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

Abstract

The world’s rivers deliver 19 billion tonnes of sediment to the coastal zone annually, with a significant fraction being sequestered in large deltas, home to over 500 million people. Most (>70%) 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 in preventing 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 (TCs) 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 TC climatology, and that a significant portion (32%) of the suspended sediment load reaching the delta is delivered by runoff generated by TC-associated rainfall. Furthermore, we estimate that the suspended load to the delta has declined by 52.6 ± 10.2 Mt over recent years (1981-2005), of which 33.0 ± 7.1 Mt is due to a shift in TC climatology. Consequently TCs play a significant 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 TC climatology affect trends in fluvial suspended sediment loads and thus are also key to fully assessing the risk posed to vulnerable coastal systems.

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Identifiers

ORCID for Stephen Darby: orcid.org/0000-0001-8778-4394

ORCID for Julian Leyland: orcid.org/0000-0002-3419-9949

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