Wissota Hydro Plant Automatic Spillway Gate Studies Phase I

Northern States Power Company (NSP) is examining various alternatives to modify the automatic spillway gates at the Wissota Hydro Plant on the Chippewa River at Chippewa Falls, Wisconsin. The gates are called "stauwerke" gates. The main purpose of the study is to assure that the automatic spillway gates operate properly during a probable-maximum-flood (PMF) as required by the Federal Energy Re&ulatory Commission (FERC) and the Department of Natural Resources (DNR). The position of the gates must be controllable so that they can be fully lowered or raised with Lake Wissota at its normal elevation of 898.0 ft. The gates must also be fully open during a PMF. The spillway has 13 automatic gates each 64 feet wide in the configuration shown in Fig. 1. Figure 2 shows a photo of the prototype spillway with gate 1 starting at the south abutment on the right. Each gate is connected to a 144.5 ton counterweight through linkages and arms as shown in Fig. 3. Records of openings and closings from 1982 to 1985 show that the gates have operated automatically. Gates 2 through 13 open when the lake elevation is about 0.6 ft above the normal elevation of 898 ft and close when the elevation drops about 0.5 ft below normal. Gate 1 is controlled by a chain hoist and can be pulled down to about 6 ft and used to somewhat control the lake elevation. When the gates open automatically, they open a distance of 3.1 to 3.6 feet. The maximum opening of the gates was 6 feet in the 1941 flood. The 13 gates were expected to be capable of passing 263,000 cfs. With the PMF of 363,000 cfs, other means have to be provided to pass the extra 100,000 cfs. Therefore, the questions are whether or not the gates will operate properly, and if the anticipated flow can be passed. Presently, in the fully lowered position the gates will be 10 feet below pool El. 898 ft. By removing curbs around the gate arm openings, the gates would be horizontal and thus down 10.83 feet. It is not possible to test the prototype gates in the fully lowered position. Therefore, NSP proposed to run field tests on Gate 1 of the Wissota Dam spillway, correlate the results with a physical hydraulic model at the St. Anthony Falls Hydraulic Laboratory, and expand the model studies to test conditions not possible in the prototype.