Enhancing Flood Hazard Assessments in Coastal Louisiana Through Coupled Hydrologic and Surge Processes

Abstract

Traditional coastal flood hazard studies do not typically account for rainfall-runoff processes in quantifying flood hazard and related cascading risks. This study addresses the potential impacts of antecedent rainfall-runoff, tropical cyclone (TC)-driven rainfall, and TC-driven surge on total water levels and its influence in delineating a coastal flood transition zone for two distinct coastal basins in southeastern Louisiana (Barataria and Lake Maurepas watersheds). Rainfall-runoff from antecedent and TC-driven rainfall along with storm surge was simulated using a new rain-on-mesh module incorporated into the ADCIRC code. Antecedent rainfall conditions were obtained for 21 landfalling TC events spanning 1948–2008 via rain stations. A parametric, TC-driven, rainfall model was used for precipitation associated with the TC. Twelve synthetic storms of varying meteorological intensity (low, medium, and high) and total rainfall were utilized for each watershed and provided model forcing for coastal inundation simulations. First, it was found that antecedent rainfall (pre-TC landfall) is influential up to 3 days pre-landfall. Second, results show that antecedent and TC-driven rainfall increase simulated peak water levels within each basin, with antecedent rainfall dominating inundation across the basin's upper portions. Third, the delineated flood zones of coastal, transition, and hydrologic show stark differences between the two basins.

An Examination of Compound Flood Hazard Zones for Past, Present, and Future Low-Gradient Coastal Land-Margins

“…The model solves the generalized wave-continuity equation, a form of the depthintegrated shallow water equation, for water surface elevation and velocity at every node of an unstructured finite element mesh (Luettich et al, 1992). The simulations performed had the following specifications: one second time step, wetting and drying activated with a 5 cm threshold, the baroclinic and advection terms not activated, and a spatially-constant horizontal eddy viscosity equal to 50 m 2 /s (Bilskie et al, 2021). Bilskie et al (2021) developed a rain-on-mesh module within the ADCIRC model source code to account for rainfall-runoff.…”

“…The simulations performed had the following specifications: one second time step, wetting and drying activated with a 5 cm threshold, the baroclinic and advection terms not activated, and a spatially-constant horizontal eddy viscosity equal to 50 m 2 /s (Bilskie et al, 2021). Bilskie et al (2021) developed a rain-on-mesh module within the ADCIRC model source code to account for rainfall-runoff. This module applied rainfall over individual mesh nodes and elements depending on their current wet/dry status.…”

“…When two or more flooding mechanisms occur in close succession or simultaneously, the event is known as a compound flood (Santiago-Collazo et al, 2019). Flood inundation maps produced from a compound flood study can delineate flood hazard zones due to rainfall-runoff, storm surge, and their combination (Bilskie et al, 2021). However, simulating compound flooding presents several challenges that extend from computational modeling and the prerequisite environmental drivers.…”

“…This research effort entails a detailed flood hazard assessment that encompasses multiple drivers of compound flooding. The methods presented by Bilskie et al (2021) were adopted to delineate compound flood hazard zones in two Louisiana coastal watersheds: Barataria and Lake Maurepas. The methods are described next, including the selected hydrodynamic model, the model inputs, and the technique for delineating the compound flood hazard zone.…”

“…The compound flood hazard zones are based on the flooding mechanisms that dominated the region, which are divided into hydrologic, coastal, and transition hazard zone. To assess the evolution of the compound flood hazard zones, the coastal landscapes (e.g., c. 1890-2090) developed by Siverd et al (2019b) are used in the compound flood modeling framework of Bilskie et al (2021). The spatial extent and rate of change in the coverage area of each compound flood hazard zones are quantified for each coastal land-margin condition, from which the evolution of the compound flood hazard zones are evaluated.…”