Risk of compound flooding substantially increases in the future Mekong River delta

Abstract. Floods are consistently identified as the most serious global natural hazard, causing devastating loss of life and economic damages that run into multiple billions of dollars each year. At the coastline, many flood disasters are in fact compound flood events, with two or more flood drivers occurring concurrently or in quick succession. In coastal regions the combined effect of fluvial (river) and coastal (storm-tides – storm surges plus high astronomical tides) floods together has a greater impact than if each occurred separately. Deltas in south-east Asia are particularly exposed to coastal compound floods as they are low-lying, densely populated regions subject to both intense rainfall and tropical cyclone (TC) derived storm tides. For our study we used a sophisticated 1D river model, combined with 2D storm tide levels, to analyse past/present and future compound flood hazard and exposure for the Mekong River delta, one of the most flood-vulnerable deltas in the world. We found that with compound flooding a greater area of the delta will be inundated, some parts will flood to greater flood depth. Central areas around Ang Giang and the Dong Thap provinces would be particularly impacted. In the future delta, the impact of compound flooding is potentially more significant, as compound floods inundate a greater area, to greater depth in many locations, and floods last longer too. Compound flooding therefore has clear implications for flood managers of the future delta, who will need to ensure that existing and future flood defences are to the right standard and in the right locations to offer effective protection against this future risk.