Keywords: subsurface flow, cut-off wall, wall opening, impact analysis, Herbert Hoover Dike, WASH123D, GMS.

This paper presents a large-scale subsurface flow model used for an impact analysis of the cut-off wall proposed to help mitigate the maintenance problems associated with the Herbert Hoover Dike (HHD) in Florida, USA resulting from inherit seepage and piping erosion. To accommodate expeditious construction of the HHD rehabilitation, the cut-off wall construction contracts will stop on either side of each dike structure (e.g., C-14) leaving a temporary "gap" that will be closed when the structures are modified. There is concern that the temporary opening will result in concentrated groundwater flow and generate higher-velocity flows through the opening, which may increase the opportunities for piping of dike materials that could lead to a dike stability problem. It is thus necessary to investigate the wall opening impact on local groundwater flow prior to construction of the cut-off wall adjacent to such dike structures.

The purpose of this modelling work was to perform order-of-magnitude and conservative estimates on the change of subsurface flow due to the cut-off wall at a sub-regional scale and due to the wall opening at a local scale. The results from this study were expected to provide a general indication on the degree and extent to which subsurface flows around Lake Okeechobee would be influenced by the cut-off wall and temporary wall opening. The US Department of Defense Groundwater Modeling System (GMS 6.5) and the WASH123D numerical model were used to construct model runs and compute subsurface flow, respectively. A two-stage approach was employed to investigate the change of subsurface flow sub-regionally and locally. The total head and the velocity simulation results from various model runs were compared to develop order-of-magnitude estimates of the cut-off wall and wall opening impact. The simulation results showed that the cut-off wall impact on groundwater flow was limited to varying distances from the HHD on the land side depending on the scenario condition analyzed and that the impacts were greater in Reach 3 than in Reach 1. The maximum Darcy velocity generated due to the localized wall opening occurred around the cut-off wall edge. With the sub-regional hydro-geologic model constructed in this study, the computed hydraulic gradient in the vicinity of the wall opening was small and is not anticipated to cause a material boiling or piping issue. For more accurate estimates, however, it is recommended to incorporate additional local geological heterogeneity into the model along the HHD alignment should it become available.

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