Salt marsh effects on flood hazard metrics under compound coastal flooding

Abstract

Coastal flooding can affect communities in different ways, mainly depending on the flood drivers involved in each event and the exposed assets. To reduce flood risk, managers and decision makers need a variety of tools to help make informed and scientifically backed decisions. In this sense, the present work uses numerical modeling to simulate tropical cyclone conditions in Mobile Bay, Alabama for the purpose of applying the results to flood risk management strategies. To those ends, this work focuses on three main contributions: 1) showing the importance of compounding interactions of unsteady fluvial and coastal sources of flooding in accurate hurricane-driven flood hazard assessment; 2) assessing the influence of salt marsh vegetation represented through bottom roughness parameterization on coastal flooding through changes in hydrodynamic energy and depth–velocity conditions; and 3) demonstrating an enhanced safety thresholds framework based on a Composite Flow Intensity indicator to identify flood hazards by combining multiple hydraulic characteristics of flooding. Using metrics that account for flood depth, flow velocity, and wave conditions, we find that the flood attenuation benefits associated with marsh vegetation are limited during extreme hurricane conditions. However, under a reduced-forcing storm scenario, flood severity metrics were reduced by approximately 21% across both marshes and the adjacent inland areas. Although this scenario represents less severe storm conditions than the Hurricane Isaac baseline simulation, the results suggest that the modeled marsh vegetation scenarios can provide meaningful reductions in flood intensity across a range of storm conditions and may be particularly effective during moderate flood events.