Abstract
<p>River deltas are low lying areas that will likely experience significant land loss because of relative sea-level rise. Most future projections of delta land loss, however, assume passive coastal inundation (using so-called “bath-tub” models) and as such they tend to be unvalidated and exclude morphodynamic processes such as sedimentation. To improve future projections of delta land area change, here we apply a morphodynamic model of delta response to RSLR to all 10,000 deltas globally. We use historic RSLR, sediment supply, and observed delta land area change from 1985-2015 to calibrate and validate this model for all these deltas. Applying our model using future RSLR scenarios, we find that by the end of this century deltas globally will have lost land under all RCP scenarios. Land loss is aggravated by river dams that have diminished sediment supply to many deltas. RSLR expected under RCP8.5 will force delta land loss at at rates exceeding 900 km<sup>2</sup>/yr by 2100. We predict cumulative land loss under RCP8.5 up to 2100 of ~35,000 km<sup>2</sup>, or about 4% of total global delta area.</p>
Highlights
Most future projections of delta land loss from relative sea-level rise (RSLR) ignore the potential for erosion and sedimentation [6], following a so-called “bath-tub” or passive flood mapping approach (e.g., 7, 8) that remains unvalidated by observations
Our model suggests that for a global mean RSLR of 10 mm yr-1, about 20% of the global fluvial sediment flux will be deposited on the delta plain (Fig. 4a)
Our predictions show substantial risk of land loss from climate-change driven RSLR
Summary
River deltas are low-lying coastal landforms created by fluvial sediment deposition. Delta shorelines can retreat landward through relative sea-level rise (RSLR) and advance seaward through sedimentation and delta plain aggradation. SROCC predictions for sea-level rise under RCP8.5 [23] and assuming fluvial sediment supply and subsidence rates remain unchanged, we find that delta land loss will exceed 808 ± 80 km2yr-1. Model results suggest that without sea-level rise, river damming, deforestation, or subsidence, deltas would globally gain about 250 km2yr-1. Of those four drivers, dams and subsidence have dominated the observed reduction in delta land gain from 1985-2015 compared to pristine conditions (Fig. 3a). Our model suggests that for a global mean RSLR of 10 mm yr-1, about 20% of the global fluvial sediment flux will be deposited on the delta plain (Fig. 4a) Such a RSLR rate would leave 80% of the fluvial sediment supply available at the river mouth for redistribution along coasts. Our results highlight that these approaches can protect deltas against some of the consequences of climate-change driven RSLR and should be encouraged
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