Search

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. [ABSTRACT FROM AUTHOR]

• Developing 2.5D CVT grids and refinement with unstructured vertical discretization. • Curve-fitting matches Voronoi cell edges with geological traces in horizontal projection. • Centroidal Voronoi grids in MODFLOW 6 for complex groundwater modeling. • Linking multiple unstructured-grid models in MODFLOW 6. An unstructured grid is a powerful data structure for a proper depiction of various geological features such as faults and pinch-outs and for groundwater simulation in a complex aquifer system. This study presents a mesh generation methodology based on 2.5D centroidal Voronoi tessellations (CVTs), which provides a great flexibility in specifying grid connections in 3D, matching irregular geometries, and adding high levels of refinement in areas of interest. Lloyd relaxation approximation is introduced to effectively guarantee the generation of centroidal Voronoi grids. A curve-fitting Voronoi gridding approach is developed to construct Voronoi cell edges conforming to intricated geological traces in the horizontal 2D projection with unstructured vertical discretization. A controlled refinement in spatial grid resolution has been demonstrated to prevent adverse impacts on groundwater flow simulations. The methodology is applied to the Southern Hills aquifer system in the Capital Area of Louisiana, USA. Significant groundwater depletion has occurred due to continuous and excessive withdrawals over decades for public supplies and industries. The Baton Rouge fault and the Denham-Springs Scotlandville fault are leaky barriers and have a significant influence on groundwater flow. A curve-fitting centroidal Voronoi grid is developed to simulate the effects of the two faults as leaky barriers. CVT-based high-fidelity unstructured multi-model groundwater modeling is conducted using MODFLOW 6 to allow more flexibility in model design. Complex groundwater flow in the Southern Hills aquifer system was revealed through the centroidal Voronoi unstructured grid. [ABSTRACT FROM AUTHOR]