Large-scale suspended sediment transport and sediment deposition in the Mekong Delta

摘要

Sediment dynamics play a major role in the agricultural and fisheryproductivity of the Mekong Delta. However, the understanding of sedimentdynamics in the delta, one of the most complex river deltas in the world, isvery limited. This is a consequence of its large extent, the intricate systemof rivers, channels and floodplains, and the scarcity of observations. Thisstudy quantifies, for the first time, the suspended sediment transport andsediment deposition in the whole Mekong Delta. To this end, a quasi-2Dhydrodynamic model is combined with a cohesive sediment transport model. Thecombined model is calibrated using six objective functions to represent thedifferent aspects of the hydraulic and sediment transport components. Themodel is calibrated for the extreme flood season in 2011 and shows goodperformance for 2 validation years with very different flood characteristics.It is shown how sediment transport and sediment deposition is differentiatedfrom Kratie at the entrance of the delta on its way to the coast. The mainfactors influencing the spatial sediment dynamics are the river and channelsystem, dike rings, sluice gate operations, the magnitude of the floods, andtidal influences. The superposition of these factors leads to high spatialvariability of sediment transport, in particular in the Vietnamesefloodplains. Depending on the flood magnitude, annual sediment loads reachingthe coast vary from 48 to 60% of the sediment load at Kratie. Depositedsediment varies from 19 to 23% of the annual load at Kratie in Cambodianfloodplains, and from 1 to 6% in the compartmented and diked floodplainsin Vietnam. Annual deposited nutrients (N, P, K), which are associated withthe sediment deposition, provide on average more than 50% of mineralfertilizers typically applied for rice crops in non-flooded ring dikefloodplains in Vietnam. Through the quantification of sediment and relatednutrient input, the presented study provides a quantitative basis forestimating the benefits of annual Mekong floods for agriculture and fishery,and is an important piece of information with regard to the assessment of theimpacts of deltaic subsidence and climate-change-related sea level rise ondelta morphology.