The Federal Energy Regulatory Commission (FERC) directed this confidential client to increase spill capacity at three of its hydroelectric facilities in order to pass the probable maximum flood (PMF). Barr performed a feasibility study to identify a cost-effective solution for modifying the plants to pass the PMF. We developed several possibilities for each plant, along with preliminary cost estimates.
For the first plant, the client selected Barr’s recommended plan to raise the embankment corewall and modify the tainter-gate spillway hoisting to increase the gate opening. The design was approved by FERC and constructed in 2008. In addition, we designed a downstream-embankment earthen buttress and concrete retaining wall to increase the embankment stability and meet FERC criteria for the PMF load case.
For the other two plants, Barr evaluated several major options, including constructing a new concrete spillway, installing a new headgate in an existing abandoned bay, and making significant modifications to the tainter-gate hoist. The work was initially estimated to cost hundreds of thousands of dollars, but by thoughtfully considering site features and conducting a detailed review of the hydraulics necessary to pass the required flow, we were able to develop a simplified plan that still achieved the primary goals of protecting the powerhouses from flooding and passing the PMF flow without overtopping the embankments. The design consisted of a small floodwall to divert some excess flow around the powerhouse and back to the river at the downstream end of the spillway. This solution cost a fraction of the amount of the original options. FERC approved the proposed plans and modifications, and construction took place in 2020.