Use of the ADCIRC Storm Surge Model for Hurricane Katrina Surge Predictions and Levee Forensic Studies

Recognizing that coastal Louisiana is super susceptible to storm surges, researchers from LSU started providing emergency managers with storm surge forecasts generated by the ADvanced CIRCulation hydrodynamic model (ADCIRC) in 2002. During Katrina LSU issued five surge forecasts over the two days prior to landfall. Model output was made available in near-real-time (www.hurricane.lsu.edu/floodprediction). One advisory was run post landfall. Hurricane Katrina made landfall in Louisiana as a moderately fast-moving Category 3 hurricane 29 th of August, 2005. Of the populated areas that constitute Greater New Orleans (GNO), 80 percent of Orleans Parish, 99 percent of St. Bernard Parish, and approximately 40 percent of Jefferson Parish were flooded, in some cases for weeks. This flooding cost the lives of more than 1,500 residents. Over 100,000 families were rendered homeless. High water marks (HWMs) acquired outside the flood protection levees permitted assessment of the accuracy of the LSU ADCIRC forecasts. The surge forecast associated with the most intense storm prediction pre-landfall produced the best overall match to observed values, resulting in a predicted GNO mean peak elevation of 3.65 m, 0.18 m lower than the observed (3.83 m). The RMS error overall was 0.60 m. The post-storm simulation produced a mean elevation (3.75 m) only 0.08 m lower than the observed mean, but with a larger RMS error (0.72 m) than the best pre-landfall. Surge forecasts show where levees are challenged and can also provide an estimate of the volume of water that would have been introduced if the levees and floodwalls had not failed. Overtopping predicted by ADCIRC would have delivered far less water than what actually entered the city, ranging from 11 percent in St. Bernard to 37 percent in Orleans East. Just under 60 percent of the nearly 23,000 ha that were submerged would not have experienced standing water. On average, breaches are estimated to have transmitted nearly 84 percent of the observed GNO flood volume. Because breaching played such a dominant role, processes that led to the initiation and expansion of breaches greatly influenced the volume of water introduced into each polder.