Llanfair Talhaiarn proposed flood risk management scheme
Read the background and proposals for Llanfair TH...
November 2012 saw further flooding to the village of Llanfair Talhaiarn (TH) when the Nant Barrog, swollen by extreme rainfall, overwhelmed the Water Street culvert and flooded 19 homes and businesses.
This followed previous flooding in the village, during April 2012 along with numerous other reported incidents in the past 50 years.
Based on modelling, the November 2012 flood was estimated to have had a one in 50 annual chance of happening. But, as evidenced by the frequency of flooding incidents in the village, Llanfair TH is actually thought be at risk from floods on the Nant Barrog with a one in 20 annual chance of happening due to overwhelming the Water Street culvert.
This risk of flooding is exacerbated by blockage of the trash screen, across the inlet of the Water Street culvert.
The River Elwy flows along the northern edge of the village. The November 2012 flood on the Elwy was reported as being between a one in 100 and one in 200 annual chance. Existing flood defences protected Llanfair TH from further flooding from the Elwy during November 2012 - the village was solely affected by flows on the Nant Barrog.
Works since 2012
Since the 2012 flood events, we've undertaken work in the village to improve resilience to flood risk and reduce the risk.
We have also completed an appraisal of the flood risk, using the services of consultant GBV to assess the flood risk and come up with measures that will further reduce this risk.
Our aim is to improve the capacity of the Water Street culvert and provide water storage upstream, to protect the village against a flood with a present day one in 100 chance of happening in any given year.
The village is thought to be well protected from the River Elwy and no improvements are planned to the village’s Elwy defences.
A drop-in session was held in Llanfair TH in March 2016 to give the community an update on our work and to identify our preferred option for a flood risk management scheme.
The consultation material from the drop in is available to view here – including four drawings outlining:
- The background
- Progress since 2012
- Natural flood risk management
- Proposed scheme overview
- a frequently asked questions document
The preferred option will provide a one in 100 annual chance standard of protection for Llanfair Talhaiarn.
The preferred option (proposed work) in Llanfair TH includes replacing 55metre length of the Water Street culvert. This section of the culvert is undersized and hydraulically ‘rough’. It will be replaced with a smoother and larger culvert sections to improve its capacity to deal with flowing water.
Additional works will be undertaken to improve existing flood walls, up and downstream of the Water Street culvert. Improvements to the current weed-screen arrangement at the upstream end of the culvert will also be included.
Upstream of the village, it is envisaged that two areas will be used to form small water storage areas. The works would involve constructing small earth bunds across overland runoff paths, to reduce the rate at which water runs off from the land above Llanfair TH in periods of very heavy rainfall.
Due to the relatively small number of properties at risk, it is not possible to design the scheme to allow for any further improvements to the defences or culvert to manage the effects of climate change.
However, a number of additional Natural Flood Risk Management (NFRM) opportunities have been identified for the upper catchment including hedge planting to reinstate field boundaries and in-channel debris “dams”. These measures could provide mitigation against future climate change and potential funding models to deliver these works outside of any capital funding programme have been proposed.
The preferred option for the scheme was selected using the decision making criteria for Wales given in the Flood and Coastal Erosion Risk Management – Appraisal Guidance (FCERM-AG). This is a five stage approach considering 1) benefit cost ratio; 2) Identify lead FCERM;Option 3). Contributions; 4) Uncertainty; and 5) Wider Objectives and Outcomes.
The option chosen is considered the most technically, environmentally and economically feasible.
July 2016 Update
We have now received internal approval to progress the detailed design of the preferred option and will work on this over the next eight months.
We will be consulting directly with those impacted by the proposed works in order that their concerns and comments are captured and fed into the design process.
A further consultation event will be held once we have completed detailed design and we will keep the community advised throughout the process.
The programme for delivery of the construction work itself has not been confirmed and will depend upon gaining the necessary approvals and funding.
We will be liaising closely with Conwy County Borough Council, as the culvert is a highway structure. We will aim to work in partnership with Conwy Council to deliver the scheme in the future.