Mississippi River Levee Underseepage Assessment, North of Baton Rouge, Louisiana

The Profit Island vicinity levee (PIVL) is located within the Mississippi River Alluvial Plain along the western bank of the Mississippi River, north of Baton Rouge, Louisiana. In 1995, the U.S. Army Corps of Engineers constructed an array of 84 relief wells along a levee reach of 3.54 km (2.2 miles) to reduce pore water pressure, control sand boils, and increase the levee factor of safety (FOS). Many of the relief wells produce relatively low discharges during high river stages, which raises the concern of levee safety. This study developed a three-dimensional seepage model using USGS MODFLOW-USG to evaluate efficiency of the relief wells. A quad-tree grid was built starting at 50-m cells and was gradually refined to 3-m cells along the levee where relief wells are located. A hydrostratigraphy of clay and sand facies for the seepage model was built from drillers' logs and geotechnical borings by using indicator kriging method for subsurface interpolation. A combination of a Connected Linear Network (CLN) with a Drain (DRN) package was used to simulate relief well flow. The seepage model was calibrated with piezometric head data and relief well discharge data from a flood event in March-May 1997. The study found that low or no discharges from those relief wells are because of high top elevation of

their riser pipes. The 3-D seepage model was applied to various scenarios with or without relief wells. Given relief wells at the 1997 condition, the seepage model shows that all reaches of the PIVL have FOS higher than 1.5 for all scenarios. The results demonstrate the importance of having the relief wells for the Profit Island vicinity levee. For the extreme flood (17.07 m NAVD 88) and the design flood (15.97 m), the seepage model shows that a few reaches may have FOS higher than 1.5 even without relief wells. For the 2017 April-July flood event, the simulated relief well discharges are qualitatively consistent with the field observations. For the 2008 flood, the simulated discharge rates are higher than the measured discharge rates at several relief wells, which indicates potential clogging at these wells. In conclusion, new piezometric data and well discharge data are needed in order to update the seepage model and to have up-to-date FOS for the current 84 relief wells.