Fluvial sediment supply to a mega-delta reduced by shifting tropical-cyclone activity
Nature Publishing Group
© 2016 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
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The world's rivers deliver 19 billion tonnes of sediment to the coastal zone annually, 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, and a sustainable supply of fluvial sediment is therefore critical to prevent deltas being 'drowned' by rising relative sea levels. 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, and anticipating future, 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.
This study was supported by awards NE/JO21970/1, NE/JO21571/1 and NE/JO21881/1 from the UK Natural Environmental Research Council (NERC) and the Academy of Finland funded project SCART (grant number 267463). We thank the Mekong River Commission for access to hydrological and suspended sediment data and the Department for Hydrology and Water Resources in Cambodia for aDcp data and their logistical support. J.L.B. was also in receipt of a University of Southampton Diamond Jubilee Fellowship and National Great Rivers Research and Education Centre Fellowship that aided completion of this work.
This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.
Vol 539, 276–279 (10 November 2016) doi:10.1038/nature19809
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