An assessment of sediment-transport processes in the Lower Mekong River based on deposit grain sizes, the CM technique and flow-energy data

The Lower Mekong River has been an important research topic for at least 15 years, notably in the fields of hydrology, fluvial geomorphology, and the impact of dams. Recent papers refer mostly to the impact of the Lancang chain of hydroelectric dams constructed on the Chinese section of the river. Among the pending scientific questions are (1) the upstream-downstream variations in the concentration and yield of suspended sediment and (2) the relative importance of sand in the total yield. The general consensus among the scientific community is that the relative importance of sand in suspended load is not the main scientific concern despite its extensive presence in the Mekong River channel, as noted by geomorphologists, and despite its extreme importance for the stability of the delta shoreline in Viet Nam. This paper contradicts the general consensus. Its objective is to present new information on the processes of sand transport in the Lower Mekong channel. Imagery, field observations, sampling on the river banks, and grain size analysis of sand deposits have been carried out between Chiang Sean (downstream of the Chinese border) and the delta. The C-M image technique served to discriminate between the various transport processes (bedload and different types of suspension). This technique helps in understanding the changing processes responsible for downstream sediment transfer in river channels. The results of this study are the following. (1) Discrepancies in the values of annual suspended-sediment load detected by different gauging stations have not yet been satisfactorily explained. The CM image technique indicates that grain size and the quantity of sand transiting in suspension vs. bedload vary from upstream to downstream. (2) These upstream-downstream differences may be related to the energy of floods. Energy helps to explain longitudinal variations in the relative importance of the different modes of transport and serves to distinguish between bedload, graded and uniform suspension, and wash load. (3) The Mekong is seen as a river carrying mostly silt and clay; however, the results suggest that sand plays a major role in downstream sediment transfer during floods. This result is in line with the large amounts of sand present in the channel. Its relative importance in the total yield must be reassessed, as suggested by Walling (2005, 2008). As may be expected, the role of sand in the stability of the delta shoreline is of prime importance. (C) 2013 Elsevier B.V. All rights reserved.