River deltas will likely experience significant land loss because of relative sea‐level rise (RSLR), but predictions have not been tested against observations. Here, we use global data of RSLR and river sediment supply to build a model of delta response to RSLR for 6,402 deltas, representing 86% of global delta land. We validate this model against delta land area change observations from 1985–2015, and project future land area change for IPCC RSLR scenarios. For 2100, we find widely ranging delta scenarios, from +94 ± 125 (2 s.d.) km2yr−1for representative concentration pathway (RCP) 2.6 to −1,026 ± 281 km2yr−1for RCP8.5. River dams, subsidence, and sea‐level rise have had a comparable influence on reduced delta growth over the past decades, but if we follow RCP8.5 to 2100, more than 85% of delta land loss will be caused by climate‐change driven sea‐level rise, resulting in a loss of ∼5% of global delta land.
- Award ID(s):
- 1810855
- NSF-PAR ID:
- 10387110
- Date Published:
- Journal Name:
- Annual Review of Earth and Planetary Sciences
- Volume:
- 51
- Issue:
- 1
- ISSN:
- 0084-6597
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.
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Abstract Lowland deltas experience natural diversions in river course known as avulsions. River avulsions pose catastrophic flood hazards and redistribute sediment that is vital for sustaining land in the face of sea‐level rise. Avulsions also affect deltaic stratigraphic architecture and the preservation of sea‐level cycles in the sedimentary record. Here, we present results from an experimental lowland delta with persistent backwater effects and systematic changes in the rates of sea‐level rise and fall. River avulsions repeatedly occurred where and when the river aggraded to a height of nearly half the channel depth, giving rise to a preferential avulsion node within the backwater zone regardless of sea‐level change. As sea‐level rise accelerated, the river responded by avulsing more frequently until reaching a maximum frequency limited by the upstream sediment supply. Experimental results support recent models, field observations, and experiments, and suggest anthropogenic sea‐level rise will introduce more frequent avulsion hazards farther inland than observed in recent history. The experiment also demonstrated that avulsions can occur during sea‐level fall—even within the confines of an incised valley—provided the offshore basin is shallow enough to allow the shoreline to prograde and the river to aggrade. Avulsions create erosional surfaces within stratigraphy that bound beds reflecting the amount of deposition between avulsions. Avulsion‐induced scours overprint erosional surfaces from sea‐level fall, except when the cumulative drop in sea‐level is greater than the channel depth and less than the basin depth. Results imply sea‐level signals outside this range are removed or distorted in delta deposits.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1146/annurev-earth-031621-093732
