Managing flood risk in Ynysybwl

Overview

Natural Resources Wales is working to reduce the risk of flooding in Ynysybwl.

Ynysybwl was badly affected during Storm Dennis in 2020. Flood waters from the Nant Clydach overtopped the highway wall, which runs along the length of Clydach Terrace, flooding 17 properties.

The Welsh Government has instructed us to undertake a full Business Case process following the Welsh Governemnt's Flood and Coastal Erosion Risk Management Business Case Guidance.

Project updates

Flood risk

Clydach Terrace lies on the natural floodplain in a very constrained section of the valley and has historically suffered from severe flooding. 

The community has a unique flood risk due to its position. When it floods, it does so quickly and to a great depth, which means there is a risk to life.

There are currently no flood defences in Ynysybwl, although we carry out regular maintenance work to manage the flood risk.

We have removed more than 700 tonnes of river shoal material from the river channel adjacent to Clydach Terrace since 2020.

Options to manage the risk

Following Storm Dennis, we undertook an initial assessment to better understand the flood risk at Clydach Terrace using a rainfall and flow model.

We have identified a range of measures to manage the risk. We want to use the wealth of knowledge that the local community has about the history of flooding to develop and appraise each option for its benefits and risks, considering technical, environmental, social and cost factors of each one.

Stages of the project

The project will be delivered in stages and will follow the HM Treasury’s appraisal and evaluation process.

The first stage will identify a long list of options. We will appraise the benefits and risks of each by considering technical, environmental, social and cost factors.

We will also need to evaluate the options of doing nothing and maintaining the current level of flood risk management through our maintenance programme.

All viable options will be short-listed and further scrutinised at the outline business case stage.

If viable, we will recommend a preferred option and ask for feedback from the local community. If agreed, it will be recommended for delivery within our full business case.

Project Timeline

Our aim is to engage with the community throughout each stage of the project.

• Winter 2023 – Spring 2024: Preparation of the Strategic Outline Case to WG (longlist of options)
• Spring 2024 – 2025: Preparation and delivery of Outline Business Case to WG (shortlist of options)
• 2025 – 2026: Preparation and delivery of Full Business Case to WG (recommended option for delivery)
• 2026 – 2028: Construction or Delivery of selected option
• 2029: Forecasted project closure