Rate Of Relative Sea-level Rise Research Articles

Vertical accretion trends project doughnut-like fragmentation of saltmarshes

Coastal saltmarshes keep pace with sea-level rise through in-situ production of organic material and incorporation of allochthonous inorganic sediment. Here we report rates of vertical accretion of 16 new sediment cores collected proximal to platform edges within saltmarshes located behind four barrier islands along the southeast United States coast. All but two of these exceed the contemporaneous rate of relative sea-level rise, often by a factor of 1.5 or more. Comparison with 80 additional measurements compiled across the Georgia Bight reveals that marshes situated closer to inlets and large bays generally accrete faster than those adjacent to small creeks or within platform interiors. These results demonstrate a spatial dichotomy in the resilience of backbarrier saltmarshes: marsh interiors are near a tipping point, but allochthonous mineral sediment fluxes allow enhanced local resilience along well-exposed and platform-edge marshes. Together, this suggests that backbarrier marshes are trending towards rapid, doughnut-like fragmentation.

Upland forest retreat lags behind sea-level rise in the mid-Atlantic coast.

Ghost forests consisting of dead trees adjacent to marshes are striking indicators of climate change, and marsh migration into retreating coastal forests is a primary mechanism for marsh survival in the face of global sea-level rise. Models of coastal transgression typically assume inundation of a static topography and instantaneous conversion of forest to marsh with rising seas. In contrast, here we use four decades of satellite observations to show that many low-elevation forests along the US mid-Atlantic coast have survived despite undergoing relative sea-level rise rates (RSLRR) that are among the fastest on Earth. Lateral forest retreat rates were strongly mediated by topography and seawater salinity, but not directly explained by spatial variability in RSLRR, climate, or disturbance. The elevation of coastal tree lines shifted upslope at rates correlated with, but far less than, contemporary RSLRR. Together, these findings suggest a multi-decadal lag between RSLRR and land conversion that implies coastal ecosystem resistance. Predictions based on instantaneous conversion of uplands to wetlands may therefore overestimate future land conversion in ways that challenge the timing of greenhouse gas fluxes and marsh creation, but also imply that the full effects of historical sea-level rise have yet to be realized.

A ∼200-year relative sea-level reconstruction from the Wellington region (New Zealand) reveals insights into vertical land movement trends

The sea-level rise threat to New Zealand's coastal cities is regionally exacerbated due to spatially varying vertical land movement (VLM). At Wellington, the capital city, situated adjacent to a major active plate boundary, strong regional spatial and temporal variability of VLM is indicated by the relatively short (∼25 year-long) continuous Global Navigation Satellite Systems (GNSS) network, but until now longer records of VLM have been lacking. Here, a ∼ 200-year-long relative sea-level reconstruction is presented from Pāuatahanui salt marsh in the northern Wellington region. The foraminifera-based relative sea-level reconstruction indicates that ∼1 ± 0.45 m of sudden uplift occurred during the 1855CE Mw 8.2 Wairarapa earthquake. Following this, Pāuatahanui has experienced a mean rate of relative sea-level rise (1855CE to present) of 1.5 ± 0.6 mm/yr, or 2.4 ± 0.8 mm/yr since the start of the twentieth century, consistent with ongoing subsidence in concert with climate-driven sea-level rise. Further acceleration to >3 mm/yr since the 1990s (with 4 mm/yr also possible if the full 95% confidence range is taken into consideration) is consistent with the globally documented acceleration in sea-level rise, although low model precision hampers confidence in this interpretation. This record is the first of its kind from a tectonically complex setting in New Zealand, shedding light on the effects of the historically significant 1855 earthquake, and fills a gap between millennial-scale and contemporary records of VLM with important implications for future sea-level projections in the region.

Sediment Compaction in Experimental Deltas: Toward a Meso‐Scale Understanding of Coastal Subsidence Patterns

AbstractWe present the first investigation of subsidence due to sediment compaction and consolidation in two laboratory‐scale river delta experiments. Spatial and temporal trends in subsidence rates in the experimental setting may elucidate behavior which cannot be directly observed at sufficiently long timescales, except for in reduced scale models such as the ones studied. We compare subsidence between a control experiment using steady boundary conditions, and an otherwise identical experiment which has been treated with a proxy for highly compressible marsh deposits. Both experiments have non‐negligible compactional subsidence rates across the delta‐top, comparable in magnitude to our boundary condition relative sea level rise rate of 250 μm/hr. Subsidence in the control experiment (on average 54 μm/hr) is concentrated in the lowest elevation (<10 mm above sea level) areas near the coast and is likely related to creep induced by a rising water table near the shoreface. The treatment experiment exhibits larger (on average 126 μm/hr) and more spatially variable subsidence rates controlled mostly by compaction of recent marsh deposits within one channel depth (∼10 mm) of the sediment surface. These rates compare favorably with field and modeling based subsidence measurements both in relative magnitude and location. We find that subsidence “hot spots” may be relatively ephemeral on longer timescales, but average subsidence across the entire delta can be variable even at our shortest measurement window. This suggests that subsidence rates over a short time frame may exceed thresholds for marsh platform drowning, even if the long term trend does not.

Early–mid Holocene relative sea-level rise in the Yangtze River Delta, China

Regional-scale Holocene sea-level reconstruction is the key to understanding natural climatic variability. The tidal flat, salt marsh, and tidal floodplain in the Yangtze River Delta were very sensitive to morphological changes and sea-level variation during the early Holocene. In this study, the lithology, radiocarbon ages, sediment grain size, benthic foraminifera, and ostracods of three new cores were analyzed. Twenty-four sea-level index points were extracted from incised-valley fills beneath the westernmost part of the Yangtze River Delta and used to construct a detailed relative sea-level curve for 11.03–7.25 ka. The relative sea level were − 38.90 ± 3.48 and 1.59 ± 3.28 m at 11.03 and 7.25 ka, respectively, and the average rate of sea-level rise was 10.71 mm/yr. The relative sea level gradually increased at 11.03–10.10 and 9.29–8.33 ka from 4.80 to 13.30 and 3.19 to 19.59 mm/yr, respectively. The rate of relative sea-level rise gradually decreased from 19.59 mm/yr at 8.33 ka to 13.40 mm/yr at 7.82 ka and further decreased to 5.53 mm/yr at 7.25 ka. The Yangtze River Delta recorded accelerations in the rate of sea-level rise from 8.72 to 8.18 ka of 14.2–19.59 mm/yr, corresponding to the pre-8.2 ka event. A comparison of the sea-level histories of the Yangtze River Delta and other regions indicates tectonics, glacial isostatic adjustment, and coastal levering effects caused by the marine inundation of the continental shelves. These new sea-level data contribute to the understanding of the difference in the sea-level rise rate during the early–mid Holocene.

Coastal response to Holocene Sea-level change: A case study from Singapore

The deceleration of early to mid-Holocene (10–7 cal. ka BP) relative sea-level rise (RSLR) played a key role in transforming coastal systems from estuaries to deltas. Palaeoenvironmental reconstruction of coastal evolution provide case studies that can help project the response of modern coastal systems to future RSLR. The response of deltas to future RSLR is particularly important to South, Southeast and East Asia which collectively contain 71% of the global coastal population living below 10 m in elevation and 75% of the global coastal floodplain population. However, few Holocene studies of equatorial delta systems exist.Here, we investigate the early to mid-Holocene coastal response to decelerating rates of RSLR through paleoenvironmental reconstruction of the Kallang River Basin, Singapore. We produce a multi-proxy (sedimentology, stable carbon isotope, XRF elemental ratios) record from sediment core MSBH01B to compare with the Holocene relative sea-level record for Singapore. We identify different phases of coastal response through the interplay between accommodation space (A) driven predominantly by RSLR and sedimentation rate (S).In the early Holocene rapid RSLR coupled with low sedimentation rates (A/S = 5.1 ± 0.3) led to mangrove disappearance in the Kallang River Basin coastal area within ∼300 years (9.5–9.2 cal. ka BP). Estuarine sediments were deposited from 9.2 to 8.8 cal. ka BP during continued high rates of RSLR coupled with highest sedimentation rates (A/S = 3.1 ± 0.8) as the coastline retreated. Prodelta sediments were deposited from 8.8 to 8.2 cal. ka BP during decreasing rate of RSLR and high sedimentation rates (A/S = 4.6 ± 5.2). Delta front sediments were deposited during this delta initiation phase from 8.2 to 7.6 cal. ka BP as during a period of low and consistent RSLR and sedimentation rates (A/S = 1.7 ± 0.2). Finally, a prograding delta started forming from 7.6 to 7.2 cal. ka BP during lowest rates of RSLR and sedimentation rates (A/S = 1.7 ± 0.2). Our record provides a case study of possible responses of modern delta systems under a spectrum of predicted sea-level rise scenarios and accompanying sedimentation rates. This study provides an estimated threshold A/S value of 1.7 for coastal retreat to inform policy and mitigation/adaptation measures for Singapore and a simple methodology to obtain local threshold values of other equatorial cities built on floodplain and/or delta systems.

Widespread retreat of coastal habitat is likely at warming levels above 1.5 °C.

Several coastal ecosystems-most notably mangroves and tidal marshes-exhibit biogenic feedbacks that are facilitating adjustment to relative sea-level rise (RSLR), including the sequestration of carbon and the trapping of mineral sediment1. The stability of reef-top habitats under RSLR is similarly linked to reef-derived sediment accumulation and the vertical accretion of protective coral reefs2. The persistence of these ecosystems under high rates of RSLR is contested3. Here we show that the probability of vertical adjustment to RSLR inferred from palaeo-stratigraphic observations aligns with contemporary in situ survey measurements. A deficit between tidal marsh and mangrove adjustment and RSLR is likely at 4 mm yr-1 and highly likely at 7 mm yr-1 of RSLR. As rates of RSLR exceed 7 mm yr-1, the probability that reef islands destabilize through increased shoreline erosion and wave over-topping increases. Increased global warming from 1.5 °C to 2.0 °C would double the area of mapped tidal marsh exposed to 4 mm yr-1 of RSLR by between 2080 and 2100. With 3 °C of warming, nearly all the world's mangrove forests and coral reef islands and almost 40% of mapped tidal marshes are estimated to be exposed to RSLR of at least 7 mm yr-1. Meeting the Paris agreement targets would minimize disruption to coastal ecosystems.

Reducing vertical bias and error in tidal marsh digital elevation models with machine learning and LiDAR derivatives

The vertical bias and uncertainty of LiDAR-derived elevation data in tidal marshes is a major challenge for researchers in these critical ecosystems. Small positive bias errors in land surface elevation can lead to large underestimates of coastal inundation. Previous studies have shown that the bias and uncertainty of elevation data can be reduced using a variety of statistical techniques. However, many studies cover a relatively small extent, cover a large extent at coarse resolution, or rely on local data products that may not be widely available or are of unknown quality. This study aimed to develop and compare bias reduction approaches for digital elevation models (DEMs) using LiDAR derivatives in tidal marshes along Delaware Bay, Delaware, USA, a region that experiences high rates of relative sea-level rise and frequent coastal flooding. In this study, several statistical and machine-learning approaches were evaluated based on their ability to reduce vertical DEM error using field GPS survey data and LiDAR and terrain derivatives as inputs. The evaluated approaches for reducing DEM error included ensembles of multiple linear regressions, kernel-K Nearest Neighbors, gradient boosted regression trees, random forests, and deep neural networks (DNNs), which were compared based on statistical performance metrics. An ensemble of deep neural networks performed best at removing vertical DEM bias and reducing DEM uncertainty, though other approaches also performed well. GPS survey data indicated a mean absolute error, mean bias, and root mean square error in GPS surveys of 11.9 cm, 10.9, and 14.8 cm, respectively, in the original DEM. These error metrics were reduced to 3.5 cm, −0.2 cm, and 5.1 cm in the DNN ensemble-corrected DEM. The DNN ensemble was then applied to all tidal marshes throughout the study area. This corrected DEM clearly showed how interpretations of marsh platform inundation based on uncorrected DEM surfaces could be misguided. The approach used in this study only requires LiDAR-derived products and field surveys, so it may be applied readily in other regions.

Long‐Term Monitoring of Coupled Vegetation and Elevation Changes in Response to Sea Level Rise in a Microtidal Salt Marsh

AbstractTight interplays between physical and biotic processes in tidal salt marshes lead to self‐organization of halophytic vegetation into recurrent zonation patterns developed across elevation gradients. Despite its importance for marsh ecomorphodynamics, however, the response of vegetation zonation to changing environmental forcings remains difficult to predict, mostly because of lacking long‐term field observations of vegetation evolution in the face of changing rates of sea level rise and marsh vertical accretion. Here we present novel data of coupled marsh elevation‐vegetation distribution collected in the microtidal Venice Lagoon (Italy) over nearly two decades. Our results suggest that: (a) despite increasing absolute marsh elevations (i.e., above a fixed datum), vertical accretion rates across most of the studied marsh were not high enough to compensate for relative sea‐level rise (RSLR), thus leading to a progressive marsh drowning; (b) accretion rates ranging 1.7–4.3 mm/year are overall lower than the measured RSLR rate (4.4 mm/year) and strongly site‐specific. Accretion rates vary largely at sites within distances of a few tens of meters, being controlled by local elevation and sediment availability from eroding marsh edges; (c) vegetation responds species‐specifically to changes in environmental forcings by modifying species‐preferential elevation ranges. For the first time, we observe the consistency of a sequential vegetation‐species zonation with increasing marsh elevations over 20 years. We suggest this is the signature of vegetation resilience to changes in external forcings. Our results highlight a strong coupling between geomorphological and ecological dynamics and call for spatially distributed marsh monitoring and spatially explicit biomorphodynamic models of marsh evolution.

Holocene relative sea-level histories of far-field islands in the mid-Pacific

Holocene relative sea-level (RSL) data from far-field islands in the mid-Pacific have been used to validate the ice-melting histories of glacial isostatic adjustment (GIA) models. However, a lack of quality control in the reconstruction of RSL hinders the understanding of regional variability that can constrain ice-equivalent sea-level changes. Here, we present a standardised database of Holocene RSL data from five regions in the mid-Pacific (Cook Islands, Tuamotu Islands, Christmas (Kiritimati) Island, Gilbert Islands and Fiji). We categorised the data as high or low quality based on the susceptibility of samples to age and/or elevation errors. Of the 614 data points that were reviewed, 25% were rejected and 100 sea-level index points (SLIPs) were reinterpreted as limiting data. The new database consists of 141 SLIPs and 262 marine and 56 terrestrial limiting data points reconstructed from a variety of sea-level indicators (e.g., coral microatolls, mangrove peat, beachrock, and beach ridges), of which 71% provide high-quality constraints on RSL. The early to mid Holocene RSL evolution in the Cook Islands, Gilbert Islands and Fiji are poorly constrained due to a lack of high-quality SLIPs and limiting data during this period. The Tuamotu Islands provided the only record of early to mid Holocene evolution of RSL, indicating rapid RSL rise from between −22.9 m and −15.2 m at ∼9.0 ka to between −0.2 m and 0.5 m by ∼6.5 ka, at rates as high as 9.8 ± 5.1 mm/a, with a slowdown in the rate of RSL rise sometime between ∼8.2 ka and ∼6 ka. The Christmas (Kiritimati) Island record indicates stable RSL within ∼1.5 m of present-day levels over the past ∼6.6 ka. In the late Holocene, the Cook Islands record suggests a gradual fall in RSL over the past ∼2.9 ka at rates below 0.1 ± 4.3 mm/a. SLIPs at Fiji also indicate a slight fall in RSL at rates of less than 0.5 ± ∼4.4 mm/a at ∼4 ka, following which RSL fell from above 0.9 m at ∼3 ka to between −0.3 m and 0.6 m by ∼2.5– ∼2.1 ka. We highlight the importance of standardisation and quality control to critically evaluate the processes controlling RSL and validate GIA models. Indeed, the new standardised database has implications for the timing of the mid- Holocene highstand, which has been used to support the ICE-4G and ICE-7G_NA models. Due to the poor constraints of data in the mid-Pacific islands, particularly in the early Holocene, there remains no unique solution for a global ice-melting history.

Recent Acceleration of Wetland Accretion and Carbon Accumulation Along the U.S. East Coast

AbstractThe long‐term stability of coastal wetlands is determined by interactions among sea level, plant primary production, sediment supply, and wetland vertical accretion. Human activities in watersheds have significantly altered sediment delivery from the landscape to the coastal ocean, with declines along much of the U.S. East Coast. Tidal wetlands in coastal systems with low sediment supply may have limited ability to keep pace with accelerating rates of sea‐level rise (SLR). Here, we show that rates of vertical accretion and carbon accumulation in nine tidal wetland systems along the U.S. East Coast from Maine to Georgia can be explained by differences in the rate of relative SLR (RSLR), the concentration of suspended sediments in the rivers draining to the coast, and temperature in the coastal region. Further, we show that rates of vertical accretion have accelerated over the past century by between 0.010 and 0.083 mm yr−2, at roughly the same pace as the acceleration of global SLR. We estimate that rates of carbon sequestration in these wetland soils have accelerated (more than doubling at several sites) along with accelerating accretion. Wetland accretion and carbon accumulation have accelerated more rapidly in coastal systems with greater relative RSLR, higher watershed sediment availability, and lower temperatures. These findings suggest that the biogeomorphic feedback processes that control accretion and carbon accumulation in these tidal wetlands have responded to accelerating RSLR, and that changes to RSLR, watershed sediment supply, and temperature interact to determine wetland vulnerability across broad geographic scales.

Long-term performance and impacts of living shorelines in mesohaline Chesapeake Bay

Shorelines in Chesapeake Bay, and many other estuaries and coastal embayments, are rapidly eroding, with even more rapid loss expected in the future from drivers like urbanization and accelerated relative sea-level rise (RSLR). Past efforts to stabilize shorelines using approaches like riprap or bulkheads generally have resulted in negative ecosystem impacts, resulting in a rise of ecosystem-based approaches using natural and nature-based features (NNBF) such as living shorelines, defined here as narrow marsh fringes with adjacent sills. Living shorelines provide similar ecosystem services as natural marshes but are threatened by the same stressors of environmental change, raising questions about their long-term resiliency and effectiveness in reducing shoreline erosion. Questions also remain about their potential impacts on benthic habitats in adjacent waters. These questions are especially relevant to the Chesapeake, where relatively rapid rates of RSLR and declining sediment supplies have led to widespread marsh loss, and submersed aquatic vegetation (SAV) is a critical indicator for water clarity. This study addresses these questions through field observations in ∼10-year-old living shorelines and SAV habitats adjacent to them, along with observations at nearby reference (unaltered) shorelines. In general, shoreline erosion continued at or above historical rates at reference shorelines, but living shoreline installation builds shorelines seaward and results in net shoreline accretion. While the sand and organic content of bottom sediments in adjacent waters changed at many sites after living shoreline installation, changes were site-specific and typically in the same direction at both living and reference shorelines. These changes did not appear to impact SAV distributions, which followed regional trends likely linked to water clarity. Sediment and nutrient burial in the coastal zone, which includes both intertidal marsh and subtidal SAV habitats, was highest for living shorelines due to the addition of marsh habitat. While this study did not consider direct replacement of SAV with living shorelines, these results suggest that discouraging living shoreline installation in areas with SAV may miss an opportunity to enhance nutrient burial in the coastal zone.

Simulating the Impacts of an Applied Dynamic Adaptive Pathways Plan Using an Agent-Based Model: A Tauranga City, New Zealand, Case Study

Climate change and relative sea-level rise (RSLR) will increasingly expose coastal cities to coastal flooding, erosion, pluvial and fluvial flooding, episodic storm-tide flooding and eventually, permanent inundation. Tools are needed to support adaptive management approaches that allow society to adapt incrementally by making decisions now without creating path dependency and compromising decision-making options in the future. We developed an agent-based model that integrates climate-related physical hazard drivers and socio-economic drivers. We used it to explore how adaptive actions might be sequentially triggered within a low-elevation coastal city in New Zealand, in response to various climate change and socio-economic scenarios. We found that different adaptive actions are triggered at about the same RSLR level regardless of shared socio-economic pathway/representative concentration pathway scenario. The timing of actions within each pathway is dictated mainly by the rate of RSLR and the timing and severity of storm events. For the representative study site, the model suggests that the limits for soft and hard protection will occur around 30 cm RSLR, fully-pumped water systems are viable to around 35 cm RSLR and infrastructure upgrades and policy mechanisms are feasible until between 40 cm and 75 cm RSLR. After 75 cm RSLR, active retreat is the only remaining adaptation pathway.

Coastal wetland adaptability to sea level rise: The neglected role of semi‐diurnal vs. diurnal tides

AbstractTidal marshes and mangroves are threatened by relative sea level rise (RSLR) in certain regions on Earth. Elsewhere, these coastal wetlands can adapt through sediment accretion and resulting surface elevation gain. Studies identifying drivers of the global variability in coastal wetland adaptability to RSLR ignored the role of the tidal pattern, varying from semi‐diurnal to diurnal globally. Here, we present a meta‐analysis, including 394 marsh and mangrove sites worldwide, and demonstrate that the tidal pattern explains ~ 25% of the variability in wetland elevation response to RSLR. Using a numerical model, we illustrate that less frequent, diurnal tides trigger lower sediment accretion rates, hence higher wetland vulnerability to RSLR, for various values of RSLR rates, tidal range and sediment supply. Our findings reveal a previously overlooked but relevant driver of coastal wetland adaptability to RSLR and call for new research as tidal patterns may affect other wetland ecosystem functions and services.

Anthropogenic impacts on tidal creek sedimentation since 1900.

Land cover and use around the margins of estuaries has shifted since 1950 at many sites in North America due to development pressures from higher population densities. Small coastal watersheds are ubiquitous along estuarine margins and most of this coastal land-cover change occurred in these tidal creek watersheds. A change in land cover could modify the contribution of sediments from tidal creek watersheds to downstream areas and affect estuarine habitats that rely on sediments to persist or are adversely impacted by sediment loading. The resilience of wetlands to accelerating relative sea-level rise depends, in part, on the supply of lithogenic sediment to support accretion and maintain elevation; however, subtidal habitats such as oyster reefs and seagrass beds are stressed under conditions of high turbidity and sedimentation. Here we compare sediment accumulation rates before and after 1950 using 210Pb in 12 tidal creeks across two distinct regions in North Carolina, one region of low relief tidal-creek watersheds where land cover change since 1959 was dominated by fluctuations in forest, silviculture, and agriculture, and another region of relatively high relief tidal-creek watersheds where land-use change was dominated by increasing suburban development. At eight of the creeks, mass accumulation rates (g cm-2 y-1) measured at the outlet of the creeks increased contemporaneously with the largest shift in land cover, within the resolution of the land-cover data set (~5-years). All but two creek sites experienced a doubling or more in sediment accumulation rates (cm yr-1) after 1950 and most sites experienced sediment accumulation rates that exceeded the rate of local relative sea-level rise, suggesting that there is an excess of sediment being delivered to these tidal creeks and that they may slowly be infilling. After 1950, land cover within one creek watershed changed little, as did mass accumulation rates at the coring location, and another creek coring site did not record an increase in mass accumulation rates at the creek outlet despite a massive increase in development in the watershed that included the construction of retention ponds. These abundant tidal-creek watersheds have little relief, area, and flow, but they are impacted by changes in land cover more, in terms of percent area, than their larger riverine counterparts, and down-stream areas are highly connected to their associated watersheds. This work expands the scientific understanding of connectivity between lower coastal plain watersheds and estuaries and provides important information for coastal zone managers seeking to balance development pressures and environmental protections.

Coastal switching of dominant depositional processes driven by decreasing rates of Holocene sea-level rise along the macrotidal coast of Gochang, SW Korea

ABSTRACTDecreasing rates of eustatic sea-level rise during the Holocene accompanied the deposition of transgressive coastal deposits worldwide. However, unraveling how transgressive deposition varies in response to different rates of relative sea-level (RSL) rise is limited by the scarcity of long (10+ m) well-dated cores spanning the entire middle to late Holocene record along macrotidal coasts. To investigate the sedimentary response of this macrotidal coast to decreasing rates of RSL rise, we acquired four cores up to 32 m in length and Chirp seismic profiles along the west coast of Korea. Core sediments were analyzed in terms of sedimentary texture, structure, and facies. Nineteen optically stimulated luminescence (OSL) and fourteen 14C accelerated mass spectrometry (AMS) ages constrain the timing of deposition of the sandy sediments. This relatively dense distribution of ages is used to determine how deposition rates changed through time. We also use a compilation of previously published RSL indices for the southwestern Korean coast in order to better constrain RSL changes through time. Results show that the evolution of the Gochang coastline switched from a tide-dominated environment to a wave-dominated environment during the latter stage of transgression as the rate of the sea-level rise decreased. Rugged antecedent topography likely led to the development of tidal currents and the formation of a tide-dominated tidal flat during rapid RSL rise from 10 to 6 ka. As the tidal channels filled with fine-grained sediments from 6 to 1 ka, tidal amplification likely waned leading to a greater role of wave energy in shaping the formation of the sandy open-coast tidal flat. Since 1 ka, wave-dominated environments formed sand-rich tidal beaches and flats. Decreasing changes in rates of the RSL rise resulted in changes in depositional environments from a tide-dominated intertidal flat to an open-coast tidal flat and finally a wave-dominated tidal beach. This study highlights the important role that rates of RSL rise play on not only sedimentation rates in a shelf setting but also playing a role in the switch from a tide-dominated to a wave-dominated setting.

Benefits of subsidence control for coastal flooding in China

Land subsidence is impacting large populations in coastal Asia via relative sea-level rise (RSLR). Here we assesses these risks and possible response strategies for China, including estimates of present rates of RSLR, flood exposure and risk to 2050. In 2015, each Chinese coastal resident experienced on average RSLR of 11 to 20 mm/yr. This is 3 to 5 times higher than climate-induced SLR, reflecting that people are concentrated in subsiding locations. In 2050, assuming these subsidence rates continue, land area, population and assets exposed to the 100-year coastal flood event is 20%-39%, 17%-37% and 18%-39% higher than assuming climate change alone, respectively. Realistic subsidence control measures can avoid up to two thirds of this additional growth in exposure, with adaptation required to address the residual. This analysis emphasizes subsidence as a RSLR hazard in China that requires a broad-scale policy response, utilizing subsidence control combined with coastal adaptation.

Integrated coastal subsidence analysis using InSAR, LiDAR, and land cover data

Land subsidence is an important cause of relative sea-level rise along the Gulf Coast. There is a lack of effective monitoring of coastal subsidence with high accuracy and high spatial resolution for improving coastal risk assessment and mitigation. This study is the first attempt to integrate satellite interferometric synthetic aperture radar (InSAR) and airborne light detection and ranging (LiDAR) methods to investigate the spatiotemporal pattern of coastal subsidence. The study area is around Eagle Point, Texas, a region known for its fast rate of relative sea-level rise in recent decades. From 2006 to 2011, the line-of-sight velocities were up to −33 mm/year based on ascending ALOS-1 PALSAR-1 images. From 2016 to 2021, the vertical velocities were up to −34 mm/year based on ascending and descending Sentinel-1 images. Additional details of the subsidence pattern were revealed by incorporating the surface difference derived from 1-m airborne LiDAR results. Comparisons of the InSAR-derived velocities from image time series and the LiDAR-derived surface changes from time-lapsed observations were conducted at different spatial levels with linkages to land cover patterns and topography. The results showed that local subsidence rates could vary significantly below the spatial resolution of InSAR results, indicating a valuable role of airborne LiDAR results in extending InSAR results to parcel and building levels and explaining subpixel uncertainties. Also, subsidence appeared to be stronger in vegetated areas than in developed areas and negatively correlated with surface imperviousness. The magnitude of subsidence was not correlated with elevation along selected transect lines. Overall, this study demonstrated the benefits of combining InSAR results with other geospatial datasets to characterize coastal subsidence. In particular, the high vertical accuracy of InSAR results and the high spatial resolution of airborne LiDAR results could be complementary, highlighting the necessity of multi-resolution data fusion to support studies on coastal flood vulnerability, infrastructure reliability, and erosion control.

Accelerated sea-level rise is suppressing CO 2 stimulation of tidal marsh productivity: A 33-year study

Accelerating relative sea-level rise (RSLR) is threatening coastal wetlands. However, rising CO2 concentrations may also stimulate carbon sequestration and vertical accretion, counterbalancing RSLR. A coastal wetland dominated by a C3 plant species was exposed to ambient and elevated levels of CO2 in situ from 1987 to 2019 during which time ambient CO2 concentration increased 18% and sea level rose 23 cm. Plant production did not increase in response to gradually rising ambient CO2 concentration during this period. Elevated CO2 increased shoot production relative to ambient CO2 for the first two decades, but from 2005 to 2019, elevated CO2 stimulation of production was diminished. The decline coincided with increases in relative sea level above a threshold that hindered root productivity. While elevated CO2 stimulation of elevation gain has the potential to moderate the negative impacts of RSLR on tidal wetland productivity, benefits for coastal wetland resilience will diminish in the long term as rates of RSLR accelerate.

Sustainability of the coastal zone of the Ganges-Brahmaputra-Meghna delta under climatic and anthropogenic stresses

The Ganges-Brahmaputra-Meghna (GBM) delta is one of the world's largest deltas. It is currently experiencing high rates of relative sea-level rise of about 5 mm/year, reflecting anthropogenic climate change and land subsidence. This is expected to accelerate further through the 21st Century, so there are concerns that the GBM delta will be progressively submerged. In this context, a core question is: can sedimentation on the delta surface maintain its elevation relative to sea level? This research seeks to answer this question by applying a two-dimensional flow and morphological model which is capable of handling dynamic interactions between the river and floodplain systems and simulating floodplain sedimentation under different flow-sediment regimes and anthropogenic interventions. We find that across a range of flood frequencies and adaptation scenarios (including the natural polder-free state), the retained volume of sediment varies between 22% and 50% of the corresponding sediment input. This translates to average rates of sedimentation on the delta surface of 5.5 mm/yr to 7.5 mm/yr. Hence, under present conditions, sedimentation associated with quasi-natural conditions can exceed current rates of relative sea-level rise and potentially create new land mass. These findings highlight that encouraging quasi-natural conditions through the widespread application of active sediment management measures has the potential to promote more sustainable outcomes for the GBM delta. Practical measures to promote include tidal river management, and appropriate combinations of cross-dams, bandal-like structures, and dredging.