Innertkirchen compensation basin outlets – Flap gate combined with small stilling basin
In the context of an update of their hydropower plants, the Hydroelectric Company KWO plans an adaption of their powerhouses Innertkirchen 1 and 2. One aim of the project is, among many others, an optimized restitution regime of the turbined water to the Aare River. Therefore, a compensation basin is planned to reduce the currently pronounced hydro-peaking. The basin is situated downstream of the Hasliaare and the Gadmerwasser confluence, next to the outlets of the two powerhouses. The spatial situation and the geology implicate strong restrictions regarding the volume of the compensation basin, allowing for a storage capacity of approximately 20’000 m3. This volume is more than doubled by a voluminous tailrace tunnel between the turbines and the basin. The regulation of the basin is assured by a flap gate and a radial gate. The basin intents to limit hydro peaking, and thereby ameliorates the ecology of the Aare River between the basin and Lake of Brienz. It will facilitate to access the Gadmerwasser for the fishes, which is known as excellent spawning ground. The efficiency and reliability of the basin regulation structures is a key item for the operation of the two powerhouses. The basin outlet gates were thus model-tested at the Laboratory of Hydraulic Constructions of EPFL, Switzerland, to assure an sufficient discharge capacity even for elevated water levels in the Aare River, to avoid unwanted erosion on the area, to avoid sedimentation of the structure by the bed load of the Aare River, and to generated acceptable conditions for fish passage along the Aare River. The modeled hydraulic perimeter covered the basin outlet structure, a length of some 250 m of the Aare River, as well as a part of the compensation basin. The model is set-up with a geometrical scale factor of 1:40 and operated under the similitude of Froude. In the experiments, the efficiency of the basin regulation gates is tested. Additionally, water levels, flow velocities and the erosion of the river bed are measured. The paper describes this particular case study with some optimization steps; finally leading to a solution with satisfies the principle requests. Given the importance of the ecological aspects and the Swiss legislation, the present compensation basin can provide ideas and concepts for similar structures to foresee in the near future.