Nature-based solutions: Natural flood management on the Nant Barrog

Protecting Llanfair Talhaiarn from flooding

The village of Llanfair Talhaiarn in Conwy, Wales has a history of flooding due to its location at the confluence of the Nant Barrog and River Elwy. In February 2020, Storm Ciara brought significant rainfall to the area and several properties in the village were flooded again.

Natural Resources Wales, the Welsh body for environmental regulation and advice, appointed us to complete a business justification case, and to design and deliver natural flood management (NFM) measures in the upper Nant Barrog catchment. This project is being part funded by the Welsh government’s innovative £2M NFM programme.

Innovative flood management strategies

Following a site walkover, geomorphological assessment and geotechnical investigation, we identified leaky barriers and riparian planting as the best solution to complement a catchment-wide planting scheme being delivered by a local charity, Coed Cymru.

Twelve leaky barriers will be installed in the upper Nant Barrog. The locations have low-risk gradients (less than 4%) and channel widths (less than 3m) in line with leaky barrier best practice guidance. The locations of the barriers avoid access to a farm and an overhead high-voltage line. They also avoid a section of the watercourse that is currently experiencing high rates of erosion.

The barriers will consist of large, woody debris placed in the bottom half of the channel, with the top half formed of woven hazel and brash hurdles weaved between vertical hazel posts. Overall, the barriers will have a relatively informal arrangement, resulting in a natural aesthetic. There is a preference for barrier material to be sourced from the site, helping to minimise the carbon footprint of the scheme.

Riparian planting will cover an area of approximately 5000m2 in the upper reaches of the Nant Barrog, at locations close to the proposed leaky barriers. There are no leaky barriers planned for the lower and middle reaches of the Nant Barrog, so our solution avoids riparian planting in these areas as any small leaf litter material could block the trash screen in Llanfair Talhaiarn. The locations for the planting were also selected to avoid disturbance to the Coedydd Derw Elwy, a site of special scientific interest (SSSI) as well as a site of importance for nature conservation and an ancient woodland.

Enhancing flood resilience

Once installed, the woody dams will attenuate the river flows and encourage water onto upstream floodplains, slowing the movement of water from the headwaters to the community in Llanfair Talhaiarn.

The measures selected will complement woodland and hedgerow cross-field planting being undertaken by Coed Cymru. We have worked closely with the charity’s project team to build relationships with landowners and farmers and generate support for NFM.

Our carbon assessment team estimate the scheme will have a carbon footprint of 20.75tCO2e, offset by the riparian planting benefits of 306tCO2e, delivering carbon sequestration of 221.1tCO2e. This wider benefit will contribute to Welsh government’s commitments under its 2021-25 Working together to reach net zero plan.