Assessment of the Evolution of Coastal Louisiana Storm Surge and Flood Risk 1850 - 2110

To prevent avulsion of the Mississippi River to the Atchafalaya River, Mississippi and Red River discharge diverted to the Atchafalaya has been controlled at 30% since 1963 (Wellner et al., 2005; Wells et al., 1982). The result is a constant supply of land-building sediment to the Wax Lake-Atchafalaya and Balize deltas while sediment delivery is interrupted elsewhere across the Louisiana coast (Twilley et al., 2016). ADvanced CIRCulation (ADCIRC) hydrodynamic storm surge models representing historic and future eras of the Louisiana coastal landscape are developed to examine the evolution of flood risk (i.e. the product of probability and consequence) in coastal communities stemming from this interruption of sediment delivery. Specifically, the impact of historic and future land loss on hurricane storm surge (e.g. maximum water levels and inundation time) and the associated human response is investigated. Land:Water (L:W) isopleths (Gagliano et al., 1970, 1971, Twilley et al., 2016) are calculated along the coast from Lake Sabine to the Pearl River for each era. These isopleths are utilized to develop comparable, simplified representations of the Louisiana coastal landscape (e.g. bathymetry, topography, bottom roughness) for ADCIRC storm surge model meshes circa 1850, 1890, 1930, 1970, 1990, 2010, 2030, 2050, 2070, 2090, and 2110 (Siverd et al., 2018).

The goal is to analyze the relationship between change in sediment delivery and change in flood risk via change in storm surge characteristics across coastal basins 1850-2110. Storm surge maximum water levels and inundation time are quantified within both hydrologic unit code 6 (HUC6) coastal basins and the smaller HUC12 sub-watersheds. Sediment-rich HUC6 Atchafalaya-Vermilion is compared to sediment-poor HUC6s Terrebonne and Barataria to analyze the evolution of storm surge across sediment-rich vs. sediment-poor coastal basins 1850-2110. Results will demonstrate the impact of land loss due to change in sediment delivery and the possible benefit to coastal communities of restoration efforts such as increased sediment delivery on the basin scale. This presentation will close with a discussion of how these findings can benefit agencies and academics in pursuit of convergence research results.