Upstream thinking provides public space benefit
Working with Merthyr Tydfil County Borough Council, Salix reviewed the options for improving water control and biodiversity at Nant Llwynog stream – an old reclaimed tip site close to the village of Bedlinog in the Taff Bargoed Valley.
The area’s Public Right of Way was frequently being eroded by overflows from the concrete channel bringing water down from the mountains.
Rolf Brown, Countryside Officer for Merthyr Tydfil Country Borough Council, explains:
“The aim of the project was to better manage water through the park in a way that was environmentally, ecologically and visually beneficial.
We approached Salix to devise a sustainable system that would minimise erosion in the valley and improve the landscape. Environment Agency Wales – now Natural Resources Wales – favoured creating a more natural appearance to the existing engineered drainage channels, and grant monies were offered by them for reducing flash flooding impacts by slowing down water runoff by creating more ponds and wetlands, and naturalising the stream course.”
Salix proposed holding more of the water up at the top end of the Park by desilting the large top pond and smaller middle pond, and by creating three new ponds linked to the main channel to attenuate floodwater. Salix also recommended reshaping the main stone lined channel to create a grass and reed lined swale.
Pond desilting
The large pond at the top of the park that captures the catchment water had become silted up. A channel through the middle of the large top pond was desilted and its exit to the swale was raised to increase the attenuation capacity. This enlarged pond now fills up when a flood occurs and backs up via a new shallow ditch into a second newly created pond, increasing capacity at the top of the park.
A new area of wet woodland was planted next to the new attenuation pond to further enhance biodiversity and reduce runoff.
To preserve the footpath, the second existing pond was also dredged and a new pipe installed taking excess water beneath the footpath to a new attenuating pond. Water from this wetland pond can pass back into the main channel further down the hill via another underground pipe.
Naturalising the channel
The 430-metre length of principal drainage in a hard engineered channel was transformed into a green swale, without the potentially prohibitively costly removal of the concrete channel. Site sourced soil was used to raise the invert. It was then seeded with native grasses and clovers and protected with an anti-erosion turf reinforcement mat.
Sections of the channel were edged with stone blocks to be re-positioned to different locations as part of the ongoing water management design.
A particularly problematic area of erosion – the steep grouted stone side channel leading down from one of the higher ponds – was also remodelled to become a grass and reed lined swale. It was lined with a separation fabric and filled with soil.
Prior to soil filling, a series of two-metre long Rock Rolls were laid across the bed at one-metre intervals to act as baffles, preventing the new soil bed slumping. This stabilised soil was seeded and then protected from erosion with the same anti-erosion turf reinforcement mat as the main channel.
Hugh Ellis, Salix’s erosion control and bioengineering specialist, explains:
“Merthyr Tydfil County Borough Council asked us to advise on how to address the costly damage to the park’s landscape as it was constantly being washed out, and how to make the drainage channel function more like a natural stream. We looked at how the flashy catchment was working to cause the problem in the park and our proposals were fully endorsed by Merthyr Tydfil CBC Engineering Department.
By focusing our efforts upstream, we have removed the need for hard engineering by controlling the velocity of the water in the Park. This has improved public amenity and biodiversity at the same time, and reduced erosion downstream of the Park.”
Rolf adds:
“Several years on the installed system continues to prevent erosion, has ‘naturalised’ and is regularly visited and monitored by local residents interested in the improved biodiversity.
The significant visual impact now makes the valley appear very natural rather than an obvious industrial reclamation site, and forms a key link between the village of Bedlinog and the Merthyr & Gelligaer Common Landscape of Special Historic Interest.”
