A brilliant case study where Calderdale residents installed a suite of natural flood management measures on their land
Tell us about your project: Installed leaky dams and attenuation ponds
Where: In the woodland above our property in Todmorden
When: Installed between 2019-2020, with maintenance ongoing
Design and implementation: We were advised to install two large attenuation ponds, and 14 leaky dams in our woodland. The land is steep, and we get a lot of run-off during extreme weather events, which we witnessed plunging off down the hill to threaten neighbour’s houses, and ultimately join the floodwaters on the valley bottom. The attenuation basins, situated lower in the woodland, are essentially large troughs created by a digger, with a relatively narrow pipe at the lower end. Water collects in the basins, whilst the pipe limits the amount of flow that passes out. They were designed by an engineer to meet specifications for safety. Where the pipe emerges from the basins, we’ve piled up brash (essentially a large pile of sticks) to further slow the flow, and also allow it to dissipate rather than exiting the pipe in a concentrated flow.
The leaky dams are situated at the top of our land. To create them, we used logs, thick branches and twigs from our woodland. As part of our woodland plan, we applied for a thinning license to take down a few trees in the woodland. This should ultimately benefit the woodland by decreasing crowding and allowing more light to the woodland floor, allowing younger trees the chance to grow, and thus diversifying the age range in our wood. It also allows more light to some lovely older trees, encouraging them to spread out.
The logs were placed sideways across one of the paths down which the water flows. Smaller branches and twigs are placed in-between. The logs are wedged in or held in place by posts to keep them from moving. Like the ponds, the idea is not to stop the water from coming down, which could over-burden the dams and cause a failure, but to slow the water to such an extent that more of it is absorbed into the soil, and so that it does not all come down at such a rate that it adds to the burden at the valley bottom. The water essentially has to seep through the whole series of dams before reaching the attenuation ponds.
Benefits and constraints: The leaky dams have been the most successful, from what we can see. They visibly held back the water, but also create a yearly build-up of debris and rotting wood which creates interesting habitat, and also adds a richer humus layer. Since the wood is on a steep acid upland, the soil is quite depleted, so this can only be of benefit. They all stayed in place, and are easy to maintain. We have replaced pieces of wood where it has become too rotten. The maintenance can be time-consuming, though, and is quite physical, but we love to be out working in the wood.
The attenuation ponds clearly collect a lot of water, but we are thinking of putting in a sluice gate to shrink the aperture in the pipe to limit the flow of water through the pipe, as we’ve found that the basin never fills up beyond the level of the pipe. The basins have a built-in overflow so they would never become dangerously full, however at present these overflows are obsolete. Maintenance is easy, mainly requiring the removal of dead leaves from the basins after leaf-fall.
Further opportunities: We’re planning to add some more leaky dams across the land, as there are several places where the water comes through which we were not able to address. We are also thinking about planting more willow around wet areas.
Costs: Pond creation – £2,905
Earth bunds by ponds – £180
Woodland thinning – £1,320
Leaky dams – £2,100
Engineer – £1,350
Forest planning – £300
Project management – £300
My husband and I did the project management and tree felling, and installed the leaky dams and brash piles on the ponds. An arboricultural consultant did the woodland planning and applied for the felling license on our behalf. An engineer designed the attenuation ponds and oversaw the work, and a contractor came in to dig the ponds.