Iron Gate Dam, Spillway, and Outlet Works
Iron Gate Dam is one of five hydroelectric dams operated by PacifiCorp on the Klamath River in Northern California. The dam is a zoned earth and rockfill embankment 189 feet high, with a crest length of 740 feet. The spillway is located in bedrock adjacent to the right abutment of the dam and contains near-vertical rock cuts measuring up to 120 feet high.
An earlier FERC Part 12 study by others concluded that the existing dam would not pass the revised Probable Maximum Flood (PMF) without being overtopped. PacifiCorp initially considered enlarging the existing spillway to pass the PMF flow without overtopping the dam crest. Cornforth Consultants (CCI) was retained by PacifiCorp to evaluate the potential impacts of the PMF level on the dam crest. CCI determined that the impermeable core would be resistant to erosion and the crest would remain stable during the PMF.
Initial cost estimates indicated that enlarging the spillway to accommodate the PMF would be very expensive. As an alternative, CCI suggested that raising the dam crest to retain the PMF would be less costly than enlarging the spillway. At PacifiCorp’s request, the firm developed several dam raise alternatives. A cantilever sheetpile wall along the crest was selected as the most cost effective option. The abutments were raised using concrete stem walls and embankment fill. Hydraulic scale modeling performed by others provided estimates of flow velocity and elevations within the spillway during the PMF. The spillway was modified with fiber-reinforced shotcrete to protect the low quality, potentially erodible rock in the spillway wall. In addition, low areas along the spillway were raised using conventional concrete cantilever and gravity retaining walls. CCI prepared technical plans and specifications, an engineer’s cost estimate, and assisted PacifiCorp with construction control, including vertical rock tiedown anchors.