The princible is that someone shovels gravel into the top end of the sluice, where a sump pump hose discharges its water. The first 8 feet or so of the sluice have a roofing tin bottom. The last 3 feet are wire mesh, which allows the water and dirt to dump into a barrel which holds the pump. All in all it workes fairly well. Still, the rainwater that has finished rinsing the gravel in the pool has emerged in the deep end looking like a big cup of coffee with cream.