PUMPED HYDRO RESERVOIR LOWERED FOR ESSENTIAL CHECKS IT’S DOWN BUT NOT DROUGHT

WATER levels at a prominent reservoir in Eryri (Snowdonia) have been lowered to enable checks on its dam, tunnels and pipes.

The sides of Llyn Peris, near Llanberis, will remain exposed for part of the summer to allow drilling rigs and plant machinery on-site.

The reservoir forms part of the pumped hydro system at Dinorwig power station, long regarded as one of Britain’s greatest engineering triumphs.

Now approaching its 40th anniversary, the site is being overhauled to extend the plant’s operating life.

Levels often drop markedly in Llyn Peris as water is pumped 1,757ft up Elidir Fawr to an upper reservoir, Marchlyn Mawr.

Levels are restored when water is released back through the mountain’s network of giant pipes to a hydro plant that generates electricity.

To accommodate the refurbishment work, water in Llyn Peris is being “stored” in Marchlyn Mawr.

Levels in Llyn Peris (pictured) were dropped by 14 metres and will stay low for around 80 days while reservoir checks are carried out.

It’s left some wondering if the exposed sides are early signs of drought.

In fact, levels have since risen slightly after water was emptied from the hydro system to allow engineers access to the plant’s hydraulics.

Station manager John Armstrong, who also oversees First Hydro’s pumped power plant at Ffestiniog, said: “The reservoir levels are reasonably similar to how they are after water is pumped to the upper reservoir, which usually done overnight when electricity prices are lower.

“The main difference is that they will stay this way for a while as refurbishment work continues.”

In the 10 years it took to build Dinorwig Power Station, which began operating in 1984, it was the UK’s largest civil engineering project. Dinorwig’s old slate quarry, once the world’s second largest, was chosen because of its elevated position and existing excavations.

To preserve Eryri’s natural beauty, the hydro plant itself was located in a cavern serviced by 16km of tunnels deep inside the mountain itself. Construction involved 2,000 locals. They removed 12 million tonnes of rock, creating tunnels wide enough for two lorries to pass, as well as the massive 590ft-long “concert hall”, Europe’s biggest man-made cavern.

Dinorwig remains the UK’s fastest source of electricity. At peak output, water empties from the upper reservoir at the rate of half an Olympic-sized swimming pool every second, enough to power 1.5 million cups of tea a second.

This way, the plant helps meet peaks loads in the electricity supply network. It also plugs gaps in supply when there are surges in demand - famously, during “TV pickup” when millions switch on kettles during hit shows like Britain’s Got Talent.

Mr Armstrong said the power station remains a “feat of engineering that is of national significance”.

He said major works were necessary to “extend its operating life for the coming decades”.

Given its impressive response times, Dinorwig is expected to remains a key component of Britain’s power network.

As the country’s energy mix switches towards renewables, such as wind and solar power, power storage sites will have an increasingly important role in dealing with intermittency of supplies.