Managing flood risk in Ynysybwl
Overview
Read this page in Welsh / Darllenwch y dudalen hon yn Gymraeg
UPDATE 23/06/25: publication of the Outline Business Case in the Project Updates section.
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 Government's Flood and Coastal Erosion Risk Management Business Case Guidance.
Project updates
23 June 2025 - publication of the Outline Business Case
We’ve completed an Outline Business Case (OBC) to explore whether a flood risk management scheme could reduce the risk of flooding from the Nant Clydach to homes along Clydach Terrace in Ynysybwl.
This work builds on previous assessments and includes detailed modelling, environmental studies, and engagement with residents. The OBC examines possible options, their costs, and potential benefits, and sets out whether a scheme could meet the technical and funding requirements needed to move forward.
Read the Outline Business Case
What was considered
This OBC followed HM Treasury and Welsh Government guidance to analyse potential flood risk management options to reduce the risk from the Nant Clydach.
Our assessment shows that:
- The existing wall beside Nant Clydach offers only limited protection.
- Flooding could happen with little warning, with peak flows arriving just three hours after heavy rain.
- As climate change progresses, this risk will increase significantly, with major flooding predicted to occur every other year in the long term if no action is taken.
We looked closely at a range of possible measures and two main options were shortlisted:
- A raised flood defence wall – replacing the current highway wall with a taller and more substantial engineered structure.
- Purchase by agreement – purchase of the most at-risk properties for demolition, effectively removing them from the floodplain.
What the assessment found
Despite the community’s clear need, neither option is close to being economically viable under the UK and Welsh Government funding rules for flood risk management purposes that we must follow.
The proposed flood defence wall would need to vary along its length between 3.5 metres and 4.5 metres above road level to provide a good level of protection.
This would have a major visual impact and would still carry the risk of being overtopped in the future or catastrophic inundation of the community should it ever fail. The cost estimated for the delivery of this scheme unfortunately far outweighs the economic benefit by a significant margin.
The assessment also confirms that purchasing the most at-risk properties is not cost-effective to be eligible to receive Flood Defence Grant in Aid funding from the Welsh Government.
We explored all possible design changes and materials to try to bring costs down, and also made allowances for uncertainties, but none of the options meet the economic tests needed to move forward to the next stage of funding.
We are not walking away
The risk of flooding remains, and we understand the distress this may cause. Although the OBC concludes a full flood scheme cannot be taken forward at this time, we are not just walking away from this community.
There are still important next steps that need to be taken:
- Community Flood Action Group: We will continue to support the Community Flood Action Group and attend Multi-Agency Meetings with them.
- Continue joint working: We are working with Rhondda Cynon Taf County Borough Council (RCT Council) and other partners to explore options to manage the risk of flooding.
Next steps
We understand that living under this constant threat has a huge impact on residents and it’s not the outcome anyone hoped for. But we remain committed to supporting the community, alongside RCT Council and the Welsh Government, in deciding the most realistic and sustainable way forward. We will continue to keep the community updated as soon as we know more.
18 July 2024 - publication of the Strategic Outline Case
Here is the Strategic Outline Case, which has been approved for funding to progress to the Outline Business Case stage.
8 March 2024
Our consultant Arup has produced the following reports as part of the project:
30 November 2023
We held a public meeting on 30 November to update the local community about the project.
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.
Many of the options will involve constructing flood risk management assets, but we will also look at other options and combinations. The decision on which option will be progressed, will be based on a multitude of different criteria, and will essentially be made by Welsh Government.
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
Areas
- Ynysybwl
Audiences
- Flooding
Interests
- Flooding
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