ACCOUNTING FOR LEVEE OVERTOPPING DURATION: A TEST WITH HURRICANE KATRINA CONDITIONS

Abstract

Overtopping of earthen levees can result in lee side erosion, levee failure and severe flooding consequences. Erosion takes time and early design guidance did not account for overtopping duration. The potential overtopping durations associated with hurricanes depends on the time histories of the surges and waves and are usually shorter than for riverine or coastal flooding by other storm types. With significant advancements in hurricane-induced storm surge modeling, it is appropriate to improve the levee erosion response characteristics. This paper presents the results of a blind-folded comparison of a method developed by Dean, et al (2010) to two levee locations in the New Orleans area. The levees failed in one area where the waves were larger than the second area where the levees performed well. The method applied accounts for surge and wave time histories, levee geometry and grass cover quality. Results are consistent with failure at the location of higher wave heights; however, the method also predicts failure at the location where levee function was retained. At the latter location, for good quality grass cover and a wave period of 4 s, a required levee crest elevation of 5.9 m is determined compared to an actual elevation of 5.8 m. For poor grass quality and a wave period of 8 s, the method indicates a required crest elevation of 7.4 m. The method can be applied to optimize levee design, for example, through investigating the trade offs between crest elevation and levee side slopes.