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Future sea-level rise poses an existential threat for many river deltas, yet quantifying the effect of sea-level changes on these coastal landforms remains a challenge. Sea-level changes have been slow compared to other coastal processes during the instrumental record, such that our knowledge comes primarily from models, experiments, and the geologic record. Here we review the current state of science on river delta response to sea-level change, including models and observations from the Holocene until 2300 CE. We report on improvements in the detection and modeling of past and future regional sea-level change, including a better understanding of the underlying processes and sources of uncertainty. We also see significant improvements in morphodynamic delta models. Still, substantial uncertainties remain, notably on present and future subsidence rates in and near deltas. Observations of delta submergence and land loss due to modern sea-level rise also remain elusive, posing major challenges to model validation. ▪ There are large differences in the initiation time and subsequent delta progradation during the Holocene, likely from different sea-level and sediment supply histories. ▪ Modern deltas are larger and will face faster sea-level rise than during their Holocene growth, making them susceptible to forced transgression. ▪ Regional sea-level projections have been much improved in the past decade and now also isolate dominant sources of uncertainty, such as the Antarctic ice sheet. ▪ Vertical land motion in deltas can be the dominant source of relative sea-level change and the dominant source of uncertainty; limited observations complicate projections. ▪ River deltas globally might lose 5% (∼35,000 km 2 ) of their surface area by 2100 and 50% by 2300 due to relative sea-level rise under a high-emission scenario. Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates. 

Abstract. Flood-protection levees have been built along rivers and coastlines globally. Current datasets, however, are generally confined to territorial boundaries (national datasets) and are not always easily accessible, posing limitations for hydrologic models and assessments of flood hazard. Here, we bridge this knowledge gap by collecting and standardizing global flood-protection levee data for river deltas into the open-source global river delta levee data environment, openDELvE. In openDELvE, we aggregate levee data from national databases, reports, maps, and satellite imagery. The database identifies the river delta land areas that the levees have been designed to protect. Where data are available, we record the extent and design specifications of the levees themselves (e.g., levee height, crest width, construction material) in a harmonized format. The 1657 polygons of openDELvE contain 19 248 km of levees and 44 733.505 km2 of leveed area. For the 153 deltas included in openDELvE, 17 % of the land area is confined by flood-protection levees. Around 26 % of delta population lives within the 17 % of delta area that is protected, making leveed areas densely populated. openDELvE data can help improve flood exposure assessments, many of which currently do not account for flood-protection levees. We find that current flood hazard assessments that do not include levees may exaggerate the delta flood exposure by 33 % on average, but up to 100 % for some deltas. The openDELvE is made public on an interactive platform (https://www.opendelve.eu/, 1 October 2022), which includes a community-driven revision tool to encourage inclusion of new levee data and continuous improvement and refinement of open-source levee data. 

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.

 

Abstract. The natural wetlands of coastal Louisiana areexperiencing rapid subsidence rates averaging 9±1 mm yr−1. Recentmeasurements based on GPS data and CRMS surface elevation tables (SETs) haveshown that most of the subsidence is shallow and occurs in the uppermost 5meters. Sources of subsidence and the origin of their spatial variabilityare strongly debated. Here we use CRMS SETs together with historic maps ofcoastal Louisiana to explore two hypotheses: (i) shallow subsidence is aresult of accommodation created by (long-term) deep subsidence processes andself-weight consolidation, and (ii) changes in marsh hydrology (groundwaterand surface water flows) have led to a recent increase in shallowsubsidence. First, we find that, although self-weight consolidation would result ingenerally high observed shallow subsidence rates, it does not explain therates nor the spatial variability of the CRMS SET data. Second, based onhistoric maps, we find that shallow subsidence rates are significantlyhigher for CRMS sites where shipping canals have reduced their distance tothe marsh edge. This is potentially a result from increased sedimentdeposition, but CRMS data also show altered groundwater levels near themarsh edge. We find some indication that prolonged periods of low watercould have led to increases in effective stresses that explain some of therapid rates of shallow subsidence observed along Louisiana's coastline. 

Abstract. Barrier islands are low-lying coastal landforms vulnerable toinundation and erosion by sea level rise. Despite their socioeconomic andecological importance, their future morphodynamic response to sea level riseor other hazards is poorly understood. To tackle this knowledge gap, weoutline and describe the BarrieR Inlet Environment (BRIE) model that cansimulate long-term barrier morphodynamics. In addition to existing overwashand shoreface formulations, BRIE accounts for alongshore sediment transport,inlet dynamics, and flood–tidal delta deposition along barrier islands.Inlets within BRIE can open, close, migrate, merge with other inlets, andbuild flood–tidal delta deposits. Long-term simulations reveal complexemergent behavior of tidal inlets resulting from interactions with sea levelrise and overwash. BRIE also includes a stratigraphic module, whichdemonstrates that barrier dynamics under constant sea level rise rates canresult in stratigraphic profiles composed of inlet fill, flood–tidal delta,and overwash deposits. In general, the BRIE model represents a process-basedexploratory view of barrier island morphodynamics that can be used toinvestigate long-term risks of flooding and erosion in barrier environments.For example, BRIE can simulate barrier island drowning in cases in which theimposed sea level rise rate is faster than the morphodynamic response of thebarrier island.