Sea-level rise, subsidence, and reduced fluvial sediment supply are causing river deltas to drown worldwide, affecting ecosystems and billions of people. Abrupt changes in river course, called avulsions, naturally nourish sinking land with sediment; however, they also create catastrophic flood hazards. Existing observations and models conflict on whether the occurrence of avulsions will change due to relative sea-level rise, hampering the ability to forecast delta response to global climate change. Here, we combined theory, numerical modeling, and field observations to develop a mechanistic framework to predict avulsion frequency on deltas with multiple self-formed lobes that scale with backwater hydrodynamics. Results show that avulsion frequency is controlled by the competition between relative sea-level rise and sediment supply that drives lobe progradation. We find that most large deltas are experiencing sufficiently low progradation rates such that relative sea-level rise enhances aggradation rates—accelerating avulsion frequency and associated hazards compared to preindustrial conditions. Some deltas may face even greater risk; if relative sea-level rise significantly outpaces sediment supply, then avulsion frequency is maximized, delta plains drown, and avulsion locations shift inland, posing new hazards to upstream communities. Results indicate that managed deltas can support more frequent engineered avulsions to recover sinking land; however, there is a threshold beyond which coastal land will be lost, and mitigation efforts should shift upstream.
Coastal rivers that build deltas undergo repeated avulsion events—that is, abrupt changes in river course—which we need to understand to predict land building and flood hazards in coastal landscapes. Climate change can impact water discharge, flood frequency, sediment supply, and sea level, all of which could impact avulsion location and frequency. Here we present results from quasi‐2D morphodynamic simulations of repeated delta‐lobe construction and avulsion to explore how avulsion location and frequency are affected by changes in relative sea level, sediment supply, and flood regime. Model results indicate that relative sea‐level rise drives more frequent avulsions that occur at a distance from the shoreline set by backwater hydrodynamics. Reducing the sediment supply relative to transport capacity has little impact on deltaic avulsions, because, despite incision in the upstream trunk channel, deltas can still aggrade as a result of progradation. However, increasing the sediment supply relative to transport capacity can shift avulsions upstream of the backwater zone because aggradation in the trunk channel outpaces progradation‐induced delta aggradation. Increasing frequency of overbank floods causes less frequent avulsions because floods scour the riverbed within the backwater zone, slowing net aggradation rates. Results provide a framework to assess upstream and downstream controls on avulsion patterns over glacial‐interglacial cycles, and the impact of land use and anthropogenic climate change on deltas.
more » « less- PAR ID:
- 10447338
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Earth Surface
- Volume:
- 126
- Issue:
- 6
- ISSN:
- 2169-9003
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract River deltas grow through repeated lobe‐scale avulsions, which often occur at a location that correlates with the backwater lengthscale. Competing hypotheses attribute the avulsion node origin to either the morphodynamic feedbacks caused by natural flood discharge variability (backwater hypothesis) or to the prograding delta lobe geometry (geometric hypothesis). Here, using theory, historical flood records, and remotely sensed elevation data, we analyzed five lobe‐scale delta avulsions in Madagascar, captured by Landsat imagery. Avulsion lengths were 5–55 km, distances significantly longer than the backwater lengthscale and inconsistent with the geometric hypothesis. We show that the steep, silt‐bedded rivers of Madagascar have flood‐induced bed scour, driven by backwater hydrodynamics, that propagates farther upstream than the backwater lengthscale. The avulsion lengths are 3.1 ± 1.5 times the predicted flood scour lengths, similar to low‐gradient deltas, and consistent with backwater hypothesis. Results demonstrate that erosion initiated by nonuniform flows in the backwater zone is a primary control on delta avulsion locations.
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Abstract It is widely recognized that waves inhibit river mouth progradation and reduce the avulsion timescale of deltaic channels. Nevertheless, those effects may not apply to downdrift‐deflected channels. In this study, we developed a coupled model to explore the effects of wave climate asymmetry and alongshore sediment bypassing on shoreline‐channel morphodynamics. The shoreline position and channel trajectory are simulated using a “shoreline” module which drives the evolution of the river profile in a “channel” module by updating the position of river mouth boundary, whereas the channel module provides the sediment load to river mouth for the “shoreline” module. The numerical results show that regional alongshore sediment transport driven by an asymmetric wave climate can enhance the progradation of deltaic channels if sediment bypassing of the river mouth is limited, which is different from the common assumption that waves inhibit delta progradation. As such, waves can have a trade‐off effect on river mouth progradation that can further influence riverbed aggradation and channel avulsion. This trade‐off effect of waves is dictated by the net alongshore sediment transport, sediment bypassing at the river mouth, and wave diffusivity. Based on the numerical results, we further propose a dimensionless parameter that includes fluvial and alongshore sediment supply relative to wave diffusivity to predict the progradation and aggradation rates and avulsion timescale of deltaic channels. The improved understanding of progradation, aggradation, and avulsion timescale of deltaic channels has important implications for engineering and predicting deltaic wetland creation, particularly under changing water and sediment input to deltaic systems.
