Hurricane Matthew caused the emergency spillway for the Lake Wackena Dam to activate in October 2016, and the outflow created damage to the left side of the outlet just beyond the end-wall of the concrete chute conveyance channel. Significant erosion occurred in the rock riprap outlet channel adjacent to the end of the concrete chute. In addition, the left corner of the concrete apron was undermined. A review of the as-constructed spillway drawings indicated that the alignment of the box culverts was skewed toward the left wall of the conveyance chute. A photograph of the activated emergency spillway during Hurricane Matthew showed that cross-waves were present in the conveyance channel and that the hydraulic jump was located in the rock riprap channel beyond the end of concrete. Engineering estimates for extending the spillway to provide energy dissipation measures were $850,000. As a result of a unique design approach, final construction costs to effectively and safely dissipate the energy were only $350,000. Computational Fluid Dynamics (CFD) 3-D Computer Modeling provided by Utah State University was paramount in reducing the final costs and providing a predictable model for design. The creativity and experience of researchers at the Utah Water Research Laboratory led to the use of simple modifications to the existing spillway that had significant effects on the ability to dissipate energy. CFD modeling was used to modify the existing spillway configuration to enable all of the energy dissipation measures to be kept within the existing concrete channel at a cost that was significantly less than the original estimates. Significant to this project is that one month after completion of construction, the spillway modifications were tested when Hurricane Florence punished eastern North Carolina. The actual performance of the spillway performed as predicted by the CFD model, and the client was very pleased.
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