| Large dams have been increasingly built and planned in the Mekong basin, and the question on how dams affect salinity intrusion in the Vietnamese Mekong Delta (VMD) remains uncertain. Dams alter the flow regimes and trap the sediment in the basin directly, and together with sand mining, cause riverbed incision indirectly. Therefore, we hypothesize that dams and sand mining are the two main drivers of increasing salinity intrusion in the VMD. To clarify, we analyzed long-term trend of salinity intrusion in the VMD, then linked with riverbed incision caused by long-term discharge and sediment alterations along the Mekong because of upstream dams and sand mining. Moreover, a coupled TELEMAC-2D and Sisyphe and the Mike 11 models were employed to quantify the influence of upstream dams on riverbed incision and salinity intrusion. The results show that the flood discharges increased and the sediment loads decreased significantly in the VMD due to rapid dam development in the Mekong basin. The sediment load of the VMD decreased by 74.1%, that, together with sand mining, caused rapid riverbed incision in the VMD in 2014-2017, which was tripple the incision rate in 1998-2008. Numerical model revealed that the riverbed of the VMD would be incised by up to -2.04 m by 2026 under a reduction of 84.8% of the 2017 sediment load. Cumulative effects of altered flow, reduced sediment supply, and incised riverbed due to upstream dams and sand mining have not only caused increasing salinity intrusion in the VMD (by up to 42% in between 1998 and 2016), but also shifted salinity intrusion occurring earlier than the past about 1-2 months. Improper dam operations may increase salinity intrusion in the VMD by up to 64%. |
