Assessing impacts of dike construction on the flood dynamics in the Mekong Delta

Abstract. There is a large public concern about recent increases in flood risks in the downstream part of the Vietnamese Mekong Delta. Pronounced recent expansion of high-dike constructions in the floodplains of the upper delta are associated with observed increases in river water levels in the downstream province of Can Tho. In this paper the hydraulic impact of upstream dike construction on the flood hazards downstream the Mekong Delta is assessed through modelling of dike density scenarios in the flood hydrographs of 2011 and 2013. To do this, the existing Mekong delta one-dimensional (1D) hydrodynamic model was combined with a quasi-two dimensional (2D) approach to explore the change in water dynamics downstream under extensive high-dike developments in An Giang and Long Xuyen Quadrangle. Most studies are unable to explain where the floodwater goes. To address this, water balances have been established to trace the propagation and redistribution of flood volumes over the delta, which provides an extension of current work in this field. Model results indicate that the extensive construction of high dikes in the upstream floodplains have only limitedly impacts upon the downstream peak river water levels in Can Tho. Instead, model impacts on peak river water levels are concentrated and amplified in the upstream reaches of the delta. The water balance analysis shows the model is able to return fairly stable river water levels at Can Tho by diverting floodwater volumes away for the Long Xuyen Quadrangle in excess of the retention volume lost due the dike construction. This reduced inflow into the Quadrangle, and subsequent diversion of flood volumes to the Plain of Reeds and Cambodian floodplain can, however, not be fully validated due to a lack of monitoring data. The model's spatial re-distribution of flood volumes can be induced by the way the model is calibrated. As dike construction results, according to the model, in a reduction of floodwater volumes reaching the Long Xuyen Quadrangle, high-dike scenarios for the whole of An Giang province or for the entire Long Xuyen Quadrangle indicate only limited increases in upstream and downstream peak river water levels. Future assessments will have to be conducted on the scale of the entire Cambodia-Vietnamese Mekong Delta, with explicit calibration and validation of the Cambodian floodplain as a means to trace the re-distribution of flood volumes and peaks across the delta.