Erosion of the coastal Mekong delta: Assessing natural against man induced processes

The Lower Mekong Delta Coastal Zone (LMDCZ) is emblematic of the coastal erosion problem facing many tropical deltas. Over the last 3500 years, large river sediment fluxes expanded the delta seaward, and waves and currents formed the Ca Mau Peninsula to the southwest. Since the middle of the 20th century, the LMDCZ is affected by various human activities that include reduction of river fluxes due to damming and sand mining, land subsidence due to groundwater extraction, and reduction of protective coastal mangroves in favor of agriculture and aquaculture. Coastal erosion is observed along many sections of the delta, with a rate of up to 50 m per year in some areas. However, the role of human activities remains difficult to assess because of its complexity. The present modeling study is designed to sort out the contribution of natural hydrodynamic redistribution of sediments from man-induced erosion. The modeling system used is based on CROCO, forced by global reanalyses at the boundaries and at the surface, including wave statistics (required by the sediment transport model). Tides and realistic river forcing are also included. Calibration and validation relies on a combination of in situ and remotely-sensed observations, and laboratory experiments. Once validated, coastal dynamics are investigated by performing sensitivity experiments for both the hydro- and sediment dynamics. The results suggest that while wind is the main factor driving the coastal currents, the sediment dynamics is essentially the result of re-suspension due to wave-induced bed shear stress. The suspended sediments are then redistributed by coastal currents that are not limited to the nearshore zone. Strong seasonality of the process is observed with the northeast winter monsoon being the season of strongest re-suspension and sediment redistribution. The annual sediment budget is characterized by important local disequilibrium, with alongshore patterns that are in agreement with the observed shoreline evolution. The effect of a decrease in river sediment supply is difficult to evaluate because the estuarine zone is still in accretion, apart from the particular case of Go Cong shores. Far from the estuarine zone, subsidence is an additional strong candidate to explain erosion in areas that should naturally be accreting. Synthesizing these results, the study proposes a first attempt at a "taxonomy and geography" of processes along the coastal Mekong delta that can explain the recent observations of shoreline changes and help design protection measures.