Multidecadal fluvial sediment fluxes to major deltas under environmental change scenarios: projections and their implications

Abstract

Deltas are important climate change hot spots that provide critical locations for human populations, agriculture, and aquaculture, as well as ecosystems and the services they provide. However, they are vulnerable systems as the sediment inputs which have historically offset relative sea level rise in undisturbed deltas appear to be threatened by anthropogenic activities. This thesis contributes to the understanding of future delta sustainability by projecting sediment delivery to a set of 47 significant deltas worldwide. This is the first study of future sediment fluxes to such a large group of major deltas, and for some of these individual deltas the projection of future sediment flux has never been done before. The hydrological model WBMsed was chosen after a review of possible models to project fluvial sediment delivery to each delta annually up to the year 2100. The model was forced by scenarios of climate change (RCPs 2.6, 4.5, 6.0, and 8.5), socioeconomic change (population and GDP data derived from SSPs 1, 2, and 3), and projected dam construction. On average, across all 47 deltas, fluvial sediment delivery is predicted to decrease by 36-41%, dependent on scenario, during the 21st century. The largest overall contributor to decreasing sediment delivery is reservoir construction. However, for some individual deltas socioeconomic changes can cause a larger decline in the supply of fluvial sediment than dam construction. In comparison, climate changes have a much smaller effect than both dam construction and socioeconomic changes, but the influence of climate change is generally of opposing sign (for 39 of 47 rivers), thereby increasing future sediment supply. There are large differences between the sediment flux changes projected between individual deltas, with some increases, but the majority of deltas (32 of 47) display decreases in sediment flux on average. The significance of the sediment delivery changes for each river was assessed by developing a new Sediment Starvation Risk Index (SSRI), which considers absolute sediment flux change versus delta area. The SSRI highlights those deltas most at risk from sediment delivery losses over the 21st century and also shows a correlation between the dominant drivers of sediment flux change for each delta and the degree of sediment starvation risk. Those deltas with sediment flux changes driven by socioeconomic change are likely to be at greater risk of sediment starvation, whereas those driven by climate change are likely to be at less risk. This research assesses projected changes in sediment load to deltas, the key drivers of these changes, and the potential significance of these changes for individual deltas using globally consistent methods. The research suggests widespread challenges for delta management and sustainability due to reduced sediment inputs over the 21st century.

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Identifiers

ORCID for Frances Eleanor Dunn: orcid.org/0000-0003-3726-7158

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

ORCID for Robert Nicholls: orcid.org/0000-0002-9715-1109

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