Modeling the effects of mangrove hybrid infrastructure for coastal flood protection.

Coastal communities throughout the (sub)tropics face increasing threats from tropical cyclones, prompting the need to develop innovative resiliency strategies. While grey infrastructure such as retaining/seawalls have long been the default solution for protecting coastal properties from destructive storm surge flooding and wave impacts, coastal wetlands offer a potential enhancement that balances shoreline protection with ecological benefits. In this study, we evaluate the outcomes and implications of a narrow mangrove belt seaward of vertical seawalls, and the impacts that they would have in an urban, developed environment at the community/neighborhood scale. We simulated eighteen hybrid designs which tested three seawall crest heights (0, 1 m, and 2 m) and six mangrove forest widths (0 - 50 m) using a coupled high-resolution hydrodynamic-wave model of a shoreline in Tampa Bay, Florida (USA). For each design, performance was evaluated under three major storm surge events that caused extensive flood damage in recent years to the local community. Our results show that just 10 m of mangrove forest can attenuate wave heights by 64%, compared to just 22% wave height attenuation over the same distance of unvegetated shoreline. However, with regards to reduction of storm surge, forest widths up to 50 m were not sufficient to reduce storm surge levels. An elevated seawall improved flood protection when the crest height exceeded the storm surge level but posed drainage issues. Hybrid designs composed of a narrow mangrove belt in front of a seawall reduced wave energy reaching inland and impacting the grey infrastructure more effectively than an unvegetated seawall. These results highlight the practicability and potential benefits of integrating nature into urban resiliency strategies.