Is a Decline in Tropical Storms Leading to the Demise of the Mekong Delta?

Abstract:

The world’s largest rivers deliver ~19 billion tonnes of sediment to the coastal zone annually, with a significant fraction sequestered in the large deltas that are home to 14% of the world’s population. Most (>70%) of these large deltas are under threat from a combination of rising sea levels and ground surface subsidence, while the rivers that feed them have also been subjected to damming, substantially reducing the sediment supply that is available for delta (re)construction.

In this paper we use new measurements of suspended sediment load, combined with hydrological modelling, to reconstruct inter-annual variations in sediment load with and without tropical cyclones during the period 1981-2005. We examine in particular the results at Kratie in Cambodia, a gauging station that is located within 250 km of the apex of the Mekong delta. Our results indicate that fluvial sediment loads at Kratie have undergone a large and rapid decline (at a rate of ~2 Mt/yr, reducing the annual sediment load in this period by ~50%) during the study period. This is a remarkable finding, but we also show that this decline in sediment load is attributable almost entirely to a commensurate decline in the proportion of the Mekong’s sediment load that is forced by tropical storms.

Our results have profound implications. The Mekong’s great delta, the rice basket of SE Asia, is home to 17 million people. A sustainable supply of fluvial sediment is critical in preventing the Mekong’s delta being ‘drowned’ by rising sea levels. In recent years attention has been focused on the likely deleterious effects of hydropower dam construction in the Mekong Basin on the future sediment loads reaching the delta. However, it is evident that the delivery of sediment from the Mekong River has already undergone a substantial reduction, similar in magnitude to that associated with future dam-induced trapping, as a result of recent changes in tropical storm climatology. A continuing dearth of tropical storms may lead to the death of this great delta earlier than was previously anticipated.