Leaky Dams

Regular readers will know about the recent programme of earthworks within the Forncett Meadows Site of Special Scientific Interest (SSSI).

Sandie, co-landowner of the SSSI says,

"The meadows are natural floodplains, but historic works in the 1950s, which straightened and lowered the river Tas, also resulted in artificially high banks being created; in times of flood, the river water has not easily been able to move onto the surrounding land.  In places, the banks on the floodplain were higher than the banks of properties opposite. The recent works lowered the banks so that at times of high flow, the water can easily spill over onto the floodplain meadows, alleviating pressure on these adjacent properties. The natural meanders of the river were also restored."

 Living near to the river ourselves, we found an additional opportunity to help restore and support the natural pulse of the river:

Ditches are a common sight around the perimeter of fields and gardens, with many draining directly into the Tas.  Ditches not only drain soils, transforming them into traversable and productive land, they also have the potential to ameliorate flooding. 

Increasing awareness of nature-based flood mitigation solutions piqued our interest in the potential of 'leaky dams'. 

These are constructed within field ditches using tree trunks and branches dug into the banks at a right angle to the river to form a porous barrage. An increasing body of evidence is showing that well-built leaky dams can be an effective low-cost, low-maintenance approach to slow down water flow from a ditch into a river at times of peak water level. 

Boundary ditches run along 136 m of our property on Wash Lane, helping to drain the surrounding arable slopes and our small grazing paddock. The ditches also form the boundary between our paddock and a small mixed-species woodland sited on the most easterly flank of Forncett Meadows. On leaving our property, the ditch travels a further 73 m along the perimeter of Forncett Meadows before discharging into the Tas, about 50 m downstream of the Wash Lane ford. Water height in the ditch synchronises with periods of heavy rainfall, though never entirely drying out, courtesy of a network of underground springs that feed the Tas valley floor. 

In collaboration with Norfolk Farming and Wildlife Advisory Group (FWAG), Sandie and Trevor, (landowners of the Forncett Meadows), and a gang of willing hands from The Conservation Volunteers (TCV) organisation, we designed and implemented a cascade of 4 leaky dams within our boundary drains. Funding for the initiative was provided through the Environment Agency’s Chalk Streams Programme. In stark reality, when I say ‘we’, all credit goes to the 40+ willing volunteers who showed up over a 5-week period to harvest suitable trunks and branches from the adjacent woodland, and install the dams. Sourcing wood in this way not only provided on-site materials to be used in construction, but also created a welcome opening-up of woody vegetation that will promote new growth and a succession of trees in the copse.

We’ll monitor the performance of the dams over the coming years, particularly during wet periods, and look out for any signs of bank erosion around the immediate installation sites. Long-term research suggests that well-installed dams can avoid this risk, especially in slow moving channels such as ours.

We hope that this discreet nature-based initiative will work in harmony with the significant natural processes and functioning recently enhanced through lengthening the meanderings of the Tas, as it winds through the Forncett Meadows, and perhaps to the wider reaches of the catchment. Furthermore, improving the welfare of the Tas has the added promise of benefits to water quality and the abundance of fauna and flora that depends upon the river and its riverine environment.

This is a shortened version of Sarah's article,  edited by Christina Wakeford. The full article will be published here shortly.

 

June 2026