Journal Article

Fluvial sediment supply to a mega-delta reduced by shifting tropical-cyclone activity

Parsons, Daniel R., Best, James L., Hackney, Christopher R., Lauri, Hannu, Nicholas, Andrew P., Darby, Stephen E., Kummu, Matti, Aalto, Rolf, Leyland, Julian

2016

Overview

About a third of the sediment delivery of the Mekong River is shown to be associated with rainfall generated by tropical cyclones, suggesting that future delta stability will be strongly moderated by changes to tropical cyclone intensity, frequency and track. Cyclones shaping tropical mega-deltas The delivery of sediment to deltas is crucial for their survival, especially when faced with rising sea levels. Human activities, such as dam building and land-cover alterations, can affect sediment supply, but Stephen Darby et al . show that, for the Mekong River, about a third of the sediment delivered is associated with rainfall generated by tropical cyclones. More than half of the decline in suspended sediment supply to the delta between 1981 and 2005 arose from shifts in tropical-cyclone climatology, suggesting that future delta stability will also be strongly moderated by additional changes to tropical-cyclone intensity and track. The world’s rivers deliver 19 billion tonnes of sediment to the coastal zone annually 1 , with a considerable fraction being sequestered in large deltas, home to over 500 million people. Most (more than 70 per cent) large deltas are under threat from a combination of rising sea levels, ground surface subsidence and anthropogenic sediment trapping 2 , 3 , and a sustainable supply of fluvial sediment is therefore critical to prevent deltas being ‘drowned’ by rising relative sea levels 2 , 3 , 4 . Here we combine suspended sediment load data from the Mekong River with hydrological model simulations to isolate the role of tropical cyclones in transmitting suspended sediment to one of the world’s great deltas. We demonstrate that spatial variations in the Mekong’s suspended sediment load are correlated ( r  = 0.765, P  < 0.1) with observed variations in tropical-cyclone climatology, and that a substantial portion (32 per cent) of the suspended sediment load reaching the delta is delivered by runoff generated by rainfall associated with tropical cyclones. Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 10.2 megatonnes over recent years (1981–2005), of which 33.0 ± 7.1 megatonnes is due to a shift in tropical-cyclone climatology. Consequently, tropical cyclones have a key role in controlling the magnitude of, and variability in, transmission of suspended sediment to the coast. It is likely that anthropogenic sediment trapping in upstream reservoirs is a dominant factor in explaining past 5 , 6 , 7 , and anticipating future 8 , 9 , declines in suspended sediment loads reaching the world’s major deltas. However, our study shows that changes in tropical-cyclone climatology affect trends in fluvial suspended sediment loads and thus are also key to fully assessing the risk posed to vulnerable coastal systems.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 704/106/35 / 704/106/694/2739/2807 / 704/242 / Anthropogenic factors / Asia, Southeastern / Climate Change / Climatology / Coastal zone / Cyclones / Cyclonic Storms - history / Cyclonic Storms - statistics & numerical data / Deltas / Fluvial sediments / Geologic Sediments - analysis / History, 20th Century / History, 21st Century / Humanities and Social Sciences / Hydrologic models / Hydrology / letter / Load / multidisciplinary / Natural history / Rain / Risk assessment / Rivers / Rivers - chemistry / Science / Sea level rise / Sediment load / Sediment transport / Sediments / Suspended load / Suspended sediments / Trapping / Tropical Climate / Tropical cyclones