It was the most serious incident involving a UK down in over a decade.
It was very nearly the most serious disaster involving a UK dam in over a
century. When water started coming over the top of Toddbrook Reservoir's dam, the
torrent began to pull away at the concrete spillway. Nobody's died because
of an incident involving a UK dam in over a hundred years. But on the 1st of
August 2019 the worst wasn't just been feared.... It was expected to happen.
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I'm in Whaley Bridge and this is Memorial Park, just behind me is Toddbrook
Reservoir, it usually holds about 1.3 million tons of water. It's massive,
but back in August 2019 about a months worth of rain fell in just two days and all that
water came rushing down into the reservoir, it overtopped just behind me there,
came rushing down the concrete slabs and it started washing them away
you can see the damage behind me. Thousands of people in the villages
around here had to be evacuated. For a few days the thought this might actually burst.
Toddbrooke Reservoir overlooks the small town of Whaley Bridge, it's in
High Peak of Derbyshire and it's about 16 miles southeast of Manchester.
The Peak Forest Canal ends here at Whaley Bridge, it's at the basin where the old
transshipment warehouse is and it's where goods would be loaded from the
canal boats onto the Cromford and High Peak railway. The basin here is also
where the canal is fed from the nearby Toddbrook Reservoir it's about half a
mile up the hill just beyond the railway station. Now the reservoir is a hundred
and eighty years old, it's fed from Todd Brook, hence the name and it brings water
down from the hills. It covers about 36 acres and it can hold a staggering
1.3 million cubic meters of water.
Just a couple of miles from Whaley Bridge over a hundred and forty eight
millimeters of rain fell in the week before the first of August, that's almost
six inches of rain. Now the Canal and River Trust have
claimed that this amount of rainfall flowing into the reservoir in that short
space of time was a one in ten thousand year event and with so much rain in the
days before the unprecedented rainfall on the 31st of July was just too much.
Early on Thursday the 1st of August 2019 the reservoir reached its peak, but water
was still rushing down from the hills, there was nowhere else for it to go and
it began to spill over and cascade down the concrete auxilary spillway.
Concrete spillways like this are designed to allow a safe and controlled release of
excess water, if the levels get too high it protects the otherwise vulnerable
parts of the dam, so the problem here was the concrete panels began to collapse
under the torrent of water and behind it was almost 1.3 million tons of water
threatening to burst through any weak point in the dam and it would have
devastated Whalley bridge in the Goyt Valley.
By lunchtime on Thursday events
began to move quickly. By half-past twelve the concrete panels on the
right-hand side of the dam were beginning to collapse. The Canal and
River Trust alerted the dam maintenance contractor Kier that there was a major
issue with the dam. Less than an hour later with the spillway collapsing the
emergency reservoir drawdown plan was activated and within an hour the
Environment Agency issued an urgent warning to residents in Whaley Bridge
that there was an immediate threat to life. Over 1,500 residents were
instructed to evacuate the homes immediately. Public transport in and out
of the town was canceled and roadblocks were set up to stop people coming into
Whaley Bridge. That afternoon it was a virtual ghost town, roads and bridges
were closed as they were all the way down the Goyt Valley in fear that they
might collapse if they got hit by debris should the dam collapse.
Experiences engineers from around the country were racing towards Whaley Bridge. I've spoken
to one of them who had a police escort driving him at 130 miles an hour.
By 2 o'clock dozens of engineers were on site,
plus over a hundred service personnel, police from the Derbyshire Constabulary,
teams from Derbyshire Fire and Rescue Service, the Environment
Agency and the Canal and River Trust and volunteers from the community were all
swarming towards the dam desperate to do what they could to prevent the down from
a catastrophic collapse.
The priority was to stop the dam from bursting and save Whaley Bridge.
It normally takes weeks or even months to empty a reservoir of this size, but the
engineers had literally hours to get those levels down and make the dam safe,
But there is nowhere for the water to go. With all the recent rainfall, Whaley Bridge
and the Goyt Valley was already flooded, so what were they gonna do.
There reservoir has got to sluices that allow water to be released either into the
Peak Forest Canal down at Whaley Bridge or into the River Goyt, but neither of
these sluices were anywhere near big enough to drain off the vast amounts of
water needed to reduce the level of the reservoir and ease the risk of the dam collapsing.
So the engineers brought in 11, 12 inch diameter pipes and some huge pumps,
each one pump in a colossal 200 litres a second, that's like 12 bathtubs
every second of water. Over three kilometres of pipes were laid each one
taking huge amounts of water out of the reservoir and Todd Brook was diverted at
its source at the beginning of the reservoir to stop any more water coming in.
So it's so much extensive damage to the concrete spillway and a large hole
in the right side of it, why didn't the whole dam collapse.
Enbankment dams like like Toddbrook
are built from Earth, but they've got a clay core. Now the problem
with this is that if ever the water flows over the top of the embankment, if
it's just earth it can be washed away pretty fast and that's why they built
the concrete auxilary spillway.
Now fast-moving water works in a similar way
to fast-moving air. Just imagine an aircraft, two wings, fast-moving air
underneath it is enough to lift the weight of a jumbo jet.
Fast moving water acts in a similar way. It creates pressure reduction like a suction, so if
any of the concrete slabs on the spillway weren't sealed properly the
soil underneath could be sucked out as the water flowed over it and that
creates a void underneath which the concrete slabs would eventually collapse into.
So part of the investigation I'm guessing we'll look at the inspection
and maintenance of these concrete slabs. A local residents showed me photographs of
the spillway before it collapsed and you can clearly see vegetation growing from it.
This photo shows vegetation rights at the point where the concrete actually collapsed.
Luckily the damage was mostly confined to the concrete and
earth fill underneath it and the vital clay core of the dam the strength of it
seems to be intact.
The next morning, Friday, thousands of
people were waking up, if they got any sleep, in a local school or friends or
relatives houses. There was still uncertainty about the structure and
integrity of the dam, but engineers have been working throughout the night to get
those levels down as much as possible.
Engineers had managed to pump out over a
hundred thousand square meters of water in just 12 hours, but the Met Office had
issued another yellow warning for more floods in the days ahead.
Meanwhile an RAF Chinook helicopter was deployed and it began dropping one
ton bags of aggregate, each one containing sand and gravel and stones,
into the collapsed void in the dam wall. Even though the clay core was intact,
the load on the core itself had to be replaced to maintain the balance and
eventually 600 tons of aggregate was dropped along with about 10,000 sandbags.
During the Chinook's operation a section of the main 'A' road the a6 was closed and
it became a holding point for the Chinook to pick up bags of aggregate
that were been filled by trucks and then taken over to Toddbrook.
So this is what six hundred bags of aggregate looks like. WOW! The orange
stuff you can see seeping between them is like a waterproof expanding sealant.
It looks like some weird industrial melted chewing gum, but it's holding everything
together and making sure that the load on that vital clay core is balanced and stable.
It's not until you get really close up to this that you appreciate the
actual size and scale of it, how huge each of these bags are on the size of
the concrete slabs that have just disappeared and the sheer size of the
hole in this damn wall. The collapsed section doesn't look that big when you
watch it from TV, it's only when you get this close that you kind of realize that
hang on this 600 one-ton bags of aggregating here and hundreds of smaller
sandbags, you kind of get the sense of just how much danger this dam was in.
Six days after the near disaster began and engineers had managed to reduce the
level of the reservoir by nine meters, it was safe and residents were finally
given the all-clear to go home and start getting back to normal.
Levels were eventually reduced to 12 meters below the normal level of the reservoir and
the pumps are fitted with monitors and watched really closely by staff 24 hours
a day 7 days a week. They're in place to make sure that what levels don't rise
and if they do, the pumps will kick in and just pump the water away.
Now with over a billion litres of water pumped out the reservoir a new home had to be
found for the estimated 30 thousand fish. They were rescued in phases and they
were transported in oxygenated tanks to other reservoirs, including the Upper Bittell Reservoir
in Worcestershire and there's still a few in here and they're
being cared for, air has been pumped into what's left of the reservoir to maintain
healthy levels of oxygen and the Canal and River Trust say they hope to
remove the final few fish over the coming weeks.
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There is a meeting a few weeks ago in Whaley Bridge and the Canal and River Trust
told residents it could take years three more years to repair and rebuild
Toddbrook Reservoir to bring it back into use. An independent investigation
has been launched and the initial findings are going to come back to the
Environment Agency before the end of this year. The public will learn in 2020
what happened. Some people put it down to climate change, freak weather events like
the ones we saw in July and August are getting more common. Other people are
more skeptical, saying it's to do with inspection and maintenance. Maybe it's a
combination. I guess we'll find out. One thing that is for sure is the heroics of
the Armed Forces, the engineers, the police and fire service and of course the
volunteers. People from Whaley Bridge and beyond who came to help. That typical
British stubbornness, the fortitude to get in and get your hands dirty, get the
job done and I think they saved Whaley Bridge.
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