# The Impact of Leveee Openings on Storm Surge: A Numerical Analysis in Coastal Louisiana
## 3 Model Results and Flux Analysis
### Storm Surge Results  
Figure 4 shows storm surge results at four CRMS stations. The top two and the bottom two are near the upper-crossing and the lower-crossing locations, respectively. The left  
Figure 3: Distributions of hurricane winds (m/s) during Isaac (2012). White lines denote contour lines every 5 m/s. Black arrows denote wind direction. Two red symbols denote two levee-opening locations. (**a**) 03:00 UTC 29 August. (**b**) 08:00 UTC 29 August. (**c**) 11:00 UTC 29 August. (**d**) 17:00 UTC 29 August.  
two and the right two are located at Barataria Basin and Breton Basin, respectively. It can be seen that the model results in the base case with existing conditions agreed well with the measurements, which further validated the model performance. Due to the blockage of the levees and the slow movement of the storm, the maximum surge at two Breton stations reached 3.5 m, which was about 1.5 m higher than that at two Barataria stations. After the removal of levees at both crossing stations, it seems that the impact at two Breton stations was minor, while at two Barataria locations with the approaching of Isaac, the levee removal caused more than 0.5 m of higher surge comparing to the base case in the beginning. After 12:00 UTC 29 August, the difference was diminished. In terms of the maximum surge, the impact was little at all four stations. When all MR&T levees were removed in the model, this impact was significant, especially for the two stations near the upper-crossing location. Breton Sound and Barataria Basin were connected via the MR when surge height was higher than local topographies along both sides of riverbanks, which caused more than 0.5 m of surge reduction at CRMS0132 in Breton and more than 0.5 m of surge increase at CRMS4103 in Barataria.  
Figure 5a shows the calculated distribution of maximum surge by Isaac in the base case under existing conditions. Although Isaac was only a Category 1 hurricane, it induced more than 4 m of surge in the upper Breton Sound due to its low forward speed, as well as the blockage of MR&T levees on the northwest and southwest sides of the basin. Storm surge in Barataria Basin was about 2 m. After the removal of levees at the both-crossing locations (see Figure 5b,d), the maximum surge changed little. When comparing Figure 5b to Figure 5a, visually the only difference is that the 2 m contour line enclosed area in Barataria Basin was merged in the both-crossing case. As shown in Figure 5d, the surge reduction area (e.g., up to 0.1 m of surge decrease) in Breton Sound was very limited and closely confined to the two levee-opening locations, while there were more surge increase areas (e.g., up to 0.1 m of surge increase) near the west bank of MR in Barataria Basin. When removing all MR&T levees in the model (see Figure 5c,e), the impact on storm surge was significant in both Breton Sound and Barataria Basin as expected. The maximum surge was decreased (e.g., by 0.9 m at the northwest corner) in Breton, while it was increased (e.g., by 0.9 m at the northeast corner) in Barataria. This situation is actually the optimized result to be achieved for coastal Louisiana in terms of storm hazard mitigation, but it is not practical to remove all MR&T levees. It seems that it was not very effective to reduce storm surge in one basin by removing local levels and connecting the basin with another one via the MR.  
Figure 4: Comparisons of water level (m, NAVD88) time series between the observed (red circle) and the modeled phenomena in three scenarios (blue line for the base case, black line for the both-crossing case and green line for the no-levee case) at four CRMS stations during Hurricane Isaac (2012).