Sand dynamics in the Mekong River Channel and export to the coastal ocean
Two field campaigns were conducted in the tidal and estuarine reach of the Song Hau distributary of the Mekong River to examine the dynamics of sand transport and export to the coastal ocean. This study examines variations in suspended sand concentration and net flux of suspended and bedload sand with respect to changes in discharge between the October 2014 high discharge and March 2015 low discharge season. During the high discharge season, ebb and flood currents are a primary control on suspended sand concentrations. Ebb tidal flows are more capable of sand transport than flooding flows, due to river discharge augmenting tidal currents. Ebb flows are the primary reason why we find a net annual flux of sand from the Song Hau to the ocean. The majority of sand in suspension is derived locally from bed material sand. Limited bedload transport estimates derived from repeat bathymetric mapping of dunes suggest that bedload sand transport is less than 10% of net suspended sand flux. Isokinetic sampling shows very low concentrations of suspended sand sediment during low discharge. This is likely caused by (1) a reduction in maximum ebb tide shear stresses associated with less freshwater input, and (2) mud mantling in the bed associated with upstream migration of estuarine circulation, that inhibits local sourcing (resuspension) of bed sand. An integrated Mekong River net annual sand flux to the ocean is estimated at 6.5 Mt yr-1 based on the 2014-2015 observational studies and numerical modeling. The Dinh An subdistributary accounts for 32% of this total while the smaller Tran De subdistributary accounts for only 9%.