Hydrodynamic storm surge model simplification via application of land to water isopleths in coastal Louisiana

Abstract

The Mississippi River Delta ranks the seventh largest delta in the world. It provides a habitat for the Louisiana seafood industry, navigation canals and rivers that support five of the 15 largest cargo ports by volume in the United States, and hurricane storm surge protection for coastal cities and oil and gas industry infrastructure that facilitates 90% of the outer continental oil and gas extraction. Due to substantial coastal wetland loss since 1900, the risk of damage to these industries and infrastructure has increased through time. The goal of this research is to develop a methodology to analyze the historical and future evolution of coastal hazards, such as hurricane storm surge, across a complex, low-lying coastal landscape. To accomplish this task, the change in coastal hazards is analyzed through historical changes in coastal wetlands. Specifically, isopleths, defined as lines on a map indicating a constant value of a given variable, are developed to describe areas of constant values of the ratio of land to water (L:W) across coastal Louisiana.In this analysis, a methodology is developed that utilizes land to water (L:W) isopleths to simplify the modern day Louisiana coastal landscape as represented in a state-of-the-art high resolution storm surge model. L:W isopleths are derived for the year 2010 and used to construct 36 storm surge models, each featuring variations of three distinct coastal zones: “High” (i.e. high wetland), “Intermediate” (i.e. wetland), and “Submersed” (i.e. region between open water and wetland). The ADvanced CIRCulation (ADCIRC) code is used to compute water surface elevations and depth-averaged currents forced by hurricane wind and pressures from Hurricanes Rita, Gustav, and Katrina for each model. Peak water levels and volume of inundation are quantified within hydrologic unit code watersheds (HUC12) in order to compare storm surge models featuring high resolution and simplified coastal landscapes.A L:W isopleth permutation of 99%–90%–40%–1% with areas labeled “High” (99%–90%), “Intermediate” (90%–40%) and “Submersed” (40%–1%) is found to best represent simulated storm surge that most closely reproduces the high resolution storm surge model. Simulation results reveal the methodology developed in this analysis is effective in identifying an isopleth permutation that accurately simplifies a high resolution storm surge model. This result may lead to future analyses of the historical evolution of storm surge attenuation in the Mississippi River Delta (MRD) as well as other complex, low-lying deltas. These possibilities include developing storm surge models for the years 1930 and 1970, for instance, with the same isopleth permutation to examine the changes in storm surge attenuation through time. This analysis could also be applied in other similar low-lying coastal regions to conduct past and future analyses of the evolution of coastal hazards.