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The Labyrinth in the McMurdo Dry Valleys of Antarctica is characterized by large bedrock channels emerging from beneath the margin of Wright Upper Glacier. To study the morphodynamics of large subglacial channels cut into bedrock, we develop herein a numerical model based on the classical theory of subglacial channels and recent results on bedrock abrasion by saltating bed load. Model results show that bedrock abrasion in subglacial channels with pressurized flow reaches a maximum at an intermediate distance up-ice from the glacier snout for a wide range of sediment grain sizes and sediment loads. Close to the snout, the velocity is too low and the sediment particles cannot be mobilized. Far from the snout, the flow accelerates and sediment is transported in suspension, thus limiting particle impacts at the channel bottom and reducing abrasion. This non-monotonic relationship between subglacial flow and bedrock abrasion produces concave up bottom profiles in subglacial channels and potential cross-section constrictions after channel confluences. Both landforms are present in the bedrock channels of the Labyrinth. We therefore conclude that these geomorphic features are a possible signature of bedrock abrasion, rather than glacial scour, and reflect the complex interplay between transport rate, sediment load, and transport capacity in subglacial channels. [ABSTRACT FROM AUTHOR]

Deltas are crucial for land building and ecological services due to their ability to store mineral sediment, carbon and potential pollutants. A decline in suspended sediment discharge in large rivers caused by the construction of mega‐dams might imperil deltaic flats and wetlands. However, there has not been clear evidence of a sedimentary shift in the downstream tidal flats that feed coastal wetlands and the intertidal zone with sediments. Here, integrated intertidal/subaqueous sediment samples, multi‐year bathymetries, fluvial and deltaic hydrological and sediment transport data in the Nanhui tidal flats and Nanhui Shoal in the Changjiang (Yangtze) Delta, one of the largest mega‐deltas in the world, were analysed to discern how sedimentary environments changed in response to the operations of the Three Gorges Dam. Results reveal that the coarser sediment fractions of surficial sediments in the subaqueous Nanhui Shoal increased between 2004 to 2021, and the overall grain size coarsened from 18.5 to 27.3 μm. Moreover, intertidal sediments in cores coarsened by 25% after the 1990s. During that period, the northern part of the Nanhui Shoal suffered large‐scale erosion, while the southern part accreted in recent decades. Reduced suspended sediment discharge of the Changjiang River combined with local resuspension of fine‐grained sediments are responsible for tidal flat erosion. This study found that the spatial pattern of grain‐size parameters has shifted from crossing the bathymetric isobaths to being parallel to them. Higher tide level and tidal range induced by sea‐level rise, an upstream increase in bed shear stress and larger waves likely further exacerbated erosion and sediment coarsening in deltaic flats. As a result, this sediment‐starved estuary coupled with sea‐level rise and artificial reclamations have enhanced the vulnerability of tidal flats in Changjiang Delta, this research is informative to the sedimentary shift of worldwide mega‐deltas. [ABSTRACT FROM AUTHOR]

Recent climate change is causing most lakes on the Tibetan Plateau to grow at an unprecedented rate. Changes in the physical properties and water storage of the lakes are now relatively well documented. Yet the impacts on their water quality remain poorly understood. Turbidity is a well-established optical water-quality indicator related to suspended particulate matter concentration which can affect vertical light attenuation and ecosystem functioning. Here, we use remotely sensed data to assess the seasonal and long-term variations in turbidity in Siling Lake, one of the fastest growing lakes on the Tibetan Plateau, and to identify potential driving mechanisms of this change. The lake experiences two distinct peaks of turbidity during the year: one in August (warm season) caused by the seasonal influx of sediments from the Zagya Zangbo River, and one in December (cold season) caused by the wind-driven resuspension of sediments along the lakes' shorelines. The analysis further revealed a persistent increasing trend that doubled the average lake turbidity between 2000 and 2017. Evidence suggests this rise in turbidity results from a climate-driven increase in sediment supply from the Zagya Zangbo River, and from sediment resuspension associated with the erosion of shorelines recently submerged during the rapid expansion of the lake (paleoshorelines). Our results highlight the vulnerability of the Tibetan Lakes' water quality to climate change. Unlabelled Image • Seasonal dynamics of Siling-Lake turbidity is characterized by two distinct peaks. • The lake turbidity has risen steadily since 2000, leading to a doubling by 2017. • Turbidity increase is related to a climate-driven increase in sediment supply. • Turbidity increase is also related to sediment resuspension on paleo-shorelines. [ABSTRACT FROM AUTHOR]

Abstract Salt marshes are deemed as one of the most dynamic and valuable ecosystems on Earth. Recently, salt marsh deterioration and loss have become widespread because of anthropogenic stressors and sea level rise. Long-term acquisition of spatial information on salt marsh vegetation communities is thus critical to detect the general evolutionary trend of marsh ecosystems before irreversible change occurs. Medium resolution imagery organized in inter-annual time series has been proven suitable for large-scale mapping of salt marsh vegetation. For long-term monitoring purpose, the challenge still lies in developing time series based on data with sparse and uneven temporal distribution. This paper proposes a flexible Monthly NDVI Time-Series (MNTS) approach to achieve multi-temporal classification maps of salt marsh vegetation communities in the Virginia Coast Reserve, USA, by utilizing all viable Landsat TM/ETM + images during the period 1984–2011. Salt marsh vegetation communities are identified on a reference MNTS spanning 12 months with an overall accuracy of 0.898, approximately 0.107 higher than classifications using single images. Utilizing a flexible selection process based on the reference MNTS, a significant inverse hyperbolic relationship emerges between overall accuracy and average length of the time series. Based on these results, eight classification maps with average accuracy of 0.844 and time interval of 2–5 years are acquired. A spatio-temporal analysis of the maps indicates that the upper low marsh vegetation community has diminished by 19.4% in the study period, with a recent acceleration of losses. The conversion of marsh area to vegetation communities typical of low elevations (37.7 km2) is more than twice the conversion to vegetation communities typical of high elevations (18.3 km2), suggesting that salt marsh ecosystems at the Virginia Coast Reserve are affected by sea level rise. Highlights • Monthly NDVI time-series is capable to accurate discriminate salt marsh vegetation. • Flexible monthly NDVI time-series is applied to all viable Landsat TM/ETM + images. • A hyperbolic relationship exists between accuracy and timespan of classification maps. • Temporal evolution of salt marsh vegetation in Virginia Coast Reserve is discerned. [ABSTRACT FROM AUTHOR]

Estuarine beaches near large rivers are dynamic systems constantly shaped by tides, waves, and fluvial sediment inputs. However, little research has been done on the intrinsic characteristics of these geomorphic systems. Using eleven high resolution bathymetries, our results show that human disturbance mingled with natural forcings have induced bathymetric changes in Nanhui beach in the Changjiang estuary, China. Isobaths display a fractal geometry, with a lower fractal dimension when tides smooth the bathymetry and a higher dimension when waves dominate. Rates of sediment accretion and erosion present a Gaussian distribution driven by tidal and wave action. Episodic extreme wave forcing or frequent land reclamation is responsible for the intermittent adjustment of the estuarine beach bathymetry. After these events the distribution of erosion and accretion becomes power-law, possibly indicating disequilibrium. The fractal dimension of isobaths and the distribution of erosion and deposition rates can therefore be used as metrics to determine the dominant processes in estuarine beaches and whether the system is close to equilibrium or not. [ABSTRACT FROM AUTHOR]

Abstract: Cheniers are sandy ridges parallel to the coast separated by muddy deposits. Here we explore the development of erosional chenier plains, which form by winnowing during storms, through dimensional analysis and numerical results from the morphodynamic model Delft3D‐SWAN. Our results highlight that wave energy and inner‐shelf slope play an important role in the formation of erosional chenier plains. We further show that different sediment characteristics and wave climates lead to three alternative coastal landscapes: sandy strandplains, mudflats, or the more complex erosional chenier plains. Low inner‐shelf slopes are the most favorable for mudflat and chenier plain formation, while high slopes decrease the likelihood of mudflat development and preservation, favoring the formation of strandplains. The present study shows that erosional cheniers can form only when there is enough sediment availability to counteract wave action and for a specific range of shelf slopes. [ABSTRACT FROM AUTHOR]

Time-series analysis of Landsat imagery was used to evaluate trends in species zonation in a restored mangrove forest in the Mekong Delta, Vietnam. Dating back throughout the primary expansion of the forest, the Landsat archive provides a unique opportunity to examine the evolution of a restored forest in all stages of maturity. Through temporal trend analysis, areas of the forest were divided into four development stages: Pre-mangrove water, initial mangrove colonization, a period dominated by Sonneratia spp., and the arrival and zonation of secondary species. Field inventory data was used in conjunction with satellite data to investigate the geomorphic and hydrologic influences behind the species zonation. We hypothesize that the development of Sonneratia spp. facilitates initial sedimentation. The trees mature at higher soil elevations and a region develops with low forest density, high light availability, and reduced tidal inundation. Multi-species zonation then develops through the timely exploitation of the geomorphic conditions suitable for the establishment of secondary species. [ABSTRACT FROM AUTHOR]

Mangroves colonize tropical shorelines, protecting coastal communities and providing valuable ecosystem services. Mangroves associated with deltas cope with a very dynamic environment characterized by strong gradients in salinity, deposition triggered by sediment inputs, and erosion caused by waves and currents. Mangroves are adapted to this ever-changing landscape, with different species colonizing different elevations in response to inundation frequency. A series of feedbacks between hydrodynamics, sediment transport, and mangroves was observed in a fringe forest of the Mekong Delta, Vietnam. Sonneratia spp. rapidly encroach upon sandy areas because the stable substrate favors seedling establishment. In contrast, fewer seedlings are present in muddy locations where currents and waves frequently rework the bottom. Along muddy shorelines that are eroding, turbulence increases local scour near roots and trunks, undercutting the trees. Enhanced sediment accumulation due to delta progradation can smother the mangrove roots and lead to forest dieback. We find clear evidence that mangroves affect both hydrodynamics and sediment transport, thus engineering the landscape and enhancing sediment trapping and delta progradation. Sonneratia spp. are replaced by Aegiceras corniculatum, Avicennia marina, and Nypa fruticans when the seabed becomes high enough, indicating that ecological succession is present in a fast prograding deltaic environment. Thus, it is imperative to determine the small-scale feedbacks between mangroves, hydrodynamics, and sediment transport in order to build quantitative ecogeomorphic models of deltaic sedimentation that can be used to explain the distribution of mangrove species, the forest structure, and largescale dynamics in a tropical deltaic setting. [ABSTRACT FROM AUTHOR]

The temporal evolution of suspended sediment concentration (SSC) in a river debouching into the ocean provides vital insights into erosion processes in the watershed and dictates the evolution of the inner continental shelf. While the delivery of sediment from rivers to the ocean has received special attention in the recent past, few studies focused on the variability and dynamics of river SSC, especially in the Changjiang (Yangtze) river, China, the longest river in Asia. Here, variations in SSC delivered by the Changjiang River to the East China Sea and possible causes of its variability were detected based on a long-term time series of daily SSC and monthly water discharge measured at the Datong gauging station. The SSC data are further compared to a hydrological analysis of yearly precipitation covering the entire catchment. The results indicate the presence of a decline in SSC in the period 1956–2013, which can be divided into three phases: (i) high SSC (0.69 kg/m 3 ) in the wet season and low SSC (0.2 kg/m 3 ) in the dry season from 1956 to 1970; (ii) relative high SSC (0.58 kg/m 3 ) in the wet season and low SSC (0.15 kg/m 3 ) in the dry season from 1971 to 2002; and (iii) low SSC (0.19 kg/m 3 ) in the wet season and very low SSC (0.09 kg/m 3 ) in the dry season after 2002. These three periods have a mean yearly SSC values of 0.62, 0.42, and 0.18 kg/m 3 , respectively. Compared with 1956–1970, the slope of the rating curve between SSC and water discharge decreased, respectively, by 2% and 30% during the period 1971–2002 and 2002–2013. Soil erosion, dam construction, and banks reinforcement along the Changjiang River are the main causes of SSC variations. Fluctuations in water discharge are also controlling the SSC long-term variations. Specifically, from 1956 to 1970, the effect of soil erosion overrules that of dam impoundment, which is likely responsible for the high SSC; during the period 1970–2002, the influence of dam impoundment increases while that of soil erosion decreases, which together produce a small reduction in SSC. Since 2002, the impact of soil erosion further decreases and large-scale sediment trapping behind the Three Gorges Dam is responsible for the occurrence of extremely low SSC. The results presented herein for the Changjiang River can inform a better management strategy of sediment resources and water quality for both the river and the coast. Our conclusions can be well applied to other rivers discharging in the ocean subject to similar human activities. [ABSTRACT FROM AUTHOR]

Many tidally-dominated estuaries of the world are experiencing variations in bottom topography due to changes in natural forcings and intensive human activities. Here we focus on the morphological evolution of the North Branch (NB), a tidally-dominated distributary of the Changjiang estuary. Our analysis is based on long-term bathymetric and hydrological data collected between 1950 and 2010. The results show that mean water depth, channel volume below 0 m, and channel volume below − 5 m have respectively decreased by 43%, 53% and 92% in the last 50 years. A reduction of the whole estuarine surface with aggradation in elongated tidal sand bars and erosion at the mouth are the main morphological variations of the NB, while a decrease in channel volume below − 5 m due to infilling is the second mode of morphological change. While the drastic decrease in sediment load from upstream is likely unrelated to the silting of the NB, local land reclamation along the banks is directly responsible for the reduction of estuarine surface area and related tidal prism. Between 1958 and 2013, enhanced flood-tide currents resulted in a large import of sediments from offshore into the NB, triggering a sustained decrease in channel volume below 0 m. It is argued that the recovery of the funnel-shaped configuration of the estuary by restoring mud flats over 0 m, dredging the southern part of the estuary bend and forbidding land reclamation could prevent the silting of the NB, otherwise the NB will likely vanish in few decades. [ABSTRACT FROM AUTHOR]

We present high-resolution field measurements of five sites along the United States Atlantic Coast, and cellular automata simulations, to investigate the erosion of marsh boundaries by wave action. According to our analysis, when salt marshes are exposed to high wave energy conditions their boundaries erode uniformly. The resulting erosion events follow a Gaussian distribution, yielding a relatively smooth shoreline. On the contrary, when wind waves are weak and the local marsh resistance is strong, jagged marsh boundaries form. In this case, erosion episodes have a long-tailed frequency magnitude distribution with numerous low-magnitude events, but also high-magnitude episodes. The logarithmic frequency magnitude distribution suggests the emergence of a critical state for marsh boundaries, which would make the prediction of failure events impossible. Internal physical processes allowing salt marshes to reach this critical state are geotechnical and biological, and related to the nonhomogeneity of salt marshes whose material discontinuities act as stress raisers. [ABSTRACT FROM AUTHOR]

• Brackish groundwater affects tree sapling regeneration in a coastal forest. • Rainfall events affect groundwater level and salinity in the short-term (hours). • Storm surges are felt in the salinity trend as slow increments. • Evapotranspiration and groundwater fluxes are significant in the long-term. • Salinity at the roots depth is high when the water table is high. The migration of salt marshes into forests along coastal regions is nowadays well documented. Sea level rise and storms threaten coastal forests by increasing groundwater levels and salinity. Salinization is the main cause of forest conversion to salt marsh in North America. In this paper we study groundwater levels and salinity in two wells installed at the border between forest and salt marsh in the lower Delmarva peninsula, USA. The upper well is located in the regenerative forest, where recruitment is still possible, while the lower well is located in the persistent forest, where only mature trees survive. Groundwater in the upper well is fresh at the root depth, while in the lower well the mean salinity is 8 ppt. Our data suggest that rainfall has an instantaneous effect on salinity and groundwater levels, but it does not affect salinity and groundwater levels on longer periods (weeks to months). Groundwater levels and salinity reflect the hydraulic gradient toward the marsh (a proxy for outgoing water fluxes), the uphill hydraulic gradient (a proxy for incoming water fluxes) and temperature (a proxy for evapotranspiration). Salinity increases when groundwater levels are high. To explain this result, we put forward the hypothesis that a high water table favors the flux of surficial, fresh water to the marsh, and loss of freshwater by evapotranspiration. These losses are likely replenished by saltier water moving at depth. [ABSTRACT FROM AUTHOR]

Salt marshes are among the most productive ecosystems on Earth, providing nurseries for fish species and shelter and food for endangered birds. Salt marshes also mitigate the impacts of hurricanes and tsunamis, and sequester large volumes of carbon in their peat soil. Understanding the mechanisms responsible for marsh stability or deterioration is therefore a key issue for society. Sea level rise is often viewed as the main driver of salt marsh deterioration. However, while salt marshes can reach equilibrium in the vertical direction, they are inherently unstable in the horizontal direction. Marsh expansion driven by sediment supply rarely matches lateral erosion by waves, creating a dynamic landscape. Recent results show that marsh collapse can occur in the absence of sea level rise if the rate at which sediment is eroded at marsh boundaries is higher than the input of sediment from nearby rivers or from the continental shelf. We propose that the horizontal dynamics and related sediment fluxes are key factors determining the survival of salt marshes. Only a complete sediment budget between salt marshes and nearby tidal flats can determine the fate of marshes at any given location, with sea level rise being only one among many external drivers. Ancient Venetians understood this dynamic very well. They manipulated the supply of sediment to the Venice lagoon, Italy, in order to control the long-term evolution of the intertidal landscape. [ABSTRACT FROM AUTHOR]

High rates of wave-induced erosion along salt marsh boundaries challenge the idea that marsh survival is dictated by the competition between vertical sediment accretion and relative sea-level rise. Because waves pounding marshes are often locally generated in enclosed basins, the depth and width of surrounding tidal flats have a pivoting control on marsh erosion. Here, we show the existence of a threshold width for tidal flats bordering salt marshes. Once this threshold is exceeded, irreversible marsh erosion takes place even in the absence of sea-level rise. This catastrophic collapse occurs because of the positive feedbacks among tidal flat widening by wave-induced marsh erosion, tidal flat deepening driven by wave bed shear stress, and local wind wave generation. The threshold width is determined by analyzing the 50-y evolution of 54 marsh basins along the US Atlantic Coast. The presence of a critical basin width is predicted by a dynamic model that accounts for both horizontal marsh migration and vertical adjustment of marshes and tidal flats. Variability in sediment supply, rather than in relative sea-level rise or wind regime, explains the different critical width, and hence erosion vulnerability, found at different sites. We conclude that sediment starvation of coastlines produced by river dredging and damming is a major anthropogenic driver of marsh loss at the study sites and generates effects at least comparable to the accelerating sea-level rise due to global warming. [ABSTRACT FROM AUTHOR]

Abstract: In this paper we develop a finite volume model to solve the two-dimensional shallow water equations governing the propagation of two superimposed layers, with the upper water layer carrying a dilute sediment suspension, and the underlaying layer being a high concentration non-Newtonian fluid mud mixture. The model formulation contains non-conservative terms as well as source terms. We propose a scheme able to deal with varying topography and dry areas, providing well-balanced solutions when both water and fluid mud are quiescent. The model is tested against both exact solutions and numerical examples. The results show the ability of the model to deal with wetting and drying of both water and fluid mud layers, providing mass-conservative solutions. Moreover, the model solves discontinuities and steep fronts, computing accurate and oscillation-free solutions. [Copyright &y& Elsevier]

Tidal deltas are characterized by a dendritic network of distributaries that transport water and sediments to the ocean. Here, I show that the distributaries self-organize to uniformly redistribute the tidal prism across the entire delta system. The 2 opposite mechanisms of channel formation by avulsion and channel abandonment drive the entire delta toward a critical state at which every channel is close to the silting threshold. Under these conditions the delta reaches self-organized criticality, with changes of its planimetric channel distribution occurring across several spatial scales. [ABSTRACT FROM AUTHOR]

Abstract: During sea-level low stands continental shelves were dissected by a network of channels somewhat resembling today''s coastal plain streams. The network was subsequently buried or erased by marine processes during sea-level transgression, so that only some tracts are still conserved in the geological record. Herein we use a numerical model to study the effect of base level change by sea-level fall on the total channel incision. We find that four factors control the total incision on the shelf: (i) the presence of convex deposits; (ii) the evolution of the rivers towards equilibrium (graded) conditions; (iii) geometrical differences between coastal plain and shelf; and (iv) the exposure of the continental slope. The conceptual model is then applied to the Adriatic Sea, Italy. Simulations show that incisions in the Adriatic shelf develop in high stand fluvial deposits in the early stages of sea-level fall. At lower sea level, fluvial incision occurs in the mid-Adriatic due to the regrading of the Po River after the capture of the Apennine streams in its drainage system. [Copyright &y& Elsevier]

Abstract: Field observations and satellite images indicate that tsunami waves exhibit specific patterns during flooding and recession forming characteristic incisions in the coastal landscape. To study these incisions we analyze high resolution remote sensing images of the coastline of Indonesia, Thailand and Sri Lanka impacted by the tsunami of December 26th, 2004. The analysis sheds light on the different mechanisms by which currents scour incisions during the flooding and receding phases of a tsunami. During flooding the high velocity flow indents the levees of existing tidal channels and bays, leaving short flood scours. The receding water then dissects the coastline with equally spaced return channels widening toward the coast. [Copyright &y& Elsevier]

Abstract: The evolution of a barrier island and its stratigraphic architecture is investigated with the numerical model BIT (barrier island translation). The model simulates, with simplified equations, the effects of various processes (wind waves, storm surges, sea-level oscillations) on sediment location and characteristics. The presented formulation is able to reproduce both the cross-shore profile and the distribution of sediment facies in time. The model is based on two appropriate simplifying assumptions: (i) the conservation of the total mass of sediments and (ii) the conservation of the equilibrium cross-shore beach profile. Particular attention is devoted to the treatment of sediment reworking by wind waves and barrier overwash, which is induced by extreme events like storms and hurricanes, and causes a relative transport of sediments from the body of the sand barrier to the top of the island and to the back-barrier area. The model is applied to Sand Key, Florida. Model results show a satisfactory reproduction of the geometry and stratigraphy of the inner shelf, with the correct position of every stratigraphic facies within the barrier island. Simulations under different scenarios of sea-level rise during the last 8000yr indicate that the rate of overwash and lagoonal deposition are critical for the survival of the barrier island under past sea-level oscillations. [Copyright &y& Elsevier]

The morphology of passive continental shelves is dictated by the input of sediments from rivers and their redistribution by waves, currents, and gravity-driven flows. The pathways followed by sediments sculpt a landscape whose diversity is rarely matched on Earth's surface. Sediments are released to the shelf from triangularly shaped, elongated, and dendritic deltas. Barrier islands rise from gently sloping areas, tidal channels dissect flats and saltmarshes, fine sediments form broad convex deposits, and shallow submarine valleys convey sediments and water to the deep ocean. This morphological diversity is based on two main building elements: water and sediments. Fluxes of water and sediments are particularly suitable to be modeled with numerical methods based on the continuum hypothesis and hydrodynamics theory. In recent years, a series of models have been developed to explore and understand the formation of shelf landforms from the dynamics of sediment transport. Herein we present an overview of the most recent results on the modeling of deltaic and inner-shelf morphodynamics. [ABSTRACT FROM AUTHOR]

Shallow tidal basins are characterized by extensive tidal flats and salt marshes that lie within specific ranges of elevation, whereas intermediate elevations are less frequent in intertidal landscapes. Here we show that this bimodal distribution of elevations stems from the characteristics of wave-induced sediment resuspension and, in particular, from the reduction of maximum wave height caused by dissipative processes in shallow waters. The conceptual model presented herein is applied to the Venice Lagoon, Italy, and demonstrates that areas at intermediate elevations are inherently unstable and tend to become either tidal flats or salt marshes. [ABSTRACT FROM AUTHOR]

A discontinuous Galerkin method for the solution of the dam-break problem is presented. The scheme solves the shallow water equations with spectral elements, utilizing an efficient Roe approximate Riemann solver in order to capture bore waves. The solution is enhanced by a projection limiter that eliminates spurious oscillations near discontinuities. The main advantage of the model is the flexibility in approximating smooth solutions with high-order polynomials and resolving at the same time discontinuous shock waves. Furthermore, the finite element discretization is capable of handling complex geometries and producing correct results near the boundaries. Both the h- and p-type extensions are investigated for the one-dimensional dam break, and the results are verified by comparison with analytical solutions. The application to a two-dimensional dam-break problem shows the efficiency and stability of the method. [ABSTRACT FROM AUTHOR]

The discontinuous spectral Galerkin method uses a finite-element discretization of the groundwater flow domain with basis functions of arbitrary order in each element. The independent choice of the basis functions in each element permits discontinuities in transmissivity in the flow domain. This formulation is shown to be of high order accuracy and particularly suitable for accurately calculating the flow field in porous media. Simulations are presented in terms of streamlines in a bidimensional aquifer, and compared with the solution calculated with a standard finite-element method and a mixed finite-element method. Numerical simulations show that the discontinuous spectral Galerkin approximation is more efficient than the standard finite-element method (in computing fluxes and streamlines/pathlines) for a given accuracy, and it is more accurate on a given grid. On the other hand the mixed finite-element method ensures the continuity of the fluxes at the cell boundaries and it is particular efficient in representing complicated flow fields with few mesh points. Simulations show that the mixed finite-element method is superior to the discontinuous spectral Galerkin method producing accurate streamlines even if few computational nodes are used. The application of the discontinuous Galerkin method is thus of interest in groundwater problems only when high order and extremely accurate solutions are needed. [Copyright &y& Elsevier]

A numerical model that couples water flow and sediment transport over a non-cohesive bed is developed to simulate the initiation and evolution of transportational cyclic steps. Transportational cyclic steps are periodic bedforms characterized by corresponding hydraulic jumps that form when a supercritical flow erodes and deposits bottom sediments on a steep bed. The flow transition at the hydraulic jump leads to bed erosion on the supercritical part and deposition on the subcritical part of the step, with consequent upstream migration while the form is preserved. The numerical scheme developed to study transportational cyclic steps utilizes an approximate Riemann solver to capture the exact location of the hydraulic jumps present in the domain. Simulations of step growth from small perturbations on an initially flat bed are carried out and partly answer the question of how transportational steps form. Results show that both an initial infinitesimal bottom perturbation and a random distribution of bottom elevations evolve in a series of steps migrating upstream. Moreover, numerical simulations clearly show that the final configuration strongly depends on initial conditions. [Copyright &y& Elsevier]

Mangrove forests are critical coastal ecosystems that provide great socio‐ecological services, which are also highly vulnerable to climate change, particularly to sea level rise (SLR). Here we assess changes in mangrove forests in four distinct river/tide/wave‐dominant large deltas along the Indo‐Pacific coast based on 1,336 remote sensing images by machine learning techniques. We find that mangroves are migrating seaward at a rate of 18% ± 12% m/yr, which can offset landward mangroves loss, 67% of which caused by land use conversion. The fact that mangroves are expanding seaward with accretion rates exceeding SLR suggests that climate change has not yet triggered substantial loss in deltaic mangrove forests. Assuming that present environmental conditions do not change and that sediment and organic deposition in the deltaic topsets match SLR rates, we project that 90% of deltaic mangrove forests may start to retreat after 132–194 years. Early inundation of mangroves will occur in wave‐dominated delta. Plain Language Summary: Mangrove forests provide significant ecological and societal services, and mitigation global warming. However, large‐scale loss in mangroves could be induced by anthropogenic drivers and sea level rise (SLR). Our study based on deltas along the Indo‐Pacific coast, highlight that mangroves are expanding seaward at a rate of 18% ± 12% m/yr, indicating that there is little impacts from SLR and has not been substantial loss in mangrove forests in these deltas so far. Mangrove expansion here can efficiently offset 67% landward mangrove losses indicates that our new model project that 90% of mangrove shorelines will may start retreating within 132–194 years. We conclude that favoring mangroves expansion seaward would enhance coastal protection and reduce the need of landward mangrove restoration. Key Points: Mangroves along typical deltas are expanding seaward of about 18% ± 12% m/yr, indicating that there is little impacts from sea level riseMangrove expansion here can efficiently offset 67% landward mangrove lossesNew model project that 90% of mangrove fringes may start retreating within 132–194 years, and wave‐dominated delta present early inundation [ABSTRACT FROM AUTHOR]

Salt marshes are vulnerable to sea-level rise, sediment deficits, and storm impacts. To remain vertically resilient, salt marshes must accrete sediment at rates greater or equal to sea-level rise. Ice-rafted debris (IRD), sediment that has been moved and deposited from ice sheets, is one of many processes that contribute to salt marsh sediment accretion in northern latitudes. On 4 January 2018, a winter storm caused major ice mobilization in the Plum Island Estuary (PIE), Massachusetts, USA, which led to large deposits of ice-rafted sediment. We aimed to quantify the volume and mass of deposited sediment, and evaluate the significance of IRD to sediment supply in Plum Island using pixel-based land-cover classification of aerial imagery collected by an Unmanned Aircraft System (UAS) and a Digital Elevation Model. Field measurements of patch thickness, and the area of IRD determined from the classification were used to estimate annual sediment accretion from IRD. Results show that IRD deposits are localized in three areas, and estimates show that IRD contributes an annual sediment accretion rate of 0.57 ± 0.14 mm/y to the study site. New England salt marsh accretion rates typically vary between 2–10 mm/y, and the average PIE sediment accretion rate is 2.5–2.7 mm/y. Therefore, this event contributed on average 20% of the annual volume of material accreted by salt marshes, although locally the deposit thickness was 8–14 times the annual accretion rate. We show that pixel-based classification can be a useful tool for identifying sediment deposits from remote sensing. Additionally, we suggest that IRD has the potential to bring a significant supply of sediment to salt marshes in northern latitudes and contribute to sediment accretion. As remotely sensed aerial imagery from UASs becomes more readily available, this method can be used to efficiently identify and quantify deposited sediment. [ABSTRACT FROM AUTHOR]

Coastal restoration is considered a key solution to counteract coastal degradation and mitigate climate change. In recent years, China's coastal restoration projects have multiplied, with more than one billion US dollars spent during 2016–2019. Due to the sudden die-off of marsh vegetation in the Liaohe River Estuary, local managers have spent more than 30 million dollars to restore these salt marshes since 2015. However, these projects either failed or yielded limited results. Combining long-term remote sensing data to ground-based surveys, we found that salt marshes in the Liaohe River Estuary have experienced two dramatic diebacks separated by a recovery period, and the rapid changes in vegetation canopy were primarily driven by freshwater availability, especially river discharge. We suggest that mismatch between degradation drivers at the watershed-scale and restoration efforts at the local-scale hinders the success of coastal restorations. Efforts to restore these salt marshes will always be thwarted unless freshwater availability is replenished. Therefore, we propose a watershed-based solution, incorporating water replenishment from the river basin in estuarine restoration projects. Given the declining trends in river freshwater inputs in low-middle latitude coastlines, such wetland deterioration might also occur in many world's estuaries. Lessons from China's large-scale coastal restoration projects can help other regions to avoid similar shortcomings, and improve our restoration practices before too many resources are lost. Image 1 • Vegetation community in the Liaohe River Estuary has experienced two dramatic collapses and one recovery periods. • Freshwater availability, especially river discharge, plays a crucial role in marsh vegetation dynamics. • Without improving the river runoff, efforts to restore the marsh were always limited or often failed. • The watershed-based solution should be considered to enhance the success of estuaries restoration and conservation. [ABSTRACT FROM AUTHOR]

Massive die-off in salt marshes is one of the most common examples of widespread degradation in marine and coastal ecosystems. In salt marshes, tidal channel networks facilitate the exchange of water, nutrients, sediments and biota with the open marine environments. However, quantitative analyses of the role of channel networks in alleviating vegetation die-off in salt marshes are scarce. Here we quantified the spatial-temporal development of marsh vegetation die-off in the northern Liaodong Bay by analyzing aerial images before, during, and after a drought (from 2014 to 2018). We found that Suaeda salsa marshes have recently experienced large-scale die-off. The extent of vegetation die-off increases with increasing distance from the channel network. Moreover, our results suggested that efficient tidal channel networks (high drainage density, low mean unchanneled path length) can mitigate die-off at the watershed scale. We presented possible abiotic & biotic processes in channel networks that explain this spatial dynamic. Our study highlights the importance of efficient tidal channel networks in mitigating die-off and enhancing the resistance of marshes to droughts, and call for incorporating theses dynamics in coastal restoration and management. Unlabelled Image • Suaeda salsa marshes in the northern Liaodong Bay have experienced large-scale die-off. • Vegetation along channel networks was less degraded than that in the interior of the marsh. • Salt marshes with efficient tidal channel networks can mitigate vegetation degradation by droughts. [ABSTRACT FROM AUTHOR]

Sediments in deltaic tidal flats regulate physical and chemical processes. Grain‐size distribution plays an important role in determining sediment dynamics and substrate properties. However, it is challenging to quantify large‐scale depositional environments in intertidal flats, due to time‐consuming grain‐size analyses and sparse sedimentary information extracted from scattered sediment samples. In this study, a novel terrestrial laser scanner (TLS) based method was developed to characterize the substrate of an intertidal flat. Surface sediment samples in the Nanhui flats in the Yangtze Delta, China, and the corresponding waveform amplitudes of TLS echoes at fixed sampling sites were collected for a total of 22 months. A negative logarithmic relationship was found between the sediment sand fraction, average grain size, D50, and corrected waveform amplitude of TLS echo in different hydro‐meteorological conditions. The mean of average grain size of five sediment sampling sites along a transect was 58.78 μm when measured by traditional grain‐size analysis, and 49.48 μm when calculated with the proposed logarithmic equation. The mean error at each site was up to 21.77%. The mean error for the sand and silt fraction at each location was as high as 27.28% and 21.75%, respectively. The spatial distribution pattern of TLS‐based average grain size in the entire study area was consistent with the measured pattern with a Root Mean Square Error of 13.83 μm. These errors could be caused by the accuracy of the TLS waveform amplitude correction and by limits of the method in recognizing different substrates. The effects produced by the presence of microphytobenthos (for example, cyanobacterial mats or diatom biofilms) or bedforms have not been investigated and may have affected the results. The TLS‐based grain‐size measurements can rapidly and effectively discriminate sediment characteristics, thus avoiding traditional time‐consuming measurements. It is expected that the TLS‐based method proposed here will have wide applications in shoreline studies, especially in inaccessible tidal flats. [ABSTRACT FROM AUTHOR]

Die-off of coastal wetlands has been reported worldwide. Planting habitat-forming species is an important strategy to reverse the decline of coastal wetlands. However, how abiotic environmental stresses and consumers affect the establishment of the planted vegetation species is unclear. We reported a large-scale restoration project in the Liaohe estuary, China, where native pioneer plant Suaeda salsa was planted. We evaluated the growth performance of the planted S. salsa , and identified the constraints on the establishment of planted S. salsa. Results showed that the growth performance (density, coverage and survival rate) of planted S. salsa was better in the low restored marsh than that in the high restored marsh. The death of planted S. salsa was primarily driven by crab herbivory, followed by abiotic stresses (low soil moisture and high salinity) in the high restored marsh, whereas plant death was only driven by crab herbivory in the low restored marsh. Herbivory strength in the high marsh was significantly higher than that in the low marsh. Our findings challenge the bottom-up paradigm used as the foundation for coastal restoration, and highlight the overlooked role of consumers. Therefore, protection measures against consumer pressure, especially in physically harsh conditions, should be considered to enhance the success of coastal wetland restoration. • China's largest coastal saltmarsh plantation project was initiated in the Liaohe Estuary. • The restoration effect of planted Suaeda salsa was better in the low restored site than that in the high restored site. • The death of planted Suaeda is primarily driven by herbivory, followed by abiotic stresses in the high restored site. • Consumer control should be considered to enhance the success of coastal restoration. [ABSTRACT FROM AUTHOR]

• Bar deterioration was significant especially in the upper reaches. • Channel bars tended to become less stable after the dam construction. • The most effective bar-forming discharge ranges and sediment load were determined. • The quantitative relationships between bars area and fluvial regimes were developed. The evolution of channel bars in response to upstream damming has significant impacts on channel stability, navigation, and aquatic habitats. Here, the effects of the Three Gorges Dam (TGD) operation on downstream channel bars in the Yichang-Chenglingji Reach (YCR) were comprehensively analyzed using remote sensing images, cross-sectional profiles, and hydrological datasets. The morphodynamic adjustments of channel bars in the YCR were significantly different during the pre- and post-TGD periods (i.e., before and after the construction of the TGD). Specifically, the total area of channel bars did not exhibit any significant trend in the pre-TGD period, but displayed a significant reduction following the construction of the TGD, although the morphodynamic response of each sub-reach was different. The channel bars in the YCR were relatively stable in the pre-TGD period, but became more erodible in the post-TGD period. The length/width ratio (LWR) of the bars showed an overall increase trend during the whole period from 1992 to 2017, not changing before and after the dam construction. The water discharge that led to the greatest channel bars adjustment was 27,000–30,000 m3/s (corresponding to bankfull discharge) in the pre-TGD period and 15,000–18,000 m3/s (corresponding to the medium discharge that can submerge the surface of the bars) in the post-TGD period. In addition, the grain size of non-uniform sediment with the highest replenishment degree gradually reduced downstream; these finer sediments were the main sources of material for the channel bars in the YCR. Quantitative relationships between bars area, the most effective bar-forming discharge and suspended sediment load with the highest replenishment degree, were proposed based on the improved Delayed Response Model (DRM). Results indicate that geomorphic adjustments of channel bars in the YCR are closely related to the previous four-year flow and sediment regimes, implying a delayed response of the fluvial system to damming. [ABSTRACT FROM AUTHOR]

Biotic and abiotic processes control the formation and evolution of tidal channel networks. However, which factor regulates the planimetric geometry of the network remains unclear. Here, we compare the geometric properties of tidal channel networks in 14 salt marshes along the coasts of the United States and China. Significant difference in the geometric properties of tidal channel networks was found in salt marshes dominated by different vegetation species. Physical parameters better explained these differences, while vegetation parameters had a weaker effect on the network geometry. Mean marsh elevation and tidal range were found to be the best variables to explain the variability in drainage density, mean unchanneled path length and sinuosity (R2 values range from 0.239 to 0.465), while biotic parameters (i.e., aboveground biomass, stem density, height, and diameter) were only significant predictors for less geometric properties (R2 values range from 0.005 to 0.312). We used multiple regressions to develop sample models, explaining more than 60% of the variability in mean unchanneled path length. Our findings underline the key role of physical factors in shaping tidal channel networks. We conclude that physical processes are more important than vegetation species in determining the long‐term development of tidal channels. [ABSTRACT FROM AUTHOR]

Retreat of coastal forests in relation to sea level rise has been widely documented. Recent work indicates that coastal forests on the Delmarva Peninsula, United States, can be differentiated into persistence and regenerative zones as a function of sea-level rise and storm events. In the lower persistence zone trees cannot regenerate because of frequent flooding and high soil salinity. This study aims to verify the existence of these zones using spectral remote sensing data, and determine whether the effect of large storm events that cause damage to these forests can be detected from satellite images. Spectral analysis confirms a significant difference in average Normalized Difference Vegetation Index (NDVI) and Normalized Difference Water Index (NDWI) values in the proposed persistence and regenerative zones. Both NDVI and NDWI indexes decrease after storms triggering a surge above 1.3 m with respect to the North American Vertical Datum of 1988 (NAVD88). NDWI values decrease more, suggesting that this index is better suited to detect the effect of hurricanes on coastal forests. In the regenerative zone, both NDVI and NDWI values recover three years after a storm, while in the persistence zone the NDVI and NDWI values keep decreasing, possibly due to sea level rise causing vegetation stress. As a result, the forest resilience to storms in the persistence zone is lower than in the regenerative zone. Our findings corroborate the ecological ratchet model of coastal forest disturbance. [ABSTRACT FROM AUTHOR]

Quantitative estimates of particle size in estuaries and shelf areas are important to understand ocean ecology and biogeochemistry. Particle size can be characterized qualitatively from satellite observations of ocean color. As a typical marginal sea, the Yellow River Delta (YRD) with the Bohai Sea experiences a complex hydrodynamic environment. Here, we attempt to quantify the particle size distribution (PSD) slope (ξ) based on its relationship with the particle backscattering exponent from Sentinel-3A/B OLCI. The PSD slope, ξ displays temporal and spatial variability in the YRD with the Bohai Sea. Its value varies between 3 and 4, and typically exceeds 5 in offshore areas. The lowest value of ξ occurs in the winter, indicating the presence of fine inorganic particles in the water, while high values are attained in the spring, when phytoplankton blooms increase the particle size. ξ decreases near the river mouth because of the large sediment-laden discharge debouching into the sea. We detected a slight increase in ξ when turbid waters were present in the period 2016–2022. Environmental factors, such as sea surface temperature, sea surface wave height, and wind, may control particle size and ξ in the long term. Inorganic suspended particle matter is derived along the YRD using the magnitude of ξ. The mean inorganic suspended particle matter area in winter approaches 23,900 km2 when ξ < 4.6. This study thoroughly characterizes variations in ξ in the YRD with the Bohai Sea and clarifies the contributions of driving factors from human activities and climate change. [ABSTRACT FROM AUTHOR]

Salt marsh assessments focus on vertical metrics such as accretion or lateral metrics such as open‐water conversion, without exploration of how the dimensions are related. We exploited a novel geospatial data set to explore how elevation is related to the unvegetated‐vegetated marsh ratio (UVVR), a lateral metric, across individual marsh "units" within four estuarine‐marsh systems. We find that elevation scales consistently with the UVVR across systems, with lower elevation units demonstrating more open‐water conversion and higher UVVRs. A normalized elevation‐UVVR relationship converges across systems near the system‐mean elevation and a UVVR of 0.1, a critical threshold identified by prior studies. This indicates that open‐water conversion becomes a dominant lateral instability process at a relatively conservative elevation threshold. We then integrate the UVVR and elevation to yield lifespan estimates, which demonstrate that higher elevation marshes are more resilient to internal deterioration, with an order‐of‐magnitude longer lifespan than predicted for lower elevation marshes. Plain Language Summary: Salt marshes are valuable ecosystems that change in response to sea‐level rise, sediment availability, storms, and other processes. Determining how salt marshes will change in the future is difficult. In this study we show that the elevation of a marsh relative to sea level is closely related to how much area it has been losing. The relationship between elevation and marsh area is the same across four large systems, and we can use these measurements to calculate how long a marsh will survive as sea‐level rises. We found that the lowest marshes have a shorter lifespan than higher marshes, and these estimates can be used to help prioritize investments. Key Points: Geospatial analysis reveals a consistent relationship between elevation and open‐water conversion in salt marshesThe unvegetated‐vegetated marsh ratio (UVVR) consistently indicates instability above a value of 0.1Marsh lifespan can be estimated by integrating elevation and the UVVR for current and potential rates of SLR [ABSTRACT FROM AUTHOR]

Shorelines face growing threats due to climate change and diminishing sand supply. Coastal headlands, common rocky features along coastlines, are crucial in shaping hydrodynamics and sediment transport. Yet, the influence of future climate conditions, including sea‐level rise (SLR) and intensified storm energy on complex shorelines with headlands has remained relatively unexplored. In this study, we model changes in hydrodynamics and headland bypassing under different SLR and higher storm wave scenarios. Our findings reveal the formation of circulation cells on both sides of a headland, where wave energy converges around the headland zone. Future climate conditions result in larger storm waves on the beach. However, SLR enhances nearshore currents through a landward shifting of the circulation cells, while higher storm waves intensify offshore flow currents due to the seaward movement of the cells. This effect, in turn, increases the potential for headland sediment bypassing. Plain Language Summary: Coastal headlands, prominent rocky features along open coastlines, play a crucial role in protecting nearby beaches from strong waves and erosion. They also affect how sand is exchanged between different beaches. We use a model to explore how future climate conditions including sea‐level rise (SLR) and larger storm waves influence coastal waves, littoral currents, and the transport of sand around headlands. Our findings reveal that headlands converge wave energy forming circulation flow cells. SLR results in stronger nearshore currents, driven by the landward movement of the rotating flow cells. In contrast, larger storm waves can move the rotating flow cells seaward, thereby increasing offshore current strength and the potential for sand transport around the headland. Understanding how coastal waves, flow and sediment transport change under future climate conditions will help determine coastal resilience. Key Points: The formation of circulation cells and wave energy convergence around headlands during storms reduce bottom shear stress at the beachSea‐level rise increases wave heights, currents, and bed shear stress in the nearshore because of landward shifting of the circulation cellsHigher storm waves expand the surf zone, shift circulation cells seaward, and enhance potential headland sediment bypassing [ABSTRACT FROM AUTHOR]

The article focuses on the destructive waves and water levels of frequent extreme storm events, and discusses the use of storm-proofing with marshes. It notes that wave tank experiments demonstrate salt marsh vegetation that dissipates wave energy and withstands extreme storm conditions. It says that shorelines are physical and ecological interfaces which separate energetic oceans from continents.

The article presents information on the ephemeral life of a salt marsh. Salt marshes are vanishing around the world at a fast rate. A common opinion of marsh vulnerability is that, for elevated rates of sea level rise, size growth of the marsh platform is not able to keep pace with sea level leading to marsh drowning.

Muddy banks of marsh channels experience soil creep – a viscous-like slow deformation resulting in a net downslope transport. Here we present the first field evidence of soil creep in a mesotidal salt marsh using both pole displacement and high precision measurements of soil deformation taken with a vibrating-wire extensometer over two years. Soil extended 2–4 mm/m/year in the high marsh platform and 20–40 mm/m/year in the low marsh bank. This was equivalent to a soil diffusivity of about 0.2 m2/year, which is much higher than the diffusivity on hillslopes and also matches the high value predicted by long-term marsh evolution models. High precision (±0.03 mm) soil deformation measurements shed light on the dynamics of the creep process. First, the low bank compressed and extended by ~5 mm/m every tidal cycle, suggesting that the diurnal changes in water level play a role in the bank movements. Second, net soil extension took place mostly during fall. Net soil extension preceded the period of ice formation, suggesting that freezing and ice rafting are not a leading cause of soil creep. Net soil extension was likely triggered by an increase in water table during fall, which decreased the effective stress and thus destabilized the bank. This high water table was caused by a combination of an abiotic factor – the increase in atmospheric precipitation – and a biotic factor – the decrease in evapotranspiration triggered by vegetation senescence. This study confirms that creep is a significant process in marsh morphodynamics and that vegetation can reduce mass wasting in marsh channels. • The rate of soil creep in salt marshes is large compared to hillslopes. • High-precision hourly measurements revealed tide-induced fluctuations in soil deformation. • Net soil extension took place mainly in late fall and was linked to high water table. [ABSTRACT FROM AUTHOR]

The retreat of coastal forests as sea level rises is well documented; however, the mechanisms which control this retreat vary with the physical and biological setting of the interface between tidal marsh and forest. Tidal flooding and saltwater intrusion as well as flooding and wind associated with storms can kill trees. Even if these processes do not kill stands, they may halt regeneration because seedlings are more sensitive to stress. We present a case study of a coastal pine forest on the Delmarva Peninsula, United States. This forest contains a persistent but nonregenerating zone of mature trees, the size of which is related to the sea level rise experienced since forest establishment. The transgression of coastal forest and shrub or marsh ecosystems is an ecological ratchet: sea-level rise pushes the regeneration boundary further into the forest while extreme events move the persistence boundary up to the regeneration boundary. [ABSTRACT FROM AUTHOR]

Wetland shorelines around the world are susceptible to wave erosion. Previous work has suggested that the lateral erosion rate of their cliff-like edges can be predicted as a function of intercepting waves, and yet numerous field studies have shown that other factors, for example, tidal currents or masswasting of differing soil types, induce a wide range of variability. Our objective was to isolate the unique effects of wave heights, wavelengths, and water depths on lateral erosion rates and then synthesize a mechanistic understanding that can be applied globally. We found a potentially universal relationship, where the lateral erosion rates increase exponentially as waves increase in height but decrease exponentially as waves become longer in length. These findings suggest that wetlands and other sheltered coastlines likely experience outsized quantities of erosion, as compared to oceanic-facing coastlines. [ABSTRACT FROM AUTHOR]

Both submerged and emergent vegetation plays a fundamental role in coastal bays. Vegetation stabilizes the substrate, increasing resilience to storms. Vegetation also traps sediments favoring accretion and therefore counteracting sea level rise. Previous modeling studies on flow-vegetation-sediment interactions have focused on one specific vegetated community, but we lack a general understanding of the synergistic effects of multiple vegetation species. We focus our study on the Virginia Coast Reserve Long Term Ecological Research (LTER) site, USA, where we apply numerical modeling (Delft3D-SWAN) to investigate the independent and synergistic effects of salt marsh vegetation and seagrass. Our numerical results show that salt marshes and seagrass beds reduce the volume of water entering and exiting the shallow coastal bays up to 15% during each tidal cycle. Vegetation also reduces bed shear stress and hence increases sediment deposition in the bay and marshes up to 10% compared to the no-vegetated case. Our study shows the double benefits of seagrass as an ally of salt marsh in promoting bays resilience. On the one hand, seagrass helps the salt marsh to survive during storms by reducing wave energy; on the other hand, seagrass generates more friction in subtidal parts of the bay where salt marsh cannot survive. [ABSTRACT FROM AUTHOR]

Abstract: Seagrasses are marine flowering plants that strongly impact their physical and biological surroundings and are therefore frequently referred to as ecological engineers. The effect of seagrasses on coastal bay resilience and sediment transport dynamics is understudied. Here we use six historical maps of seagrass distribution in Barnegat Bay, USA, to investigate the role of these vegetated surfaces on the sediment storage capacity of shallow bays. Analyses are carried out by means of the Coupled‐Ocean‐Atmosphere‐Wave‐Sediment Transport (COAWST) numerical modeling framework. Results show that a decline in the extent of seagrass meadows reduces the sediment mass potentially stored within bay systems. The presence of seagrass reduces shear stress values across the entire bay, including unvegetated areas, and promotes sediment deposition on tidal flats. On the other hand, the presence of seagrasses decreases suspended sediment concentrations, which in turn reduces the delivery of sediment to marsh platforms. Results highlight the relevance of seagrasses for the long‐term survival of coastal ecosystems, and the complex dynamics regulating the interaction between subtidal and intertidal landscapes. [ABSTRACT FROM AUTHOR]

Abstract: Wind waves were measured in the Willapa Bay mudflats, Washington State, USA, for two months. Wave height, period, and bed shear stresses were modulated by water depth (0–3.5m), wind speed (0–20m/s), and fetch (1–5km). Good agreement was found between the measured waves and predictions of the wave spectral model SWAN using either simplified 1D flat bottom or 2D geometries. The relationship between bed shear stress and water depth shows a dependence on fetch: the decay of bed shear stress with increasing water depth is gradual for long fetch and rapid for short fetch. This difference is explained by the coupled effects of water depth, wave height and wave period. Due to the fetch-dependent bed shear stress, different morphological consequences for tidal flats of different size are predicted. In small (∼2km) and sheltered tidal flats, waves cause the largest sediment resuspension when water levels are near mean sea level. In extensive tidal flats (∼20km) or in flats exposed to waves propagating from deep water, waves also are effective in causing substrate erosion during high tides or large storm surges. [Copyright &y& Elsevier]

Deltas are fragile ecosystems threatened by sea‐level rise, sediment starvation, and subsidence. Erosional/depositional processes in these systems mainly depend on the sediment supply and the spatial divergence in bed shear stress induced by hydrodynamic forces. Thus, quantifying the spatiotemporal variability of the flow velocity field is essential for forecasting their fate. To calibrate/validate models, field measurements alone are not sufficient because such data only characterize the hydrodynamic conditions in localized areas. Remote sensing has potential to fill this data gap. We developed a methodology to estimate flow velocities from a map of suspended sediment concentration (SSC) measured by the NASA airborne spectrometer AVIRIS‐NG within the Wax Lake Delta, Louisiana. We extracted streaklines from remotely sensed SSC estimates, and quantified water fluxes and velocities based on the distance between them. Our study demonstrates that the velocity field in deltas can be estimated by leveraging the synoptic information offered by remote sensing. Plain Language Summary: River deltas are coastal environments particularly susceptible to sea‐level rise. Sediment deposition can counteract land loss from sea‐level rise, but we have a poor understanding of where and when land is built through sediment accretion. Improving the accuracy of hydrodynamic models is pivotal to predicting the fate of sediment that rebuilds sinking land, but we lack methods to test these models against observations over large areas. Here, we introduce a new method to measure water discharge, flow direction and speed from remotely sensed data. The method takes advantage of water flow patterns revealed in data collected from a NASA instrument flown on an airplane. Our study shows the utility of remotely sensed data to improve the performance and reliability of models that are needed to predict the fate of river deltas. Key Points: We propose a novel methodology to compute water fluxes and flow velocities in deltas from maps of suspended sediment concentrationOur method quantifies flow velocities based on the distance between streaklines, under the assumption that streaklines act as streamlinesFlow velocities obtained via our methodology match well with those derived from a numerical model in a sub‐region of the Wax Lake Delta [ABSTRACT FROM AUTHOR]

Recent studies have focused on the effect of large tropical cyclones (hurricanes) on the shore, neglecting the role of less intense but more frequent events. Here we analyze the effect of the offshore tropical storm Melissa on groundwater data collected along the North America Atlantic coast. Our meta‐analysis indicates that both groundwater level and specific conductivity significantly increased during Melissa, respectively reaching maximum values of 1.09 m and 25.2 mS/cm above pre‐storm levels. Time to recover to pre‐storm levels was 10 times greater for groundwater specific conductivity, with a median value of 20 days, while groundwater level had a median recovery time of 2 days. A frequency‐magnitude analysis indicates that the percent of time with salinization is higher for Melissa than for energetic hurricanes. Given the high frequency of these events (return period of 1–2 years), and the long time needed for groundwater conditions to return to normal levels, we conclude that increasingly frequent moderate storms will have a significant impact on the ecology of vegetated shorelines. Plain Language Summary: Salinization and flooding events due to sea level rise and storm surges threaten coastal ecosystems, changing groundwater characteristics. Moderate and more frequent storm surges can have a significant impact on coastal ecology, similar to larger tropical cyclones. Salinity and water table elevation need time to recover to normal conditions. The recovery time is compared to the frequency of these moderate storm surge events to determine the effect on the coastal groundwater. Key Points: SalinizationCoastal areasGroundwater [ABSTRACT FROM AUTHOR]

Freshwater wetlands are important ecosystems experiencing rapid degradation around the world. As much as 64% of world's wetland area has been lost since 1900; the situation is even more serious in Asia, where land reclamation and anthropogenic modifications of rivers are increasing the rate of wetland disappearance. In this study, we provide a first complete estimation of daily Emergent Wetland Area (EWA) in Poyang Lake, China's largest freshwater lake, from 1955 to 2012. A wavelet analysis indicates a strong periodicity in the monthly EWA time series with two oscillations having a period of 12 and 60–72 months, respectively. A dramatic increase in mean annual EWA is detected since 2003, when the Three Gorges Dam (TGD) was completed, mainly due to the seasonal drying of 1078 km 2 of wetlands in October. It is found that the timing of wetland emergence during the dry season has been anticipated of one month, from November to October, since the establishment of TGD. It is argued that a significant increase in wetland exposure and an observable shift in the seasonal timing of flooding and drying will seriously degrade the wetland system and threaten the endangered migratory birds that inhabit it unless effective countermeasures are implemented. [ABSTRACT FROM AUTHOR]

Mangrove forests exert a strong influence on tropical deltas by trapping sediments discharged by rivers and by stabilizing the substrate with roots. Understanding the dynamics of sediments and morphology in and around mangrove forests is critical in order to assess the resilience of coastlines in a period of accelerated sea level rise. In this research, sediment samples, mangrove forest characteristics, and remote sensing data are used to investigate the relationship between mangroves and sediment substrate in the Mekong Delta, Vietnam. Our data show a significant correlation between percent of sand in bottom sediments and density of Sonneratia caseolaris forest. We ascribe this result to higher sediment disturbance in muddy areas that prevents seedling establishment. This correlation potentially allows the determination of substrate characteristics from vegetation attributes detected by remote sensing, despite the impenetrability of the forest canopy. The results presented herein suggest that a supply of sand from the river and hydrodynamic processes moving the sand ashore control the density of the Sonneratia mangrove forests at this location, promoting tidal flat colonization and canopy expansion. [ABSTRACT FROM AUTHOR]

Salt marsh losses have been documented worldwide because of land use change, wave erosion, and sea-level rise. It is still unclear how resistant salt marshes are to extreme storms and whether they can survive multiple events without collapsing. Based on a large dataset of salt marsh lateral erosion rates collected around the world, here, we determine the general response of salt marsh boundaries to wave action under normal and extreme weather conditions. As wave energy increases, salt marsh response to wind waves remains linear, and there is not a critical threshold in wave energy above which salt marsh erosion drastically accelerates. We apply our general formulation for salt marsh erosion to historical wave climates at eight salt marsh locations affected by hurricanes in the United States. Based on the analysis of two decades of data, we find that violent storms and hurricanes contribute less than 1% to long-term salt marsh erosion rates. In contrast, moderate storms with a return period of 2.5 mo are those causing the most salt marsh deterioration. Therefore, salt marshes seem more susceptible to variations in mean wave energy rather than changes in the extremes. The intrinsic resistance of salt marshes to violent storms and their predictable erosion rates during moderate events should be taken into account by coastal managers in restoration projects and risk management plans. [ABSTRACT FROM AUTHOR]