An RAF Chinook helicopter hovers over the slipped pieces of concrete, delicately dropping sandbags into a neat pile across the damaged spillway of the 100-plus year old dam holding back the Toddbrook Reservoir. Behind the damaged wall waits 1.3 million tonnes of water, ready to crash into the town of Whaley Bridge below.
About 85 per cent of the residents of the Derbyshire town’s 6,500 residents have left their homes after heavy rain damaged the dam on August 1. But to shore-up the dam, which was built in 1831, authorities are turning to brute force.
Alongside dropping the sandbags, the helicopter has also been tasked with stopping the flow of water into the reservoir by plugging the Todd Brook with one-tonne bags of aggregate, a mix of sand and gravel. Divers examined the damaged dam through touch, as turbulent water reduced visibility too much. Hoses snake out of the walled edge into 16 high-volume pumps; so far, levels have dropped by 200mm after heavy rains overfilled the reservoir, but work to stabilise the dam could take days.
Dams have two key elements, says Nicholas Howden of the University of Bristol: an impermeable barrier to hold water in, and ballast to balance the force of all that water. In some cases, both aspects are managed via large concrete barriers, which is both waterproof and heavy. With the Victorian-era Whaley Bridge dam, clay was used to build an impermeable barrier with earth banks used to hold it in place.
That’s not where the failure appears to be, however. If a reservoir has too much water, excess is discharged via a spillway. “The dam is designed such that when it gets full, excess water flows over the dam crest and down a spillway,” says Paul Shepley, lecturer in geotechnical engineering at the University of Sheffield.
In this case, the spillway appears to have been damaged by the large water flow that was sparked by heavy rain. “I can’t be certain, but it looks like what’s happened is that some of that water has got underneath some of the concrete flaps and started to wear away at the material,” Howden says.
Domenico Lombardi, lecturer at the University of Manchester agrees, saying it was likely caused by internal erosion. “The process involves transportation of soil particles by groundwater and tends to accelerate under high and prolonged hydraulic gradients, such as those caused by prolonged periods of heavy rains,” he explains. “In water-retaining structures, erosion normally begins on the downstream side and then propagates backwards upstream as the granular material is gradually washed away.”
While we’ll have to wait for full details of the cause, what it means now is there’s less material balancing the force of the dam and further damage would be caused by water flowing over the spillway. “Pumping water out […] will lessen the force on the upstream part of the dam,” Howden says. It will also offer room in the reservoir in case it does rain, as that will “prevent more water flowing over the damaged section,” Shepley explains. “The key thing here will be if there’s any more significant rainfall forecast as that will change the situation.” The Met Office is forecasting rain on Sunday.
Alongside reducing the water level in the reservoir, emergency responders are shoring up the damaged section of the spillway with more material, which is why sandbags are being piled up where the concrete has slipped. “This is to improve the stability of the earth dam structure and limit any damage to the dam core,” Shepley says. “It would also help in the instance that any more water flows over the spillway, but that’s not probably not driving that effort.”
Howden notes that the dam would have been designed to hold back more water than strictly necessary, a gap known as a factor of safety, in case of problems just like the one at Whaley Bridge. “The force holding water in place is greater than the force of the water,” he says. “There’ll be enough mass there to hold it back.”
he damage to the concrete spillway is the problem, as it creates a weak point that could lead to worse erosion if more water discharges over the spillway. If the reservoir is refilled by rain and overspills, that water will add to the damage and wash away more of the weight holding the dam together. Hence the work to pump out the reservoir to ensure that doesn’t happen.
Even though such measures are the correct ones to take, according to Shepley, an evacuation of the town below also remains necessary. “It could take days to get the water level in the reservoir down to a sufficiently low level to deem the situation resolved, but the efforts taking place are all spot on, including the evacuation of Whaley Bridge given the situation,” he says. “That the reservoir level has already dropped 200mm is promising and shows the current efforts are working. So full credit to the engineering team’s response to the situation.”
While the extreme weather hasn’t helped, similar dam failures have happened before — and the techniques described above prevented collapse. “Ulley Dam in 2007 had a similar failure, with damage to the spillway on the face of the dam leading to erosion of the dam materials,” says Shepley. Cracks were spotted in the South Yorkshire dam after heavy rain in June 2007, sparking evacuations. The dam was shored up at the time using 2,600 tonnes of limestone, but £3.8 million in engineering work followed, including a new spillway, before it was reopened three years later.
Shepley also points to the Oroville Dam in California. In 2017, erosion was spotted in the main spillway as well as a secondary, emergency spillway, leading to the evacuation of 180,000 people. After significant design flaws were revealed, the spillway was demolished using explosives — there’s a great slow motion video — and entirely rebuilt. This isn’t the first issue at Toddbrook Reservoir. Built in 1841, the dam has been hit by leakage concerns for more than a hundred years, according to New Civil Engineer. It was fully drained in 1981 after a leak was spotted in the dam banks and partially emptied in 2009 for realignment works. That doesn’t mean the dam was damaged before the reservoir flooded; it was checked twice weekly under safety regulations.
hepley says he’s not aware of any full spillway collapse. “So there’s no precedent,” he says. If it did go — and that’s a big if, as the preventative measures seem to be working, he says — the water would gush out so quickly there’d be no time to evacuate the town below, eroding more of the spillway as it went.
Howden says there are good signs that the Whaley Bridge dam isn’t about to collapse, though he stresses he isn’t on site and that anything local engineers and emergency responders say should of course take precedence — and they have warned a collapse is possible. “If you look at videos or photos of a dam that is going to fail imminently, you’ll see water flowing through it,” he says.
That water pathway through the dam will cause further erosion, washing away that ballast material. “That’s generally quite a bad sign.” But looking at Whaley Bridge, that doesn’t appear to have happened — again, he stresses that anyone in the area should listen to emergency crews on the ground, as they’ll have better information, including from monitoring devices installed in the dam. If the local authorities tell you to evacuate because a dam may fail, do what they say.
