If the natural flow of water into a reservoir is not sufficient for optimal energy production, additional water is fed into the reservoir via ducts or tunnels from water catchments that can be up to several kilometres distant. Coarse debris is held back by massive grids at the main water inlet. Smaller stones, grit and sand can accumulate in the transfer basin in front of the tunnel. Constant monitoring of the level of stones and sand in the basin ensures that no debris gets into the tunnel system.