Like other parts of Gloucestershire, the Stroud Valleys suffered extensive flooding during the summer of 2007. Every year since, has seen flooding in some parts of the Stroud Valleys, including most recently Chalford on the middle Frome, and Bridgend and Eastington on the lower Frome. One of the tributaries has been designated as a rapid response catchment, meaning it is at risk of severe flash flooding.
After the flooding in 2007, community flood action groups were established in the Slad and Painswick Valleys and also the upper & lower Frome. They have campaigned for better protection for residents and properties from flooding, but over the years, communities and authorities have realised that the River Frome and its tributaries are not suited to hard engineered solutions. This is in part due to the physical nature of the catchment and the distribution of the properties at risk but also due to the heritage and aesthetic value of the Stroud valleys.
So in 2012, using funding from the Severn and Wye Regional Flood and Coastal Committee, the Stroud District Council established the Rural Sustainable project to implement natural flood management throughout the 250 kms catchment of the Stroud River Frome. The flood action groups have been a key part of the project and a representative of the groups even sat on the interview panel to appoint the project officer.
In the two and half years the project has been running, they have built over 250 interventions across the catchment with 16 landowners and tenants. The contracts to construct the work are offered to landowners or their contractors, to encourage local ownership. All the projects are undertaken voluntarily by landowners and there are no compensation payments made by the project.
However, the ethos of the project is to build many small interventions rather than few and large, which means the land take and impact are kept to an absolute minimum for each landowner.
All the interventions are designed to slow the peak flow down the valleys from the tributaries to the areas at risk of flooding. Interventions are designed to mimic and restore natural drainage processes, using local materials sourced directly from the farms and land on which the structures are built. They range from large woody debris in water courses, to flow diversions, dry-ponds, soakaways, to disrupting surface drainage routes to encourage greater infiltration of flows.
Case study- Wick Street Farm
Pitchcombe is a privately owned, mixed livestock farm on the outskirts of Stroud. The Painswick Stream flows through the farm and several large spring sourced streams arise on the farm and flow to the Painswick Stream.
When the Stroud Rural Suds project began, the flood action group for the Painswick valley provided Chris Uttley, the project officer with some contacts for landowners they had been to speak to previously, who had agreed to help in whatever way they could.
They decided not to have a grand launch of their project in Stroud, but instead, took the decision to communicate gradually, by example and by local means of communication when they had some works on the ground.
Chris first met the owners of the farm in the Autumn of 2015 and discussed a wide range of issues, including their experience of the flooding, how the streams work, what happens to the springs during high water, the history of the farm, where they went to school, what mutual connections they had in the community, how he came to be working in Stroud, what his wife did for a living. This was the process of establishing trust and connection that is necessary before any agreements on works.
At the end of the initial meeting, they agreed that Chris could work up some proposals for them to have a look at and think about. Chris then visited the farm at least 4 more times to develop an understanding of how water works, where they could intervene in a way that would have least impact on their activity, but would contribute to the project.
Their project is designed to allow many landowners to make a contribution, so in designing interventions, they are not looking to “solve” all their flooding issues on one farm. Once Chris had a rough design, along with the owners, he walked through the proposal and talked about how they could achieve it. The owners made a wide range of suggestions and amendments, which Chris then incorporated into the design.
Once they had broad agreement to undertake the work, Chris applied for Land Drainage consent on the owner’s behalf. The whole pre-construction process took approximately 8 months.
Construction Techniques and Interventions
Once of the key aims of their project is to encourage local involvement and ownership of the work. To help this occur, Chris offered the contract to undertake the work to the owners. They discussed the labour required and the equipment that would be needed. Chris had to be satisfied that with his help, the farmer and his workforce would be able to undertake the works themselves. The benefits of this are that the landowner then knows how the structures were built and what they are designed to do. He also understands what to do with further trees that fall into the spring streams and how to leave them safely to provide benefits for flood risk and wildlife.
The main technique they used on Wick Street Farm was the installation of multiple Large Woody Debris (LWD) leaky dams within the spring fed streams flowing through the farm. They had already decided that works in the main stream would be difficult and a possible risk to upstream properties, so they focused their efforts on slowing the flow of water into the main stream and reducing the amount of sediment reaching the main stream.
The spring streams are currently unfenced, but the owner had agreed to enter into a Countryside Stewardship agreement, which included fencing spring sides to prevent cattle access. The leaky dams were constructed from tree trunks and branches sourced primarily from trees that had already fallen near the springs, but they included structures within an area of Alder coppice, using Alder stems. For most of the year at low stream flow, the water can flow freely under the structure. The aim of the work is to provide a physical barrier for high flows to create a series of small pools to attenuate flood flows and slow down the rate at which flood water progress down the valley to residential areas.
Reasons & benefits
The structures on Wick Street Farm are primarily designed to help reduce flood risk, by slowing the rate at which high flows travel to the Painswick Stream. Large Woody Debris is used to create a system of leaky dams that slow down the flow of flood peaks down the valley. They work because they provide a physical barrier to high flows or divert water onto the small floodplains that occur down the spring lines, particularly where cattle have historically congregated and caused erosion . Large woody debris has several benefits. Firstly & crucially, the structures attenuate high flows, slowing the rate at which flood peaks travel downstream. Secondly, it will eventually speed up flows downstream of each structure. As silt and sediment accumulate behind the structures, this will create a small head of water. These increased flows will clean stones and gravels downstream. Thirdly, large woody debris will cause higher flows to collect on the floodplain. This allows silt and sediment to drop out of the water column onto the flood plain, decreasing the total sediment load in the stream. Woody debris also provides a habitat in its own right and is colonised by many invertebrates, lower plants and fungi. It engineers habitat diversity, creating a system of pools & riffles which can be occupied by a range of invertebrates and fish.
Construction data and information
In total they have constructed 28 Large woody Debris leaky dams within two spring lines flowing through the farm. They used a mixture of fallen Beech, Ash, Oak, Willow and softwoods (Fig 1) for construction in the most Northerly spring, and mainly Alder & Willow for structures within the Southerly spring (Fig 2). No structures have been pinned, as there is significant streamside tree growth to prevent long timbers from moving through the system. The majority of structures were built using hand tools, including chainsaws, Crow bars, timber grips and ropes (Fig 3). For structures with good accessibility by a machine, they used a Telehandler and silage grap to move large trunks into approximate position and then hand tools for final positioning (Fig 4). Timber costs were donated and not costed, but likely to be significant for this site.
For more information, visit the Stroud Website