Brithdir Mawr reservoir
Brithdir Mawr, is situated in the eastern foothills of the Clwydians, between Loggerheads and Afon-wen. Built in 1897 to provide drinking water for the local area, it is fed by Aber Eilun river. Brithdir Mawr no longer forms part of the drinking water network with customers in the area now being supplied from the Alwen reservoir.
Even though Brithdir Mawr reservoir no longer forms part of the drinking water network, as with all our reservoirs and dams, we have continued to inspect and monitor it as part of our Dam Safety monitoring plan.
What are we doing on site?
As a not-for-profit company, maintaining and investing in reservoirs to meet current reservoir safety standards that no longer supply drinking water to customers would not make best use of customers money. Our plan is to decommission the reservoir and restore the natural watercourse – as it was before the dam was built.
We have carried out a similar scheme in nearby Cilcain where we decommissioned two of the four reservoirs near the village and restored part of the Nant Gain river.
Environment
Through careful design, it is hoped that the scheme will lead to an improvement in the biodiversity in the area, allowing wildlife to flourish. Natural Resources Wales has been consulted throughout the scheme design and is positive about its ecological benefits allowing wildlife to flourish. The reservoir is located within an Area of Outstanding Natural Beauty near Moel Famau and returning the site to nature will be in keeping with the surrounding landscape
All of the work at Brithdir Mawr will be completed with the support of a team of ecologists experienced in reservoir decommissioning and re-naturalization, ensuring that the work complies with environmental legislation.
How are we doing the work?
We plan to carry out this work in two phases. This first phase of the work will be done in the Autumn of 2024 with the second phase being carried out during in 2026.
Phase 1 – drawing down the reservoir
The first phase of the work involves draining down the reservoir. This work will be done in Autumn 2024 and should take a few weeks to be done. We will then allow the area to dry naturally until we are ready to carry out the work to remove the dam and restore the natural watercourse.
Drawing down the reservoir before carrying out the rest of the work on a later date brings with it a number of benefits.
Drawing down the reservoir will expose silt at the bottom of the reservoir, which will inevitably dry and be revegetated, limiting the potential for sediment-related impacts in future work on site.
In the long term, the removal of the reservoir will restore the natural hydrology of the inflowing streams that existed before the reservoir's construction. By re-establishing these natural watercourses, we will promote a healthier and more resilient ecosystem that supports a greater diversity of plant and animal species.
Immediately following the lowering of the water, the edges around the lake may look muddy as silt becomes exposed, but this will dry out as time goes by.
Our team will start on site in the Autumn and should be finish the first phase of the work by the end of the year.
Phase 2 – reinstating the natural watercourse
We will look to carry out the second phase of the work in 2026. This work will involve creating a notch through the dam, filling in the current spillway, reinstating the natural river course and landscaping work.
We will update this page with more information about the work that will be carried out in phase 2 closer to the time.
Our below artist impression shows how the site may look after our work has been completed, but this may change slightly as we work on our detail designs for the site.
We carried out similar work in Cilcain in 2023 where we decommissioned two adjoining reservoirs and restored the natural riverbed. Below are photos of the work carried out and the area after it has been restored.
Frequently asked questions
Brithdir Mawr is no longer connected to the local water network as customers in the area have been supplied by Alwen reservoir and Alwen water treatment works for a long time. As a not-for-profit company, maintaining and investing in reservoirs that no longer supply drinking water to would not make best use of customer money in the long term. We will therefore decommission the reservoir and restore the natural river.
All reservoirs in Wales above 10,000m³ fall under the ambit of the Reservoir Act 1975. This means owners must legally ensure the safety, ongoing surveillance, monitoring and inspections of the dams as well as any mandatory Capital works needed. The costs associated with this are very often in the £millions. Furthermore, there is always a residual risk associated with dams, particularly as assets age.
Therefore, keeping Brithdir Mawr as it is, is not a viable option. The dam structure at Brithdir Mawr will be removed and the reservoir will be returned back to its natural state as a river.
The lake is not stocked with fish and there are no fishing agreements at Brithdir Mawr. Any fish in the lake will be carefully removed and relocated to another site.
A large notch will be constructed through the dam, which the natural river channel will then flow through. We will work with specialists to construct the river channel as close to how it would have run before the dam was constructed.
Construction traffic will access site via the existing track. Construction plant size will be limited due to the narrow lanes in the area. Some minor track improvements will be required and some cutting back of overhead branches close to the site entrance will be needed.
The site will remain under Welsh Water as we will continue with regular visits to site to check and monitor the area.