We present a reduced-complexity modeling framework that couples hydrodynamics, short-term bed morphodynamics and marsh vegetation accretion to evaluate terrace-based restoration designs. Using this framework, we test a hybrid terrace configuration consisting of angled linear, chevron and circular shapes in a tidally restored impoundment. Hydrodynamics are controlled by terrace configuration, with tidal flow directed into and out of the domain, establishing simulated circulation conditions favorable for sediment retention in the back of the domain. Subaqueous depositional patterns depend on the terrace form and arrangement, while the deposition extent and magnitude are governed by the sediment type (grain size). The side slopes of the terrace configuration provide intertidal elevations favorable for marsh vegetation growth and ecosystem function. Edge habitat increases under relative sea-level rise due to vegetation-enhanced accretion. Gains in marsh edge habitat area were volumetrically equal for fresh, brackish and saline conditions but were variably distributed over subtidal, intertidal and supratidal elevations. Elevation capital produced through vegetation-enhanced marsh accretion is greatest for fresh species due to greater biomass productivity, followed by salt-tolerant brackish species, while saline marsh vegetation demonstrated greater stability of mature productivity under rising sea level. Homeostatic marsh vegetation along the terrace edges provides multiple ecological benefits, including sediment retention, habitat expansion and structural support for invertebrates, nekton and waterbirds. Our results demonstrate the reduced-complexity modeling of terrace designs can be used by restoration practitioners to assess physical and ecological outcomes to develop management plants to sustain wetland health.

Long-term monitoring is vital to understanding how coastal wetlands respond to environmental changes. Using 15 years (2008–2022) of hourly water-level and salinity data from over 300 Coastwide Reference Monitoring System (CRMS) stations in Louisiana, USA, this study quantifies annual and decadal trends in water levels and salinity across nine basins and five vegetation types. The variations and trends were evaluated using a 12-month moving average method. Water levels increased statewide at 13.9 mm yr -1, leading to an increase in inundation depth of 5.9 mm yr -1. The influence of local climatic drivers (sea surface temperature, river flow, precipitation, and wind) varies with local hydrogeomorphic settings. In marine-influenced saline and brackish marshes, water levels covaried more strongly with surface temperature in the Gulf of Mexico. Concurrently, elevated river flows and local precipitation further increased water levels in freshwater marshes and swamps. Salinity levels showed high sensitivity to freshwater inputs and steadily decreased from 2011 to 2020, consistent with increased freshwater inputs. These results indicate that salinity responses in deltaic wetlands are possibly non-monotonic and strongly modulated by freshwater variability, even under ongoing sea-level rise. This comprehensive study on one of the most vulnerable coastal wetland systems provides insights into future coastal wetland responses and resilience broadly. Also, the automated, data-driven workflow provides transferable inputs to process-based modeling, enabling scenario evaluation and resilience planning in dynamic coastal wetlands.

The relative influence of baseline hydroperiod (monthly tidal datums), event-driven high-water exceedances (predominantly associated with frontal weather systems) and tropical-cyclone extremes, on marsh vertical accretion in Louisiana (USA) microtidal systems remains uncertain. We examined their contributions at stations spanning freshwater-brackish-saline marshes in the Atchafalaya and Terrebonne basins during water year (WY) 2020-2021, which aligned hourly CRMS water-level records with contemporaneous vegetation and accretion observations and includes multiple tropical events for contrast. Our approach was a reduced-form monthly partitioning framework: hourly water levels were used only to derive monthly hydroperiod metrics and within-month exceedance tiers. By reducing hourly water levels to monthly means and within-month exceedance tiers (minor, moderate, major), we modeled distinct pathways of mineral and organic accumulation. Suspended sediment concentration (SSC) is temporally controlled by river discharge seasonality and varies spatially across the deltaic landscape, while aboveground biomass regulates vegetation-mediated trapping. The distal saline site is mineral-dominated due to its low elevation, high SSC, and elevated mean sea level, whereas both freshwater sites are organic-dominated. We found that event-driven exceedances contribute roughly 40-45% of the annual mineral accretion, primarily from frequent minor exceedances, highlighting that accretion depends more on event frequency than individual magnitude. Named tropical cyclones are treated as discrete extremes when storm peaks exceed the statistical major tier; their incremental contribution was similar to 2-3% of the annual total, reinforcing outer-bay marshes during the cool-season wind-wave regime. Vertical accretion profiles showed mineral loading tapering with height as flood depths contract, while organic productivity peaks at intermediate elevations. Independent comparisons with NOAA tide gauges and river-coastal data confirmed that these patterns reflect genuine environmental controls. Results indicate that marsh resilience depends less on tropical-cyclone extreme storms and more on the cumulative effects of frequent, shallow, sediment-bearing floods acting on a rising baseline hydroperiod.

Due to sea-level rise, small river-dominated deltas (<100 km(2)) are expected to become more exposed to tidal influences in the future. However, there remains a knowledge gap in the impending hydrodynamics of such deltas, particularly with the interactions between river and tidal flows. In addition to sea-level rise, river-tide interactions in these deltas depend on their morphology, which is influenced by the sand proportion in the particulate matter delivered by rivers. This study investigates river-tide interactions predicted for small deltas formed by different sand-to-mud ratios under various sea-level rise scenarios. Delta morphologies were generated using reduced-order complexity model (DeltaRCM), and hydrodynamic simulations were performed using advanced circulation (ADCIRC) modeling. The findings indicate that sea-level rise promotes deeper tidal penetration into deltas. Deltas formed by finer sediments exhibit deeper channels flanked by large natural levees, whereas those formed by coarser sands are characterized by shallow channels with smaller levees. Consequently, tides primarily propagate along the channels of deltas formed by finer material, while deltas formed by coarser material experience greater tidal inundation. The findings are meaningful toward the adaptive management of deltas.

Water is vital for any civilization to prosper; thus, settlements have been established within the coastal or fluvial floodplain throughout history. These communities, especially low-gradient ones, are prone to flood hazards such as nuisance flooding, storm surges, extreme rainfall, and high river discharge. However, when two or more flood drivers (e.g., coastal, pluvial, and fluvial) coincide or are in close succession, it can be classified as a compound flood event (Bilskie and Hagen, 2018). Consequently, the inundation impacts can be exacerbated (rather than being a linear superposition) due to the nonlinear interaction of the coastal and hydrologic processes (Santiago-Collazo et al., 2019). With the effects of climate change, coastal watersheds will be subjected to additional flood stressors, resulting in catastrophic effects if future planning tools do not consider the compounding behavior of these flood drivers. Numerical modeling tools may allow the design of risk-mitigation strategies but require simulations of several flooding scenarios to be considered simultaneously to account for compound floods. However, the current techniques cannot simulate multiple processes simultaneously and lose accuracy when modeling events with numerous flood drivers by coupling different modeling tools.

Coastal restoration projects are significantly important in coastal ecosystems as wetland losses accelerate. This study investigates tidal hydrodynamics and the potential impacts of a waterway opening on an existing roadway to restore and revitalise salt marshes in a small estuarine system in Virginia, USA. A depth-integrated, discontinuous Galerkin shallow-water equations model (DG-SWEM) is applied for astronomic tide simulation. The model employs a high-resolution unstructured mesh with a minimum element size of less than one meter and resolves complex tidal flows in the entire barrier island system, including the existing culvert gate and canal system. Compared to the existing system, the water exchange increased dramatically (flushing time also dramatically decreased) under the opened scenarios regardless of the opening width (22.9-, 30.5-, and 38.1-m width). By increasing the opening width, peak velocity through the proposed opening decreased 30-40%, and the maximum shear stress was reduced by more than half. The high-resolution model represented complex tidal flows, including eddies, and assisted in striking a balance of water-exchange capability, opening stability, and minimising their potential erosions for the opening design. Besides, erosion and sediment transport potentials for suspended sediments were estimated using bed shear stress and a Lagrangian particle tracking module. Such proxy modelling approach allows for the impact assessment of civil engineering and ecological waterworks in complex and highly damped tidal flow areas and is readily transferrable to other like systems (e.g. causeway construction, causeway cutting, biota passageways, and inlet modification).

Low-gradient coastal watersheds are susceptible to flooding caused by various flows such as rainfall-runoff, astronomical tides, storm surges, and riverine flows. Compound flooding occurs when at least one coastal flood driver occurs simultaneously or in close succession with a pluvial and/or fluvial flood driver, such as during a tropical cyclone event. This study presents a one-dimensional (1-D), reduced-order physics compound inundation model tested over an idealized coastal watershed transect under various forcing conditions (e.g., storm surge, astronomical tides, and rainfall) that varied in magnitude, time, and space. This study aims to evaluate each flooding mechanism and the associated hydrodynamic responses to identify generalized coastal transition zones and enhance the production of flood maps for varying regions in a coastal watershed. Compound inundation levels are affected by the magnitude and timing of each flooding mechanism. The desire is a more holistic compound inundation model that can be a critical tool for decision-makers, stakeholders, and authorities who provide evacuation planning to save human lives and enhance resilience.

Parameter estimation is an inverse problem which is crucial to reliable groundwater model predictions and management. Numerous techniques have been developed to address this challenging problem. This study aims to compare the performance of a stochastic optimization method, covariance matrix adaptation-evolution strategy (CMA-ES), and a data assimilation method, Levenberg-Marquardt based iterative ensemble smoother (ES-LM), on solving inverse problems in groundwater modeling. This study also presents a parallelization strategy to accel-erate their implementation. The comparative analysis involves synthetic inverse problems and a real-world case study of calibrating a high-resolution groundwater model. Results from the synthetic problems suggest that both the CMA-ES and the ES-LM are able to achieve desirable data fitting, but the ES-LM generally exhibits greater efficiency than the CMA-ES. Also, with appropriate stopping criteria, both methods can perform well on prop-agating uncertainty of model predictions. However, both methods can underestimate the uncertainty of model parameters derived from Bayesian posterior. The underestimation is particularly pronounced by the ES-LM when using a small ensemble size, whereas uncertainty quantification by the CMA-ES is less affected by population size. Moreover, calibration of the highly parameterized groundwater model in Louisiana and Southwest Mis-sissippi indicates that the ES-LM can outperform the CMA-ES for high-dimensional inverse problems regarding both data match and computational costs. With proper ensemble size, the two methods can produce comparable uncertainty of model parameters and predictions. This study contributes to the scientific understanding and efficient application of the CMA-ES and the ES-LM.

The effects of large-scale interior headland restoration on tidal hydrodynamics and salinity transport in an open coast, marine dominant estuary (Grand Bay, Alabama, U.S.A) are investigated using a two-dimensional model, the Discontinuous-Galerkin Shallow Water Equations Model (DG-SWEM). Three restoration alternatives are simulated for present-day conditions, as well as under 0.5 m of sea level rise (SLR). Model results show that the restoration alternatives have no impact on tidal range within the estuary but change maximum tidal velocities by ±5 cm/s in the present-day scenarios and by ±7 cm/s in the scenarios with 0.5 m of SLR. Differences in average salinity concentrations for simulated tropical and frontal seasons show increases and decreases on the order of 2 pss in the embayments surrounding the restoration alternatives; differences were larger (on the order of ±4 pss) for the scenarios with 0.5 m of SLR. There were minimal changes in average salinity outside of the estuary and no changes offshore. The size and position of the alternatives played a role in the salinity response as a result of changing the estuarine shoreline geometry and affecting the fetch within the bay. SLR was more impactful in increasing exposure to low salinity values (i.e., less than 5 pss) than the presence of the restoration alternatives. Overall, the modeled results indicate that these large-scale restoration actions have limited and localized impacts on the hydrodynamics and salinity patterns in this open coast estuary. The results also demonstrate the nonlinear response of salinity to SLR, with increases and decreases in the maximum, mean and minimum daily salinity concentrations from present-day conditions. This nonlinear response was a result of changes in the directions of the residual currents, which affected salinity transport.

Modeling ocean processes requires a discrete representation of the system geometry, which itself contains multiple degrees of freedom, especially for systems of large geographical scale. This paper assesses the unstructured meshing requirements for a large-scale, astronomic-tide model domain of the western North Atlantic Ocean, Gulf of Mexico and Caribbean Sea with telescopic focus of the South Atlantic Bight (United States southeastern seaboard) and its estuaries. The methodologies of localized truncation error analysis (LTEA), which account only for the model interior and LTEA plus complex derivatives (+CD), which account for the model interior and boundary, are exploited due to their foundations stemming directly from tidal physics. LTEA and LTEA+CD are applied for the M2 tidal harmonics resulting from linear and nonlinear solutions of the ADCIRC (advanced circulation) model based on a highly resolved uniform mesh. The resultant target element size distributions are interrogated distinctively for the inshore (estuarine) region of the South Atlantic Bight, the inlet-punctuated coastline, the shelf-slope-rise triad and deep ocean, and the open-ocean boundary. The relatively large size of the LTEA computational molecule makes it the suitable choice for the deep ocean, offshore and shelf regions where the tidal dynamics are predominantly linear and smoothly changing in space. LTEA+CD is the logical choice for the shallows and estuaries because of the spatial coverage afforded by the combined utility of interior and boundary estimators (computational molecules), as well as its accounting for the nonlinearities in the evaluation of local truncation error. The combination of the two methods enables an almost complete coverage of the domain for generating spacing requirements, with LTEA+CD extending beyond the interior-only definition of LTEA to capture the inshore regions and model boundaries. Implementing the approach with LTEA and LTEA+CD to target finer resolution in areas with sharper gradients in the hydrodynamics, while relaxing resolution in areas with smoother hydrodynamics, resulted in a mesh with a far-reduced number of elements (∼1.3M) relative to an overly resolved uniform mesh (>10M elements) that is comparable in accuracy with ±5% tidal amplitude error for 99% of the domain.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]The ground-truth about lidar-derived digital elevation models in coastal wetland regions of the northern Gulf of MexicoAuthorsJinIkedaiDPeterBacopoulosiDJeffDanielsoniDBradyCouvillionShuGaoiDStephenMedeirosiDScottHagenSee all authors Jin IkedaiDCorresponding Author• Submitting AuthorLouisiana State UniversityCenter for Coastal ResiliencyiDhttps://orcid.org/0000-0002-9364-0888view email addressThe email was not providedcopy email addressPeter BacopoulosiDLouisiana State UniversityiDhttps://orcid.org/0000-0002-5954-0429view email addressThe email was not providedcopy email addressJeff DanielsoniDOrganization Not ListediDhttps://orcid.org/0000-0003-0907-034Xview email addressThe email was not providedcopy email addressBrady CouvillionU.S. Geological Surveyview email addressThe email was not providedcopy email addressShu GaoiDLuisiana State UniversityiDhttps://orcid.org/0000-0002-4078-0422view email addressThe email was not providedcopy email addressStephen MedeirosiDEmbry-Riddle Aeronautical UniversityiDhttps://orcid.org/0000-0003-0264-5868view email addressThe email was not providedcopy email addressScott HagenLouisiana State Universityview email addressThe email was not providedcopy email address

This paper presents a forensic analysis of the coastal erosion and structural damage associated with four consecutive-year, Atlantic-basin major hurricanes. The investigation focuses on Hurricanes Matthew (2016), Irma (2017), Michael (2018) and Dorian (2019) and their impact on the sandy shorelines and coastal construction of Florida, located in the southeastern United States. The study identifies the degree of resilience and coastal protection of different coastal construction methods and beach management programs under varied conditions associated with major hurricane impacts. The study results demonstrate a coherent relationship between observed peak storm tide and surveyed measures of coastal erosion and structural damage. The storm tide generated by the natural forcing of tropical-cyclone winds is variably influenced by the wide continental shelf of Florida's gulf coast and the meso-tidal regime of Florida's east coast. As our data show, beyond the obvious metric of storm strength, as expressed in terms of maximum wind speed, coastal impacts by hurricanes are additionally (or more predominantly) driven by storm size, as expressed in terms of wind radii for tropical stormand hurricane-force winds. Of socioeconomic relevance, the results demonstrate that beach management projects of sand placement provide dune and berm protection as well as mitigation against damage to coastal property and infrastructure. The findings of this study are directly relevant to other coastal regions with recreational beach, dense populations and vulnerable infrastructure, subject to erosional and destructive events caused by tropical cyclones.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]A Physics-Based Classification of Coastal Land-Margins based on Surface FlowAuthorsFelixSantiago-CollazoMatthewBilskieiDPeterBacopoulosiDKoryKonsoerScottHagenSee all authors Felix Santiago-CollazoCorresponding Author• Submitting AuthorUniversity of Georgiaview email addressThe email was not providedcopy email addressMatthew BilskieiDUniversity of GeorgiaiDhttps://orcid.org/0000-0002-7697-7403view email addressThe email was not providedcopy email addressPeter BacopoulosiDLouisiana State UniversityiDhttps://orcid.org/0000-0002-5954-0429view email addressThe email was not providedcopy email addressKory KonsoerLouisiana State Universityview email addressThe email was not providedcopy email addressScott HagenLouisiana State Universityview email addressThe email was not providedcopy email address

Recent events worldwide demonstrate how coastal communities of integrated natural and human systems are exposed to hydrological and coastal flooding processes. Standard flood hazard assessment practices account independently for rainfall-runoff, tides, storm surge flooding and not the non-linear combination commonly defined as compound flooding. This research evaluates compound flood hazard zones for past, present, and future (c. 1890–2090) conditions of the Mississippi River Delta Plain (MRDP). The MRDP provides a low-gradient coastal land-margin representing similar landscapes around the world that are experiencing relative sea-level rise and serves as a warning beacon for our coastal settlements. A set of plausible synthetic storms and rainfall events, which account for antecedent rainfall-runoff, tropical cyclone-driven rainfall, and tropical cyclone-driven surge, are employed in a tide and surge hydrodynamic model that integrates rain over the mesh. This study demonstrates the evolution of the compound flood hazard zones from the 1890s, before major western settlement and alterations to the Mississippi River and deltaic system, to the present day and out to 2090. Furthermore, near-future projections of the compound flood hazard zones suggest that the coastal flood zone will suffer the most significant changes in coverage area due to a combination of increasing eustatic sea-level rise and alterations to the coastal land-margin during low flood events. Our results emphasize the need to establish evolution trends of compound flood hazard zones to enable more descriptive future projections under a changing climate. Such projections will aid policy-makers, stakeholders, and authorities as they pursue enhanced coastal resilience to compound flooding.

Earth and Space Science Open Archive PosterOpen AccessYou are viewing the latest version by default [v1]Hydrodynamic Assessment of Natural and Nature-based Features for Escatawpa River and Grand Bay in the Northern Gulf of MexicoAuthorsPeterBacopoulosiDKarimAlizadDavinaPasseriMatthewBilskieiDStephenMedeirosiDScottHagenSee all authors Peter BacopoulosiDCorresponding Author• Submitting AuthorLouisiana State UniversityiDhttps://orcid.org/0000-0002-5954-0429view email addressThe email was not providedcopy email addressKarim AlizadUnited States Geological Surveyview email addressThe email was not providedcopy email addressDavina PasseriUnited States Geological Surveyview email addressThe email was not providedcopy email addressMatthew BilskieiDUniversity of GeorgiaiDhttps://orcid.org/0000-0002-7697-7403view email addressThe email was not providedcopy email addressStephen MedeirosiDEmbry-Riddle Aeronautical UniversityiDhttps://orcid.org/0000-0003-0264-5868view email addressThe email was not providedcopy email addressScott HagenLouisiana State Universityview email addressThe email was not providedcopy email address

Salinity response to sea-level rise is evaluated for a low-gradient, tidally active estuary, the lower St. Johns River, Florida. A high-resolution numerical model is forced by continuous data of water levels and freshwater inflows for the offshore and upstream boundaries, respectively. The modeling approach is configured for salinity simulation over a 10-year record, 1997–2007, and validated at four salinity-gauging stations inside the river. The initial condition of salinity field was found to be a critical factor in the numerical simulation. Adjustments in the initial salinity condition of ± 10% required 6–9 months for the model salinity solution to dynamically equilibrate with the applied boundary conditions. Model predictions of salinity response to sea-level rise of 0.05, 0.15, and 0.30 m were diagnosed in terms of salinity change. Salinity was found to increase over the entire river, regardless of the magnitude of sea-level rise. Linear rates of salinity increase were predicted as high as 6 ppt m −1 inside the river. The change in salinity was nonuniform throughout the system and exhibited a moderate-to-strong nonlinear component. The results uncover a hotspot in the river where salinity was predicted to increase as much as ~ 2.3 ppt due to the nonlinear system response to sea-level rise.

A localized truncation error analysis with complex derivatives (LTEA+CD) is applied recursively with advanced circulation (ADCIRC) simulations of tides and storm surge for finite element mesh optimization. Mesh optimization is demonstrated with two iterations of LTEA+CD for tidal simulation in the lower 200 km of the St. Johns River, located in northeast Florida, and achieves more than an over 50% decrease in the number of mesh nodes, relating to a twofold increase in efficiency, at a zero cost to model accuracy. The recursively generated meshes using LTEA+CD lead to successive reductions in the global cumulative truncation error associated with the model mesh. Tides are simulated with root mean square error (RMSE) of 0.09–0.21 m and index of agreement (IA) values generally in the 80s and 90s percentage ranges. Tidal currents are simulated with RMSE of 0.09–0.23 m s−1 and IA values of 97% and greater. Storm tide due to Hurricane Matthew 2016 is simulated with RMSE of 0.09–0.33 m and IA values of 75–96%. Analysis of the LTEA+CD results shows the M2 constituent to dominate the node spacing requirement in the St. Johns River, with the M4 and M6 overtides and the STEADY constituent contributing some. Friction is the predominant physical factor influencing the target element size distribution, especially along the main river stem, while frequency (inertia) and Coriolis (rotation) are supplementary contributing factors. The combination of interior- and boundary-type computational molecules, providing near-full coverage of the model domain, renders LTEA+CD an attractive mesh generation/optimization tool for complex coastal and estuarine domains. The mesh optimization procedure using LTEA+CD is automatic and extensible to other finite element-based numerical models. Discussion is provided on the scope of LTEA+CD, the starting point (mesh) of the procedure, the user-specified scaling of the LTEA+CD results, and the iteration (termination) of LTEA+CD for mesh optimization.

Hurricane Irma’s (2017) storm tide in Florida (USA) was extreme in varied ways, breaking records of water-level extrema around Florida (15 gauging stations) with low water of − 2.34 m relative to mean sea level (MSL) and estimated return period of 283 years on the gulf coast (Cedar Key), and high water of 1.81 m (MSL) and estimated return period of 110 years on the east coast (Mayport). Five other weather systems that caused similarly extreme low and high waters around Florida are identified in the observed records. A common causal factor associated with extreme storm surge around Florida is large total storm size where tropical storm-/hurricane-force winds act over scales of 100–400 km. The peninsular shape of Florida creates a geographical setting where simultaneous extreme low and high waters can occur. The continental shelf is generally wide for the gulf coast and variable with latitude for the east coast. The curvature afforded by the gently cuspate features of the coastline (scales of 50–200 km) and protruding points and capes lend local environments to amplify the storm tide. Depending on the wind direction, the coastal and shelf geometry traps and accumulates water (extreme high waters) or it provides an openness for winds to blow out the water (extreme low waters), where both are exacerbated by the long duration of winds due to large-scale storm systems. Sea-level rise, interannual variability, seasonality and astronomic tides are all shown to be positive contributing factors of water-level extrema.

Sea-level rise impacts on salt marsh for Guana Tolomato Matanzas National Estuarine Research Reserve are investigated using field measurements (six sites within the marsh) and a tide-marsh equilibrium model (Hydro-MEM). The hydrodynamic component of the model enables for prediction of spatially variable tidal data (mean low water and mean high water), which are coupled with a marsh equilibrium model (MEM) for prediction of spatially based biomass productivity of Spartina alterniflora. The field measurements corroborate the model results by way of prediction of relatively productive marsh at four of the six sites (percent coverage of Spartina alterniflora of 30–41%, canopy height of 0.27–0.67 m and simulated biomass density of greater than 750 g m−2 over at least half of the local area within 500-m radii of the measurement sites) and relatively limited marsh at the two other sites (percent coverage of Spartina alterniflora of 3–5%, canopy height of 0.11–0.29 m, and simulated biomass density of greater than 750 g m−2 over less than one-tenth of the local area within 500-m radii of the measurement sites). The model is applied in a coupled fashion for 50 years of time into the future using ten 5-year increments, where each increment of Hydro-MEM accounts for the natural accretion of the marsh, an update of the digital elevation model and bottom-friction parameterization, and the subsequent feedback to the hydroperiod and marsh productivity. Hydro-MEM is shown to exhibit rate-sensitivity with respect to sea-level rise exceeding the marsh accretion rate, whereby a sudden loss of marsh elevation occurs in such instances of marsh destabilization. Demonstrating rate-critical transition, the model proves flexible to account for the non-homogeneous and transient nature of the fast-slow variables, whereby the marsh migrates away from the tidal creeks and further into the upland zones. Practical implication of the model results is illustrated by identifying zones of lands into which marsh will be able to migrate and where existing marsh will not survive under increasing sea level. Post-analysis compares the final model output against land use/cover zonation to correct the 50-year simulation results of marsh productivity for elevation-appropriate regions but that are developed, freshwater, or otherwise inappropriate land type for salt-marsh habitat.

An historical storm population is developed for the St. Johns River, located in northeast Florida US east coast, via extreme value assessment of an 89-year-long record of hourly water-level data. Storm surge extrema and the corresponding (independent) storm, systems are extracted from the historical record as well as the linear and nonlinear trends of mean sea level. Peaks-over-threshold analysis reveals the top 16 most-impactful (storm surge) systems in the general return-period range of 1-100 years. Hurricane Matthew (2016) broke the record with a new absolute maximum water level of 1.56 m, although, the peak surge occurred during slack tide level (0.00 m). Hurricanes and tropical systems contribute to return periods of 10-100 years with water levels in the approximate range of 1.3-1.55 m. Extratropical systems and nor'easters contribute to the historical storm population (in the general return-period range of 1-10 years) and are capable of producing extreme storm surges (in the approximate range of 1.15-1.3 m) on par with those generated by hurricanes and tropical systems. The highest astronomical tide is 1.02 m, which by evaluation of the historical record can contribute as much as 94% to the total storm-tide water level. Statically, a hypothetical scenario of Hurricane Matthew's peak surge coinciding with the highest astronomical tide would yield an overall storm-tide water level of 2.58 m, corresponding to an approximate 1000-year return period by historical comparison. Sea-level trends (linear and nonlinear) impact water-level return periods and constitute additional risk hazard for coastal engineering designs. (C) 2017 Elsevier B.V. All rights reserved.