Improving Flood Risk Estimates and Mitigation Policies in Coastal Louisiana under Deep Uncertainty

Future climate change presents a threat of increased hurricane flood risks to many coastal cities. In the wake of Hurricanes Katrina and Rita in 2005 and Hurricane Ike in 2008, the U.S. Army Corps of Engineers has provided ;A previous study of risk in coastal Louisiana estimated the 100-year flooding and economic damage within the HSDRRS by estimating 100-year surge levels at each point along the system boundary, calculating overtopping rates from this surge surface, and then passing the resulting volumes of water through an interior drainage model to calculate interior flood depths. This "surge surface" methodology can produce misleading results, as a storm producing a 100-year surge at one point along the system is unlikely to simultaneously produce 100-year surge levels everywhere around the system exterior.;The Coastal Louisiana Risk Assessment (CLARA) model instead estimates surge and wave characteristics from a large set of synthetic storms. Each storm is run through the interior drainage model separately, and the resulting flood depths are weighted by a parameterized likelihood of each synthetic storm. This results in an empirical distribution of interior flood depths that accounts for the geographic variation in surge response inherent to any individual storm.;100-year and 500-year flood depth exceedances within the New Orleans HSDRRS calculated by our new methodology differ from those estimated by the surge surface method, suggesting a need to re-evaluate current design philosophies for hurricane risk reduction systems. Rather than setting design elevations based on inputs (e.g., a "100-year storm event"), they should be set with an eye towards consequences (e.g., to reduce 100-year interior flood depths to an acceptable level). 3.