In previous posts I’ve mentioned stilling basins. These concrete structures are being constructed at two of the ponds on the Heath – Ladies Bathing Pond and Hampstead No. 1.
A stilling basin is a structure at the downstream side of a dam, designed to take away some of the energy from overtopping water flowing down the spillway, to reduce the risk of erosion of the ground near the dam and the dam itself. The shape of the basin (basically a flat slab but with walls at the end) is to encourage the water to form a hydraulic jump at the bottom of the slope, so the flow rises up at the downstream end of the basin. This reduces the energy more than letting it run flat away from the downstream toe of the dam.
The reason a stilling basin is needed at Ladies Bathing Pond is because the spillway is located on top of the dam, to protect the outer perimeter of trees around the pond, whereas at other ponds the spillway can go around the end of the dam – which is preferable. The water therefore flows faster as it is going down a steeper slope (as the slope of the dam is steeper than the natural ground either side).
Similarly the other stilling basin is at Hampstead No.1 Pond, where the new spillway culverts will follow the slope of the dam, which is quite steep.
In both cases, the stilling basin is to be filled with topsoil, so the concrete is buried. This topsoil will get washed away in a big flood, but then it can be replaced.
The stilling basin at Ladies Bathing Pond is currently under construction. These pictures were taken of the concrete pour last week.