important factor influencing the along-shelf sediment transport. This associates MRD's asymmetric evolution with an increased wave influence during the Neoglaciation. Coastal currents formed by the geostrophically balanced Mekong plume are strengthened by intensified winter monsoons. Wave and tidal mixing re-suspends previously deposited Mekong sediments, which are then transported southwestward to the Gulf of Thailand. These results link sediment dynamics and delta evolution with variations in monsoonal activities during the late Holocene.