Investigation of Compound Flooding in the Amite-Comite River basin in Southern Louisiana

Low-lying coastal population centers are threatened by the occurrence of tropical wind-driven coastal storms as well as non-tropical rainfall events. When these events coincide, they typically result in devastating impacts on coastal communities. Recent decades have seen an increase in extreme coastal events combined with heavy convective rainfalls. The combination of pluvial flooding and coastal storm surges results in a compound flooding event. Traditionally, inland flooding analyses and coastal storm surge calculations were done using specialized and separate sets of numerical models. Often these tools are completely decoupled and communicated simply through an exchange of information at their respective boundaries. The use of decoupled numerical models led to questions about their ability to capture the dynamics of compound floodings. For this reason, compound flooding is an active research area that analyzes the complex interaction between pluvial, fluvial, coastal, and groundwater flooding. In this work, we analyze compound flooding by implementing hydrodynamic models for a "transition zone" watershed (Amite-Comite) in Southeast Louisiana, USA. We utilized both Delft3D and HEC-RAS for this analysis. The models extend from the headwater of the Amite-Comite River to Breton sound in the Mississippi River Delta region. The grid resolution varied from 10 m to 3,000 m. We applied rectangular curvilinear meshes for all major channels and flexible triangular meshes for the rest of the watershed. Observed tidal water levels and atmospheric forcings such as gridded rainfall, wind, and pressure data were used in the model. Two major events were hindcasted. The first was a historical major rain event in August 2016 (~ 500-year return period), and the second was a wind event from Hurricane Isaac in 2012. The models were used to examine the following hypotheses; the transition from coastal to inland processes is not static; rather, it shifts based on the strength of events driving the hydrodynamics; simple superposition of the flood heights from coastal storms and pluvial flooding leads to significant overestimation of flood elevations. This study also provides suggestions about appropriate locations and mechanisms to couple coastal models with inland flood models.