We developed a coupled overland and river model for modeling compound flooding using the kinematic wave approximation on inland sections of an unstructured mesh, and the diffusive wave approximation on riverine sections. A finite element method is used for spatial discretization and a Crank-Nicolson scheme is used for time discretization. A wetting and drying algorithm is implemented for improved efficiency in the model. Pluvial conditions and tidal conditions are implemented as source terms in the river model. The results show that effects from compound inundation could be captured using this framework.

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.

Worldwide coastal land-margins are prone to many flood hazards such as astronomical tides, tropical cyclones, sea-level rise, and extreme precipitation events. Compound flood events, in which two or more flooding mechanisms occur simultaneously or in close succession (Santiago-Collazo et al., 2019, https://doi.org/10.1016/j.envsoft. 2019.06.002), can exacerbate the inundation impacts due to the highly non-linear interaction of coastal and hydrologic processes. Furthermore, sea-level rise will increase the hazard at low-gradient coastal land-margins when assessing future projections due to its non-linear nuance on the compound flood (Santiago-Collazo et al., 2021, https://doi.org/10.3389/fclim.2021.684035). Therefore, there is an urgent need to develop new technologies capable of comprehensively studying compound flood events and identifying hotspots prone to these inundations. This research aims to develop a technique capable of defining and classifying coastal land- margins based on physically-based criteria due to surface flow hydrodynamics. A one-dimensional (1-D) hydrodynamic model was used to quantify the hydrodynamic response of thousands of different combinations of input parameters (e.g., astronomical tides, storm surge, precipitation, and landscape) that define a coastal land-margin. This 1-D fully-coupled model, based on the shallow water equations, was applied at a national spatial scale, considering several coastal watersheds within the Gulf of Mexico and the US East coast. One of the main goals of this tool is to identify coastal land-margins vulnerable to compound flood hazards over broad spatial scales (e.g., national or global scale). Findings suggest that low-gradient (e.g., slopes less than 0.01 m km-1) coastal land-margins are more susceptible to compound flood impacts than ones with a steeper gradient under most flooding scenarios. Future research will focus on applying this tool on a worldwide basis to test its capabilities at low-resolution, scarce data regions. A worldwide classification of coastal land-margins may help authorities, policy-makers, and professionals converge on better coastal resilience measures, such as comprehensive compound flood analysis to delineate accurate compound flood hazard maps.Full online poster version at agu2021fallmeeting-agu.ipostersessions.com/Default.aspx?s=FA-1F-20-67-21-4E-E7-69-9F-89-1E-33-BB-3D-2D-40

Tropical coastal communities accommodate critical infrastructure, densely-populated urban regions, tourism-driven economies, and industrial facilities. These communities are prone to multiple flood hazards such as nuisance flooding, tropical cyclones, extreme rainfall events, and sea-level rise (SLR). Thus, governments and stakeholders Are pursuing a range of measures to enhance flood resiliency. These alternatives can be classified into structural (i.e., conventional infrastructure), non-structural, natural and nature-based features (NNBF), and hybrid systems. While there is a large body of published work on coastal risk reduction via conventional infrastructure and NNBF, there is a paucity of information on Hybrid Infrastructure Systems (HIS) in the literature, especially under multi-flood hazard scenarios. This research aims to assess various HIS under multiple flood hazards for flood reduction and wildlife and habitat benefits in the coastal community of Tybee Island (Georgia, US). This community is the most densely populated barrier island in Georgia and receives over 1 million visitors each year. The Interconnected Channel and Pond Routing (ICPR) hydrodynamic model was selected to simulate hydrologic (e.g., rainfall and infiltration) and coastal (e.g., tides, storm surge, and SLR) processes, and various combinations of HIS including several conventional infrastructures (e.g., stormwater drainage system, culverts, pump systems), inland (e.g., bioswales and pocket parks) and coastal (e.g., horizontal levees and retention ponds with smart tidal gates) NNBF. Results show that NNBF can prolong the service life of conventional infrastructure in a HIS by reducing flooding stress on these structures while promoting wildlife habitats and marsh conservation by increasing the hydraulic connectivity in the tidal river system. HIS alternatives were ranked using multi-criteria decision analysis. The local government, residents, and stakeholders will select their preferred alternative for detailed design. Local studies and modeling of multi-hazard flood processes can provide insight into the performance of HIS, thus providing the opportunity to policy-makers and government agencies to improve design standards and permitting procedures for HIS at a regional scale.