# The Impact of Leveee Openings on Storm Surge: A Numerical Analysis in Coastal Louisiana
## 1 Introduction  
Mississippi River and Tributaries' (MR&T) levees currently protect more than 4 million citizens, 1.5 million homes, 33,000 farms, and countless vital transportation routes from destructive rivering floods [1]. The levees are designed to protect the alluvial valley against a hypothetical project flood by confining flow to the leveed channel. For coastal areas, in addition to riverine flood, another threat that could result in flooding is storm surge defined as an offshore rise of water level caused by tropical cyclones. It is a severe devastation to human lives and properties throughout the world, from southeast Asia, the south and east coasts of China to the Caribbean islands, northern Gulf of Mexico and the U.S. east coast. Two recent storm surge events were Typhoon Meihua and Hurricane Ian in September 2022. They struck Hangzhou Bay and Yangtze Estuary in the east coast of China and the states of Florida and South Carolina along the southeast U.S. coast, respectively.  
The lower Mississippi River (MR) delta area and Louisiana coast (see Figure 1) are susceptible to hurricane impacts frequently as well. Historical hurricane events that devastating affected coastal Louisiana include Katrina (2005), Gustav (2008), Isaac (2012) and Ida (2021). The existence of the MR&T levees in the MR delta area (see Figure 1) could block and amplify storm surge in adjacent basins. For example, both measurements and numerical results [2] show that Isaac (2012) induced more than 4 m of storm surge against the MR&T levees, east of the river, in Breton Sound, Louisiana, due to the semi-enclosed geometry of the estuary (see Figure 1) and the slow-moving pace of the hurricane.  
In order to reduce storm surge amplification caused by these barriers, one possible solution that has been proposed is to build \"floodways\" through the mainstream MR levees. The idea is to allow storm surge during tropical events to cross the lower river peninsula through these openings/floodways from east to west or vice versa depending upon the path of a storm [3]. Previous analysis has indicated that the cost of providing adequate storm risk reduction by conventional means--contiguous linear levees parallel on the basin side to the river levees--is exorbitant and cannot be justified on a cost-benefit basis [4]. Additionally, the maintenance of the contiguous linear leveee system on either side of the river can amplify surge in the local and adjacent communities, as well as on the coast of the State of Mississippi. Further, those same levees might starve adjacent disappearing wetlands of the benefits of the annual spring overflow of the MR.  
To achieve the benefits described above, one fundamental and important question is: will this solution effectively reduce storm surge in adjacent basins (i.e., Breton Sound and Barataria Basin)? The numerical analysis presented here examines the hypothesis concerning building \"floodways\" through the mainstem MR levees in the lower MR delta. Two levee removal/opening locations (i.e., the upper-crossing location and the lower-crossing location, see Figure 1) were considered and their effects on storm surge were analyzed through a pre-validated storm surge model.  
Figure 1: The lower MR river delta area. Yellow lines denote the MR&T levees. Two double headed red arrows denote the levee removal locations. Blue squares denote water level gauges. Orange circle denotes the discharge boundary location of the MR river in the model. Magenta line with dots denotes the track of Isaac (2012). The full track is shown in the top-right subfigure (revised from [[https://en.wikipedia.org/wiki/Hurricane_Isaac_](https://en.wikipedia.org/wiki/Hurricane_Isaac_)]([https://en.wikipedia.org/wiki/Hurricane_Isaac_](https://en.wikipedia.org/wiki/Hurricane_Isaac_))(2012)#/media/File:Isaac_2012_track.png, accessed on 23 October 2022).