Slowing the flow at Cwmparc

Cwmparc and the spider in action

Cwmparc with the spider in action

With funding from the Welsh Government’s Nature Fund, we supported Rhondda Cynon Taf Council in taking steps to naturalise one of the concrete channels which brings water down from the mountains at Cwmparc, in order to slow the flow of water, reduce flood risk and improve wildlife habitats.

If successful, the measures introduced in this pilot project could be rolled out to a further 50 watercourses across the Rhondda Valley, bringing significant environmental benefits to both people and wildlife in the area.

Becky Davies, NRW’s Area based planning project lead for the Rhondda, explains:

“Flood risk is a significant issue within the Rhondda Valley, with 5,865 properties in the Rhondda that are at significant risk of flooding.

Built in the 1980s as part of the land reclamation scheme to manage runoff over a large area of coal spoil, the steep concrete or stone lined channels, like the one kilometre long watercourse here at Cwmparc, convey the water in a torrent, pouring water down the valley sides at terrific speeds. These torrents reach the bottom of the valley incredibly quickly and during peak flows, flooding is almost inevitable.

But by working creatively and smartly together, the agencies which manage the area can use the area’s rich natural resources to reduce flooding, reconnect wildlife habitats and enhance biodiversity.

The concept of using local woody debris to ‘naturalise’ and slow down watercourses in man-made channels, is both simple and an efficient use of local resources. We will be monitoring the effectiveness of the project over the coming months and if effective, the measures will be replicated across other watercourses in the Rhondda.”

David Holland, Technical Director for Salix, explains:

“By securely pinning large sections of woody debris in the watercourse to create dams, the flow becomes more like that of a natural river or stream, with pools and eddies developing, areas of sediment building up and variances in flow rates within the channel introduced.

Not only does this slow the flow, giving the catchment area downstream more time to absorb and convey water, but it also provides habitat for a wide range of invertebrates, fish and amphibians. The existing stone or concrete lined channels are largely devoid of life as the speed of the water pretty much washes everything away. If biodiversity within the steam is improved, other animals will also benefit and there is the potential for otters to feed along these watercourses in the future.”

He continues: “We are using our incredibly strong and versatile four-legged walking spider machine to climb in and out of the river and carefully position the large woody debris – made up of whole trees and large branches – at intervals within the channel.

The pieces of wood can weigh up to a tonne and are held securely with stainless steel cables that are anchored into the concrete bed. This will ensure the trees can withstand the high flows of water conveyed by the channel without washing away.”

The woody debris work at Cwmparc is part of a range of interventions which aim to make use of the area’s natural resources to reduce flood risk and restore more ecologically diverse habitats, including the restoration of 11ha of peatbog, improvements to land management and community awareness.


Becky Davies adds: “The aim of this project is to demonstrate that through a combination of interventions there can be benefits to local flood risk, by potentially taking off the top five per cent of peak flows, but also slowing down flows.”

Woody debris work began on 6 July and is expected to be completed next week. More information about the Cwmparc project will be available at www.naturalresources.wales