Hydrodynamics Modeling to Study the Impacts of Hurricanes without and with NNBFs in the Coastal Region of Mobile Bay

Abstract

The coastal communities of Mobile Bay, AL, are facing a serious problem of flooding due to storm surges. With the increasing economic and social activities along with the increasing frequency as well as the intensity of the hurricanes, the coastal regions are at greater risk than before. This short- and long-term flooding due to frequent storms, could result in substantial harm to properties and roadways, decline in tourism, increase traffic for both commercial and personal purposes, and negatively impacting the quality of life for the residents. A three-dimensional hydrodynamic model using Environmental Fluid Dynamics Code Plus (EFDC+) was developed to study the impacts of different intensities of Hurricane without and with the implementation of Natural and Nature-Based Features (NNBFS) in the coastal regions of the bay. The external driving forces for the model include the inflows from two major rivers (Mobile River and Tensaw River), atmospheric winds, tropical cyclones, and the harmonics tides or water level at the open boundaries, on the south where flow exchange takes place. Hurricane Ivan (2004) and Hurricane Sally (2020) were used for model calibration and verification. The simulated water surface elevations (WSEL) were compared against the observed data at different monitoring stations within the bay with good agreement (coefficient of determination, R2 = 0.96 and 0.92 between the observed WSEL and the modeled results at Dauphin Island station for 2004 and 2020 respectively). The calibrated EFDC+ model was used to study the response of the bay under two different hurricane scenarios, one was based on the storm events with seven different return period, provided by Hazus application, and another was based on hypothetical worst-case approach with three different hurricane track directions. The northern coastal area in Baldwin County sits at relatively higher elevation as compared to the other coastal regions in Mobile and Baldwin Counties which is why this was the least impacted region for all the scenarios. Hurricanes were modeled in such a way that the peak windspeed was achieved along the top half of the bay, which was the reason low lying northern coastal region of Mobile County was mostly flooded for all major hurricanes (Categories 3 and above). For Hurricane categories 2 and below, the flooding occurred along the track of the hurricanes whereas for major hurricanes (Categories 3 and above), the flooding occurred along the path as well as other low-lying areas away from the track. The area near the Mobile regional airport, which was the most impacted region in different scenario runs, was selected to test different NNBFs: iii Vegetations with three different plant densities (#/m2), Artificial Reefs modeled as partially blocking fixed masks, Artificial Sand Dunes modeled by increasing the bottom elevation, and also artificial sand dunes with vegetation. One storm event from each modeling scenario was used for the evaluation of the NNBFs – 100-year storm event from modeling scenario 1 and category 4 hurricane moving North from modeling scenario 2. Both of these storm events fall under the category of major hurricanes. Four different parameters: water depth, velocity, flow, and bed shear, were compared before and after the implementation of NNBFs. Among the three different NNBFs, the results for all four parameters were similar without and with the use of artificial reefs modeled as masks. Also, the water depths, before and after the implementation of vegetation, were almost similar for all three plant densities. However, there was reduction in velocity, flow, and bed shear after the installation of vegetation with increasing plant density. The installation of artificial sand dunes provided greater reduction in water depths, velocities, flows, and bed shear as compared to other tested NNBFs. The results were even better for the combined use of sand dunes along with the vegetations. This study compared the impact of hurricane events with different tracks and intensities to identify the vulnerable regions, in terms of flooding, on the coastal areas of the Mobile Bay. The coastal region of the Daphne-Fairhope cities of Baldwin County is at the least risk of flooding being at higher elevations. All other coastal areas are at relatively lower elevations which are vulnerable to flooding from extreme events depending on the storm track and intensity. The NNBFs test results showed the ability of different NNBFs to reduce storm impacts. Among the three tested NNBFs, construction of artificial sand dunes was seen to be much more effective, and the use of combination of NNBFs such as sand dunes with high density vegetations was even more effective at reducing the storm impacts.