Llyn Celyn improvement work
At Welsh Water we take our responsibility for safely maintaining and operating our dams and reservoirs seriously.
In 2020, a 10 year statutory inspection was completed by an independent Panel Engineer. It was confirmed that Llyn Celyn reservoir is in a good state, however as expected recommendations were made to ensure that the reservoir meets the latest UK guidance. The recommendations in the final report are statutory and are enforced by Natural Resources Wales. Welsh Water must respond to them within a defined timescale.
Please be assured that this does not mean there is any immediate safety concerns with the dam.
One of the recommendations was to introduce measures to deal with any overspill at the reservoir that did not rely on manual intervention. At present, the water level in the reservoir is controlled by a series of valves and pipework situated in the tower and within the dam structure and is managed by staff on site. In addition, the reservoir already has one spillway and, although this spillway will remain and is in good working order, the water at Llyn Celyn hasn’t risen to the level of this spillway since the 1970’s.
Welsh Water has considered numerous options to deal with an overspill for extreme water levels. The proposal is to build a new auxiliary spillway to deal with exceptionally high-water levels when the reservoir levels far exceed the point where the current spillway comes into use. This has a chance of occurring 0.01% in any one year. Work will also be carried out on the existing pipes and valves as part of an upgrade maintenance programme.
The proposed work will ensure resilience for future generations.
What we are doing?
We are building a new additional spillway next to the main car park off the A4212 at the crest of the dam. Over summer 2023 we carried out preparation work on site ahead of the main work. These works included constructing a new haulage road from the bottom of the dam to the top as well as diverting high voltage cables to allow the construction work to be done.
While we carry out the work on this major three-year project the carpark and path across the dam is closed to the public.
The project will see an innovative new spillway being created. With the reservoir located in Eryri National Park every effort was made to minimise the visual impact of the new spillway to help it better blend with its natural surroundings. The new spillway will be culverted under the crest of the dam and will use the natural gradient of the valley to direct flows back to Afon Tryweryn below. If the waters at Llyn Celyn reach the exceptional level of the additional spillway, water will flow over the new concrete weir of the proposed spillway into the upstream compartment but will go no further at that point as it will be held in the reservoir by a system of hydraulic gates. In extremely rare events of exceptionally high-water levels when the reservoir levels far exceed the point where the current spillway comes into use (0.01% in any one year) the gates would open on the new spillway.
This additional spillway will provide a crucial safeguard against extreme water levels that could potentially occur in the future.
This work is being carried out by our partners, Mott MacDonald Bentley (MMB), along with a number of specialist subcontractors. We will be carrying out the work in a phased approach with the five phases running into each other as well as some running concurrently. The work started in the summer of 2023 and should be finished by the end of 2025.
Phase one work near the carpark involves removing soil and securing the ground, installing underdrains as well as constructing the base, walls and roof to create the culverted spillway. Near the end of the project we will then be reinstating the soil over the outlet part of the spillway so that the visual impact is minimised as much as possible. Phase two is to create the new inlet of the auxiliary spillway, which will include removing soil and any rocks where needed. We will then build a crane pad so that our crane can work safely for us to create the new spillway base and walls.
As much of the excavated soil and rocks will be stored on site until it is put back to minimise traffic movement to site, but you may notice a number of concrete wagons coming to site over the next few months. These lorries will be coming to site to create the base, walls and roof of the new spillway via the A4212 and through the carpark at the top of the dam. To help facilitate this we will have traffic management in place.
Phases one and two will see us construct either side of the new spillway – the inlet and outlet. Phase three, four and five will then join the inlet and outlet.
With a lot of construction traffic coming and going from site a section of the A4212 near the dam has a temporary speed limit of 40mph on it and traffic management when required.
When the work to construct the new additional spillway is complete additional recreational facilities such as picnic tables and a bike rack will be placed at the entrance of the dam and information boards telling the story of Llyn Celyn will also be erected.
Work on site is expected to be completed and the carpark reopened at the end of 2025.
Frequently asked questions
The latest guidance regarding dams and reservoirs were considered as part of our 10 yearly independent survey of Llyn Celyn published in 2020. The expert statutory advice was that additional measures should be introduced to manage flooding in the unlikely event of extremely high levels of rainfall.
The current spillway will remain operational and will deal with water levels when they reach about 2m below the crest of the dam. It is in good working order and some maintenance work is being carried out on this spillway over the coming months.
No. The work to construct a new spillway will not change the operational work at Llyn Celyn. The water levels of the reservoir will stay the same and will only rise as currently happens when we experience severe rainfall.
If the reservoir water level reaches the level of the new spillway, water will flow over the new concrete weir of the proposed spillway into the upstream compartment but will go no further at that point as it will be held in the reservoir by a system of hydraulic gates. In extremely rare events in excess of the 1 in 10,000 year storm (0.01% Annual Exceedance Probability) the gates would open on the new spillway and water would be released back to the River Tryweryn downstream of the dam by flowing across open grasslands for approximately 500m.
The car park at the top of the dam will be closed and access to the pedestrian route across the top of the dam will be restricted during the construction period (2023-2025).
Welsh Water is in contact with the Snowdonia National Park Authority and the local community to erect information boards around the reservoir telling the story of Capel Celyn and Llyn Celyn. The planning application includes new information boards near the main car park at the top of the dam. We are also aware of the campaign to erect a Tryweryn memorial in the vicinity of the reservoir and an area has been designated for any potential memorial in the future.
A 1 in 10,000-year event may occur (on average) once every 10,000-years, i.e. it is associated with a probability (Annual Exceedance Probability) of 0.01% in any one year.
Llyn Celyn reservoir already has one spillway to pass water at times of high-water levels. This spillway has not been used since the 1970’s as the water level has not reached the top of the spillway. Since the 1990s, the reservoir has been operated with a target maximum level of between -1.0m or -3.0m below its top water level. This operational regime has successfully prevented the existing spillway from operating, although the reservoir came close to spilling during floods in 2015.
We started work in the summer of 2023 with work is expected to be completed by the end of 2025.
