Relief Well Evaluation: Three-Dimensional Modeling and Blanket Theory

Abstract

This study compares the differences in calculated relief well performance at the Profit Island vicinity levee (PIVL) near Baton Rouge, Louisiana, between the three-dimensional (3D) USGS MODFLOW-USG groundwater model and the blanket theory. The 3D hydrogeological model was built using indicator kriging with 192 boring logs and calibrated against measured well discharge rates and piezometric heads during the March to May 1997 flood event. The blanket theory indicates that the total heads at 19 relief wells were lower than the ground surface. However, relief well flow rates observed during the 1997 Mississippi River flood imply that the aquifer pressure was sufficient to overcome both the total hydraulic head equivalent to the ground surface and any frictional head losses. The discrepancy is mainly due to the complex geological setting, effects resulting from relief well installation, and the influence of adjacent relief wells that cause observations to diverge from blanket theory assumptions. The 3D MODFLOW model found that all 84 relief wells maintain a factor of safety (FS) above 1.5 during the peak flood stage in 1997 even though several relief wells produced negligible discharge rates. As a result, this case study indicates that 3D seepage modeling is an accountable and precise tool.