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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., coastal and pluvial) that varied in magnitude, time, and space. This study aims to evaluate each flooding mechanism and the associated hydrodynamic responses by performing a sensitivity analysis and developing a non‐linear equation that could correlate the flood drivers with the severity of its flood. Compound inundation levels are affected by the magnitude and timing of each flooding mechanism. Results highlight the need to consider momentum exchange during a compound event and the importance of reduced‐physics approaches that can improve the interaction between flood drivers when paired with a moving coupling node approach. 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. Plain Language Summary: Intense rainfall, tides, and hurricane storm surges can flood coastal regions with mild sloping terrain. When multiple flooding events occur at the exact location, the inundation effects can be aggravated by the interaction of these physical processes. This study aims to disentangle the complexities of interactive physics according to fundamental concepts. Thus, identifying regions affected by hydrological and coastal processes can improve flood mapping techniques for coastal communities in future efforts. Results show that each flood event's strength and time of occurrence affect the total inundation levels. Furthermore, the flow velocity from these flood processes is essential when they interact. Our research can serve as a foundation for further studies to enhance tools that accurately predict these flood events, resulting in better planning and preparedness for worldwide coastal communities to reduce property damage and loss of lives. Key Points: A reduced‐physics one‐dimensional flood model was developed to assess compound floods at an idealized coastal watershed transectA moving coupling node approach can capture the momentum exchange between pluvial and coastal flood driversA non‐linear equation correlating storm characteristics with flood severity was developed to examine the driver's interaction [ABSTRACT FROM AUTHOR]

Rising sea levels have increased flood risk in coastal communities on both the east and west coasts of the USA. The goal of this analysis is to approximate flood defense costs from cyclonic flooding as a partial means to evaluate the resilience of coastal communities. Storm surge models were previously constructed via an established approach to represent historical and future coastal Louisiana landscapes and associated flood patterns. Coastal flooding was also previously simulated via a suite of 14 hurricanes. Approximate levee heights surrounding Lafitte, Louisiana, are calculated from the surge and wave output of Hurricane Isaac, the predominated hurricane in the Lafitte area for all years examined (NAVD88, m): 1.68 (1930), 2.92 (1970), 3.30 (1990), 4.82 (2010), 5.93 (2030), 6.57 (2050), 7.16 (2070), 7.70 (2090), and 8.22 (2110). Approximate costs per person are also calculated (2010 USD): $49,500 (1930), $41,400 (1970), $37,500 (1990), $181,600 (2010), $223,600 (2030), $247,800 (2050), $269,900 (2070), $290,100 (2090), and $309,800 (2110). The Gulf of Mexico (GOM) migrated 7.4 km inland within the Louisiana Barataria coastal basin between 1973 and 2010. For each person in Lafitte, flood defense costs increased approximately (2010 USD) $19,000 per person per kilometer inland migration of the GOM from 1973 to 2010. The methodology developed in this case study effectively connects wetland loss with increased flood defense costs and can be applied to communities with similar challenges. [ABSTRACT FROM AUTHOR]

This paper reports on the process and results of focus groups conducted as stakeholder engagement for a two‐year, marine extension agency‐sponsored interdisciplinary project in coastal Louisiana, USA. The project involved refining the topographic features (eg flood protection infrastructure) of a computational storm surge model using local knowledge of elevation data. Coastal Louisiana experiences continual changes to the built and natural environment and persistent land loss, necessitating frequent adjustments to real‐time storm surge and restoration planning models to make them more useful for decision‐makers. Stakeholder involvement ensured accurate updates to the real‐time computational system and accessible and understandable modelling results displayed by an interactive decision‐support tool. The findings provide insights and recommendations from various practitioners that are applicable to decision‐support model and tool development for coastal hazard emergency response. [ABSTRACT FROM AUTHOR]

Storm surge models are constructed to represent the Louisiana coastal landscape circa 1850, 1890, 1930, 1970, 1990, 2010, 2030, 2050, 2070, 2090, and 2110. Historical maps are utilized to develop models with past landscapes while a continuation of recent landscape trends is assumed for future models. The same suite of meteorological wind and pressure fields is simulated with each storm surge model. Simulation results for 1850 and 1890 demonstrate minimal change in storm surge characteristics along the Louisiana coast. A mean maximum storm surge height increase of 0.26 m from 1930 to 2010 is quantified within the sediment-abundant Atchafalaya-Vermilion coastal basin, while increases of 0.34 m and 0.41 m are quantified within sediment-starved Terrebonne and Barataria, respectively. Future mean maximum storm surge heights increase across these three coastal basins by 0.67 m, 0.55 m, and 0.75 m, indicating negligible differences from 2010 to 2110, regardless of sediment availability. Results indicate that past changes in the Louisiana coastal landscape and storm surge were a consequence of local land and river management decisions while future changes are dominated by relative (subsidence and eustatic) sea level rise. Projecting landscape and surge changes beyond 50 years could aide policy makers as they work to enhance resilience across coastal Louisiana. Similar analyses could be conducted for other deltas across the world, such as the Ganges, that are experiencing challenges comparable to those of the Mississippi River Delta. [ABSTRACT FROM AUTHOR]

The co-evolution of wetland loss and flood risk in the Mississippi River Delta is tested by contrasting the response of storm surge in coastal basins with varying historical riverine sediment inputs. A previously developed method to construct hydrodynamic storm surge models is employed to quantify historical changes in coastal storm surge. Simplified historical landscapes facilitate comparability while storm surge model meshes developed from historical data are incomparable due to the only recent (post-2000) extensive use of lidar for topographic mapping. Storm surge model meshes circa 1930, 1970 and 2010 are constructed via application of land to water (L:W) isopleths, lines that indicate areas of constant land to water ratio across coastal Louisiana. The ADvanced CIRCulation (ADCIRC) code, coupled with the Simulating WAves Nearshore (SWAN) wave model, is used to compute water surface elevations, time of inundation, depth-averaged currents and wave statistics from a suite of 14 hurricane wind and pressure fields for each mesh year. Maximum water surface elevation and inundation time differences correspond with coastal basins featuring historically negligible riverine sediment inputs and wetland loss as well as a coastal basin with historically substantial riverine inputs and wetland gain. The major finding of this analysis is maximum water surface elevations differences from 1970 to 2010 are 0.247 m and 0.282 m within sediment-starved Terrebonne and Barataria coastal basins, respectively. This difference is only 0.096 m across the adjacent sediment-abundant Atchafalaya-Vermilion coastal basin. Hurricane Rita inundation time results from 1970 to 2010 demonstrate an increase of approximately one day across Terrebonne and Barataria while little change occurs across Atchafalaya-Vermilion. The connection between storm surge characteristics and changes in riverine sediment inputs is also demonstrated via a sensitivity analysis which identifies changes in sediment inputs as the greatest contributor to changes in storm surge when compared with historical global mean sea level (GMSL) rise and the excavation of major navigation waterways. Results imply the magnitude of the challenge of preparing this area for future subsidence and GMSL rise. • Land to water (L:W) isopleths are derived for storm surge model mesh years 1930, 1970, 2010. • ADCIRC storm surge model meshes are constructed featuring historic Louisiana coastal landscapes for 1930, 1970, 2010. • Hydrologic unit code 6 (HUC6) coastal basins and HUC12 sub-watersheds are utilized to quantify and compare mesh year results. • Results indicate riverine sediment input is a HUC6 coastal basin scale compensator for relative sea level rise. • Reduced sediment input leads to increased storm surge heights in sediment starved coastal basins 1970-2010. [ABSTRACT FROM AUTHOR]

The influence of including the dynamic effects of future shoreline changes associated with sea level rise into hydrodynamic modeling is evaluated for the coast of the Northern Gulf of Mexico from Mobile Bay, AL to St. Andrew Bay, FL. A two-dimensional, depth-integrated hydrodynamic model forced by astronomic tides and hurricane winds and pressures representative of Hurricanes Ivan (2004), Dennis (2005) and Katrina (2005) is used to simulate present conditions, 2050 projected sea level (0.46 m rise) with present-day shorelines, and 2050 sea level with projected 2050 shorelines. The 2050 shoreline and nearshore morphology are projected using Coastal Vulnerability Index shoreline change rates to determine the position of the new Gulf and bay shorelines, while the active beach profile is shifted horizontally according to the amount of erosion or accretion, and vertically to keep pace with rising seas. Hydrodynamic model results show that taking a dynamic approach to modeling sea level rise (as opposed to a static, or 'bathtub' approach) increases tidal ranges and tidal prisms within the bay systems. Incorporating the projected shoreline changes does not alter tidal ranges, but some bays experience changes in tidal prisms depending on whether the planform area of the bay increases or decreases with the projected erosion or accretion. Barrier islands with projected erosion are vulnerable to increased overtopping from storm surge inundation, which impels more water into the back-bays and increases the inland inundation extent and magnitude. Inundation along barrier islands with projected accretion remains relatively the same as inundation under present-day shorelines, which prevents additional overtopping and limits more water from entering back-bays. Results demonstrate that although modeling sea level rise as a dynamic process is necessary, the incorporation of shoreline changes has variable impacts when evaluating future hydrodynamics and the response of the coastal system to sea level rise. [ABSTRACT FROM AUTHOR]

Abstract: This study is focused on the integration of bare earth lidar (Light Detection and Ranging) data into unstructured (triangular) finite element meshes and the implications on simulating storm surge inundation using a shallow water equations model. A methodology is developed to compute root mean square error (RMSE) and the 95th percentile of vertical elevation errors using four different interpolation methods (linear, inverse distance weighted, natural neighbor, and cell averaging) to resample bare earth lidar and lidar-derived digital elevation models (DEMs) onto unstructured meshes at different resolutions. The results are consolidated into a table of optimal interpolation methods that minimize the vertical elevation error of an unstructured mesh for a given mesh node density. The cell area averaging method performed most accurate when DEM grid cells within 0.25 times the ratio of local element size and DEM cell size were averaged. The methodology is applied to simulate inundation extent and maximum water levels in southern Mississippi due to Hurricane Katrina, which illustrates that local changes in topography such as adjusting element size and interpolation method drastically alter simulated storm surge locally and non-locally. The methods and results presented have utility and implications to any modeling application that uses bare earth lidar. [Copyright &y& Elsevier]

We studied the hydrodynamic response caused by the four major hurricanes that struck Florida's coasts in 2004; Charley, Frances, Ivan, and Jeanne. A large-scale, shallow-water-equation model was applied so as to simulate wind and tidally driven hydrodynamics from the deep ocean into the shelf and coastal waters of Florida. Hurricane Ivan served as the calibration case, where the adjusted parameter was the wind drag coefficient. We identified an "increased" wind drag coefficient to perform best and suggest it as a first approximation to the wave contribution to the hydrodynamics. The increased drag coefficient is offered to the consulting and/or forecasting communities, who are frequently without wave-modeling resources, as a pragmatic approach to approximate for waves. Hurricanes Charley, Frances, and Jeanne serve as the validation cases in which the increased wind drag coefficient was applied. Analysis was performed by inspection of maximum water-surface elevations, which are interpreted in terms of shelf and bay dynamics for the west coast hurricane cases (Charley and Ivan) and in terms of channel hydraulics for the east coast hurricane cases (Frances and Jeanne). We conclude that the broad shelf off Florida's west coast allows the storm tide to accumulate along the open coast and that the embayments further magnify the storm tide, and that the Atlantic Intracoastal Waterway along Florida's east coast is effective in propagating storm tide, where we show compartmentalization of the storm tide in the Indian River lagoon as caused by the flow impediment of the causeway abutments. [ABSTRACT FROM AUTHOR]

This paper demonstrates a practical application of coupling a hydrodynamic model with a wave model for the calculation of storm tide elevations in the St. Johns River (Northeastern Florida). Hurricane Floyd (1999) is chosen as the storm of interest due to its track which paralleled the northeast coast of Florida without making a direct landfall on the St. Johns River. The advanced circulation (ADCIRC) numerical code is applied as the hydrodynamic model for the computation of two-dimensional circulation resulting from astronomic tides and meteorologically induced storm surge. The simulating waves nearshore (SWAN) numerical code is applied as the wave model for the computation of wind-induced waves. Two model implementations are considered in order to investigate the effect of short wave contributions on the overall storm tide water level: (1) a one-way coupling procedure that transfers gradient of wave radiation stresses from SWAN to ADCIRC; and (2) a two-way coupling procedure that builds on the former to include feedback of water levels and currents from ADCIRC to SWAN. Simulated storm tide elevations are compared to historical National Ocean Service data to result in two major conclusions: (1) wind-induced waves play a significant role in the storm tide (contributing 10–15% of the peak water level), namely with respect to the transfer of momentum from the dissipation of short waves to the long-wave motion of the storm surge; and (2) a local-scale hydrodynamic model requires the application of a hydrograph boundary condition in order to account for the remote effects of the storm surge response. [ABSTRACT FROM AUTHOR]

Abstract: Bridge scour modeling requires storm surge hydrographs as open ocean boundary conditions for coastal waters surrounding tidal inlets. These open coast storm surge hydrographs are used to accurately determine both horizontal and vertical circulation patterns, and thus scour, within the inlet and bay for an extreme event. At present, very little information is available on the effect that tidal inlets have on these open coast storm surge hydrographs. Furthermore, current modeling practice enforces a single design hydrograph along the open coast boundary for bridge scour models. This study expands on these concepts and provides a more fundamental understanding on both of these modeling areas. A numerical parameter study is undertaken to elucidate the influence of tidal inlets on open coast storm surge hydrographs. Four different inlet–bay configurations are developed based on a statistical analysis of existing tidal inlets along the Florida coast. The length and depth of the inlet are held constant in each configuration, but the widths are modified to include the following four inlet profiles: 1) average Florida inlet width; 2) 100 m inlet width; 3) 500 m inlet width; and 4) 1000 m inlet width. Results from these domains are compared to a control case that does not include any inlet–bay system in the computational domain. The Advanced Circulation, Two-Dimensional Depth-Integrated (ADCIRC-2DDI) numerical code is used to obtain water surface elevations for all studies performed herein. The code is driven by astronomic tides at the open ocean boundary, and wind velocities and atmospheric pressure profiles over the surface of the computational domains. Model results clearly indicate that the four inlet–bay configurations do not have a significant impact on the open coast storm surge hydrographs. Furthermore, a spatial variance amongst the storm surge hydrographs is recognized for open coast boundary locations extending seaward from the mouth of the inlet. In addition, a storm surge study of Hurricane Ivan in the vicinity of Escambia Bay along the Panhandle of Florida is performed to assess the findings of the numerical parameter study in a real-life scenario. The main conclusions from the numerical parameter study are verified in the Hurricane Ivan study: 1) the Pensacola Pass–Escambia Bay system has a minimal effect on the open coast storm surge hydrographs; and 2) the open coast storm surge hydrographs exhibit spatial dependence along typical open coast boundary locations. The results and conclusions presented herein have implications toward future bridge scour modeling efforts. [Copyright &y& Elsevier]

The storm surge generated by Hurricane Hugo (1989) is reproduced for the purposes of (1) evaluating the significance of including inland flooding areas and corresponding levels of spatial resolution and (2) assessing the spatial variability of near-inlet boundary forcings with respect to storm surge hydrographs. The study focuses on the coastal regions of South Carolina, incorporating inundation areas between Charleston and Shallotte Inlet and including Winyah Bay and the Waccamaw River. The region of interest is modeled with five variations of an unstructured, finite-element mesh, including a localized coarse grid with and without floodplains, a localized fine grid with and without floodplains, and a large-domain approach that includes floodplains and the continental shelf and deep ocean. Simulation results demonstrate that the inclusion of inland flooding areas significantly improves the description of storm surge generation within coastal regions. While the large-domain approach is shown to be beneficial, it is determined that the coarse mesh resolution of the floodplains is sufficient. Computed water surface elevations at specific locations surrounding Winyah Bay Inlet indicate that storm surge hydrographs are highly spatially dependent near inlets, as opposed to the astronomical tides, which exhibit minimal variance in space around the inlet environment. [ABSTRACT FROM AUTHOR]

A previously developed research-grade (e.g. high-resolution) unstructured mesh of the northern Gulf of Mexico (named NGOM3) is optimized to produce a computationally efficient forecast-grade mesh for deployment in a real-time hurricane storm surge early warning system. The real-time mesh is developed from a mesh decimation scheme with focus on the coastal floodplain. The mesh decimation scheme reduces mesh nodes and elements from the research-grade mesh while preserving the representation of the bare-earth topography. The resulting real-time unstructured mesh (named NGOM-RT) contains 64% less mesh nodes than the research-grade mesh. Comparison of (ADCIRC + SWAN) simulated times-series and peak water levels to observations between the research-grade and real-time-grade meshes for Hurricanes Ivan (2004), Dennis (2005), Katrina (2005), and Isaac (2012) show virtually no difference. Model simulations with the NGOM-RT mesh are 1.5–2.0 times faster than using NGOM3 on the same number of compute cores. [ABSTRACT FROM AUTHOR]

Extreme coastal flooding poses a major threat to human life and infrastructure. Low-gradient coastal watersheds can be vulnerable to flooding from both intense rainfall and storm surge. Here we present a comprehensive review of the most recent studies that quantify extreme flooding using variations of a compound inundation model. A compound inundation model may consist of different numerical models, observed data, and/or a combination of these. The definitions, advantages, and limitations of each joining technique are discussed with the goal of enabling and focusing subsequent research. Future investigation should focus on the development of a tight-coupling procedure that can accurately represent the complex physical interactions between storm surge and rainfall-runoff. A more accurate compound flood forecast tool can help decision-makers, stakeholders and authorities converge on better coastal resiliency measures that can potentially save human lives, aid in the design of structures and communities, and decrease property damage. • Rainfall-runoff and storm surge interact in many forms at the coastal floodplain. • Compound flooding can be estimated by joining models using different techniques. • The linking technique neglects many physical interactions of the compound flood. • Tight-coupling can enable holistic assessments within the coastal flood zone. [ABSTRACT FROM AUTHOR]

An introduction to the journal is presented in which the editors discuss various reports published within the issue including one on observed sea-level rise trends and acceleration over the last 100 years, another on the relative impacts of local sea-level rise and increasing wave heights, and another one on sea-level rise effects on storm surge and nearshore waves on the Texas coast.

Coastal and riverine flooding is one of the most common environmental hazards that affect billions of people worldwide. A coupled hydrologic and coastal storm surge simulation is required to develop an improved understanding of the individual and collective mechanisms that can cause flooding within watersheds. These simulations are dependent on an accurate digital elevation model (DEM); however, it is a challenge to include numerical model resolution as fine as contemporary DEMs due to the enormous computational cost. Therefore, significant vertical features (VFs) such as roadbeds, levees, railroads, and natural ridges must be identified and considered in developing the model representation of the DEM since the VFs can affect flow propagation. PyVF is an open-source program to extract significant VFs from a high-resolution, bare-earth, LiDAR-derived DEM automatically. This paper introduces the methods and shows the automated extraction of VFs for a coastal, urban, mountain and beach area. • PyVF is a program for identifying vertical features automatically from high-resolution topographic elevation data. • The vertical features are defined by some meaningful parameters. • A variety of landforms (e.g., low-gradient area, urban area, mountain area, beach area) can benefit from the ability of PyVF. [ABSTRACT FROM AUTHOR]

Summary: Water surface elevations in the St. Johns River (Northeastern Florida) are simulated over a 122-day time period spanning June 1–September 30, 2005, which relates to a particularly active hurricane season for the Atlantic basin, and includes Hurricane Ophelia that significantly impacted the St. Johns River. The hydrodynamic model employed for calculating two-dimensional flows is the ADCIRC (Advanced Circulation Model for Oceanic, Coastal, and Estuarine Waters) numerical code. The region of interest is modeled using three variations of an unstructured, finite element mesh: (1) a large-scale computational domain that hones in on the St. Johns River from the Western North Atlantic Ocean, Gulf of Mexico, and Caribbean Sea; (2) a shelf-based subset of the large domain; (3) an inlet-based subset of the large domain. Numerical experiments are then conducted in order to examine the relative importance of three long-wave forcing mechanisms for the St. Johns River: (1) astronomic tides; (2) freshwater river inflows; (3) winds and pressure variations. Two major findings result from the various modeling approaches considered in this study, and are applicable in general (e.g., over the entire 122-day time period) and even more so for extreme storm events (e.g., Hurricane Ophelia): (1) meteorological forcing for the St. Johns River is equal to or greater than that of astronomic tides and generally supersedes the impact of freshwater river inflows, while pressure variations provide minimal impact; (2) water surface elevations in the St. Johns River are dependent upon the remote effects caused by winds occurring in the deep ocean, in addition to local wind effects. During periods of calm weather through the 122-day time period, water surface elevations in the St. Johns River were generally tidal in response, with amplitudes exceeding 1m at the mouth and diminishing to less than 10cm 150km upriver. Considering an extreme storm event, the timing of Hurricane Ophelia occurred during the neap phase of the tidal cycle and at the mouth of the St. Johns River, the wind-driven storm surge was near equal to the tidal component, each contributing about 0.5m to the overall water surface elevation. However, 150km upriver, meteorological forcing dominated, as over 90% of the total water surface elevation was driven by winds and pressures. The simulation results replicate these behaviors well. As a supplement, it is shown that applying a hydrograph boundary condition, generated by a large domain, to a localized domain is highly beneficial towards accounting for the remote wind forcing. [Copyright &y& Elsevier]