Modeling Assessment of Groundwater Flooding and Levee Underseepage in Greater New Orleans, USA

Abstract:

As surrounded by surface waters, the New Orleans City has been suffering from flooding issues. However, potential groundwater flooding that can be triggered by relatively high surface water level is seldom studied and poorly understood. Levees adjacent to high water level are also subject to strong underseepage. This study aimed to investigate the impact of hydrogeology on potential groundwater flooding and seepage-induced hazards to levees in the Greater New Orleans area. A high-fidelity stratigraphy model was constructed using 2,873 well logs to obtain detailed shallow geology within 60 m in depth. A groundwater model was subsequently developed using MODFLOW-USG to explore groundwater dynamics and assess groundwater flooding and levee underseepage. The stratigraphy model showed that New Orleans has rich fine-grained sediments in upper 60 m. However, noticeable amounts of coarse sediments are found to connect to streambeds and lakebeds. Modeling groundwater flow of 2018 showed the strong interactions between surface water and groundwater through coarse sediments, such as beach sands and point bar sands. Taking into account model parameter uncertainties, the model results suggested that 40% of the urban area could have groundwater level above land surface more than 1.0 m. Groundwater flooding is likely to occur in the low lying areas due to thin top confining layers and high groundwater level. Levees underlain by shallow sands are likely to have strong underseepage and a low factor of safety. The groundwater model confirmed strong seepage at the sites where levees were breached during the Hurricane Katrina in 2005. Moreover, the model was able to identify potentially hazardous areas that were not discovered before. This study revealed the imperative of understanding hydrogeology for New Orleans with respect to groundwater flooding and levee underseepage, and indicated potential geo-hazards in coastal zones where similar geographical and geological conditions exist around the world.