Leaky Dams and Nature-based Solutions to Flooding of the River Tas

 The River Tas is considered to be a chalk stream. There are just over 200 on the planet and 85% of them are in England, so the Tas is a most rare and precious resource.  Anyone living by the river has a front row seat to its changing rhythms and the sights and sounds it shares with those willing to pause a while and absorb its delights. This small, quietly spoken chalk stream provides the backdrop to the seasonal pulse of fauna and flora that grace our patch of the South Norfolk and North Suffolk Claylands of East Anglia.

 Over geological time, the Tas has incised its path through the relatively flat topography of the region, weaving a corridor along the river valley bedrock of late Cretaceous chalk, overlain by glacial tills and ice age deposits of sand and gravel.

 Lying within the River Yare catchment, and rising at Carleton Fen, near New Buckenham, the Tas is fed by a wide network of minor streams and drainage ditches typical of rural East Anglia. It joins the River Yare just south of Norwich, ultimately draining into the Norfolk Broads and the North Sea beyond.

 As with most rivers in the UK, much of the Tas was physically modified and rerouted during the 20th century. In some areas this  resulted in slowing the water flow, allowing deposition of silt, and emergence of vegetation where it had previously been unable to anchor roots.

 In Forncett St Peter the river flows through the Forncett Meadows, a Site of Special Scientific Interest (SSSI). Sandie, the co-landowner, 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 (1) lowered the banks so that at times of high flow, the water can easily spill over onto the whole of the floodplain meadows, alleviating pressure on these adjacent properties.

 "As part of the habitat enhancement works (1), a length of the river has also been reinstated to its pre-1950’s course, by creating low bunds so that the river follows three original meanders (‘wanderings’). As well as providing additional channel length, the reinstated meanders are narrower and more akin to their original width. This helps maintain more energy and oxygen in the river water at times of lower flow, allowing better habitat for wildlife - especially fish and birds, such as the kingfisher.  At times of high flow, the bunds can be over-ridden by the high river water so there is additional channel capacity for flood waters."

 Ongoing management of the SSSI site is an excellent example of how nature-based initiatives can restore and enhance the health and flood mitigation properties of the Tas and its riverine corridor.

Vegetative 'litter' and encroaching shrubs and trees have the potential to choke the rate that water flows through the Tas, so periodic removal is one of the key management activities undertaken on the meadows. So too is the task of constraining the succession of the meadows from transitioning into secondary woodland habitat, thereby maintaining an open vista, and the ability of the meadow to act as a giant sponge during periods of heavy and prolonged rainfall. This is done through low intensity conservation grazing using cob-type ponies (from May to September) in combination with selective removal of areas of vegetation.

The Leaky Dam Project

 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 (2) 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 (3). 

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 restored   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.

 

Forncett Nature Matters would like to thank the author, Sarah Park, who is a locally based Agricultural Ecologist. Also, thank you to the owners and guardians of the Forncett Meadows, Sandie and Trevor, for their input into this article.

We would also like to add that further valuable work, applying nature-based principles to the care of the River Tas, is being done further downstream by other private individuals, who are supporters of FNM:

 Justin and Steve at Forncett St Mary, taking advice from the Environment Agency, have dug out scrapes on their riverside fields to take floodwater and provide habitat for wildfowl and wading birds. Read more.

 At Flordon Common, Peter manages this low lying SSSI which borders the Tas and absorbs its overflow, providing a wonderfully bio diverse habitat. Read more

 At Tasburgh,where the river flows at the end of his garden, Ross has documented the health of the water over the years and makes videos of the variety of wildlife in and around the water. Including the endangered white clawed crayfish! Read more.

References:

1 Reintroducing wiggles to river systems. Ally Rae. .2025

2  Water Friendly Farming (2020). Optimising leaky Barriers to prevent collapse and improve performance. 

3  An ongoing study conducted in collaboration with Freshwater Habitats Trust, Game & Wildlife Conservation Trust, the University of York, the Environment Agency and landowners in the three catchments. The project has mainly been funded by the Environment Agency, with Anglian Regional Flood and Coastal Committee.

June 2026

Thanks to Christina Wakeford for her editorial input.