Forestry engineers built a road to the bottom of a reservoir as they preserved an historical local landmark for future generations.
Forestry Commission Wales experts hatched a plan to save the old slate mining reservoir at Hendre Ddu in the Dyfi Forest after the Environment Agency Wales felt the reservoir was at a higher risk of flooding.
The complex operation to restore the original dam structure and features took just under a month and entailed excavating more than 200 tons of solid rock and draining thousands of gallons of water.
There are several old slate mines in the area around Abergynolwyn, Corris and Aberllefenni and the reservoir provided water to power machinery in the cutting sheds lower down the valley in Hendre Ddu when the area was alive with slate mining.
It enabled Sir Edmund Buckley to establish the Hendre Ddu Slate and Slab Company in 1864, which specialised in producing slate slabs for billiard table beds.
But when Forestry Commission Wales engineers were informed that the reservoir in the 5,500-hectare Welsh Assembly Government forest was a potential flood risk, they had two options.
Dave Farmery, Local Area Manager in Dyfi forest between the towns of Dolgellau and Machynlleth, said, “It was thought that the reservoir could not cope with a one in 1,000-year flood.
“We had two options: either carry out statutory safety repair work to retain the reservoir for historical and conservation interest, or breach the dam wall to take it out of the Reservoirs Act.
“Although this is not a designated monument, the site is of local interest so we were determined to come up with a design which made the dam safe, protected the water quality and at the same time preserved this special landscape character within the Dyfi forest.”
FC Wales’s civil engineers were helped by three different contractors to preserve the original Victorian design of the structure that was built well over 100 years ago.
The engineers had to increase the capacity of the dam wall by one metre and increase the volume of the spillway so that more water could run off from the reservoir.
However, tests showed that all was not well in the murky depths of the reservoir, which is between 10 and 15 metres deep, and the team realised they would have to drain all the water out.
It took two weeks to empty the reservoir with one of the contractors, Jones Bros of Ruthin, setting up pumps to intercept the mountain streams at one end and another pump sucking the water out at the top of the dam.
Mark Trumper, the FC Wales Area Engineer who oversaw the operation, said:
“We built an access ramp down to the reservoir as the water level went down, to enable us to get to the bottom of the lake.
“In effect, we built a road to the bottom of the lake. It was like a sludge pond. We were constantly dealing with the unknown.”
The team then had to locate a pipe in the sludge and build and fit a plastic filtration elbow on the end of the existing Victorian draw-down pipe made of cast iron.
“We were very lucky with the weather,” said Mark. “We had a lot of rain beforehand and afterwards, but if it had rained hard during the job I’m not sure if the pumps would have coped.
“Things were constantly changing, not by the day, but by the hour. I’d say it was the most challenging job I’ve had to take on. It was well out of the norm – we’re good at repairing roads and bridges, but this was something completely different!”
It’s hoped the experience will stand the FC Wales civil engineering team in good stead when they come to examine another two reservoirs in the next year or so.
