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Slow release | Two new dams for Skipton

Eller beck 3 23

The town of Skipton was made to be flooded. The steep hills and sodden moorland of the Yorkshire Dales National Park run straight down into the town of about 15,000.

Two major watercourses are the problem: Eller Beck, a stream to the north, and Waller Hill Beck to the east.

Both run on a direct path into town, then wind their way around homes and businesses via a series of winding culverts. 

As such, waist-deep water has been a regular feature in the town after heavy storms. In the 1982 flood, one life was lost and 570 properties flooded. About 230 residential and 90 non-residential properties are at risk of flooding during a 1-in-100 year event.

Talk about flood-proofing the town has gone on for decades. One Arup director recalls working on possible designs in the 1970s, but these were deemed too expensive and complex at the time.

Once the £13.95M funding was acquired (see below – How to: find flood funding) through the partnership model the next process was to decide what to build. Building major pieces of infrastructure in town was ruled out early on.

“With Skipton being a historic, old town, you can’t put the big floodwalls through, because the space is so limited,” says Environment Agency project manager Jenny Cooke. “And in terms of taking that approach as well, it’s incredibly expensive, in terms of the temporary works and impacts on buildings in town.”

So upstream storage was the way to go. 

Skipton jpg

Skipton jpg

Eller Beck Dam

For Eller Beck it was decided that an upstream dam be created to hold back water during heavy rainfall. New Civil Engineer visited the 17ha site in late July and the scale of the project is remarkable, especially considering the size of the nearby town, only 1.5km to the south. Centre stage on the larger of the two sites is the 14m high, hydraulic powered, sensored, automated Eller Beck dam. At 360m long, it contains 200,000t of fill, imported from a quarry near Halifax about 48km away.

Initial investigations revealed the ground comprising Glaciolacustrine sediments deposited from glaciers was very soft and compressible. There were a number of possible solutions to create a firm base, including piling, but digging it out down to more stable material and then backfilling it was the selected option. About 30,000t of material was removed from a 5m deep excavation.

It has been replaced by a material that’s excavated as a type of shale, then treated on-site, becoming clay-like as it gets air and moisture. The team continues testing the material to make sure it is up to the structural standard required.

The dam has a concrete culvert running through it, allowing the stream to flow along its natural alignment in normal weather conditions. The culvert was installed in seven sections linked by waterproof joints. There is an inlet screen to stop debris, and should it get blocked during a storm then water will be diverted around and further down into the culvert via an emergency pipe.

When flood conditions arise, a Programmable Logic Controller monitors receives water flow readings upstream, downstream and near the culverts. It instructs the hydraulic penstocks to be lowered over the 2.5mopening, impounding the water to form a reservoir

“The whole system is automated. It also sends information back to Environment Agency’s control centre, where they can take control remotely. Or they could come on site and operate it manually,” says Galliford Black & Veatch (GBV) project manager David West. GBV is a joint venture between Galliford Try and Black & Veatch.

At its fullest, the dam can hold back 433,000m3 water. The local golf course and about half the valley would be flooded at that stage, but residents and businesses of Skipton would continue receiving a steady 17m3/s, flow – low enough to prevent flooding.

Eller beck 3 23

Eller beck 3 23

The penstock operating system incorporates a heirarchy of contingency operations to ensure that it closes at times of flood risk.

“There’s a standby generator that kicks in on automatic,” West says. ”If that fails, then we’ve also got an external generator. If that all goes down, there’s… a big cylinder, with a compressed air section. What it does is looks at the main supply, and if it doesn’t get a power supply, there’ an allowance for the standby to kick in, the interlock opens, and gas purges into the system, forces hydraulics out, and shuts the penstock.”

And if the dam does get to 433,001m3 of water, a concrete spillway comes into action, just north of the dam. It measures 30m wide by 300m long and to the casual observer it may seem like overkill. It will likely never be used, only in an epic 1-in-1,000 year flood event. But as Cooke explains, it is required, as part of the dam’s “Category A” status under the Reservoirs Act.

“There’s a certain amount of safety features that have to be built into the dam. And when you have a population downstream of the new dam that you’re building, then you have to make sure you can spill safely, in a controlled manner, rather than it all flowing over the top of it.”

To reduce the visual impact of this concrete eyesore (in what will otherwise be grass-covered structure) a type of rubberised brick pattern has been laid into the setting concrete. It will look great, says West. “Once it’s had a bit of weather on it, which in this part of the world, won’t take long. It does rain most days here.”

It is important to mention that all this work is taking place on a golf course. As works took over, the Skipton golfers have played without their 15th and 16th holes for about a year. Initially there was strong opposition from the golf club, and a nearby landowner. But as is often the case, the dispute was resolved for the good of the town.

“It’s not a concern for us, it’s a concern for the people at the bottom of the hill. But flooding is hard to predict, and to have something in place now makes sense,” says Skipton Golf Club chairman Mick Hirst.

Eller beck1

Eller beck1

The view up the Eller Beck culvert facing north west (upstream).

A landscape designer has also been brought in to produce designs for new fairways and greens to be built on and around the dam.

Another feature of the project is an otter ledge that runs along the length of the culvert. New Civil Engineer inquired as to the prevalence of otters in the area; early ecological studies found evidence of the elusive water dweller, but the construction crew have yet to sight one.

“[The ledge] is perched about a metre above, so when the water levels get quite high it [an otter] can scurry along the river. A 90m length culvert is going to be a challenge to swim along,” says Cooke. Likewise for the fish it could be a challenge, so there are baffles (concrete blocks) up to 300mm tall to divert and slow the flow.

As you might expect in Yorkshire, weather has played havoc with the construction programme. The Eller Beck works were completely flooded out on four occasions last winter. Rain fell on 27 days in November, every day in December and most of January.

“We had probably half a dozen major flooding events during the winter, which completely flooded out our works,” says West.

“Worst of all was Boxing Day, where [local homewares store] B&M got flooded as well as a few properties, and probably just showed that the dam is needed.”

There was up to 100,000t of material stockpiled on site, and 25 wagons running about five loads per day. So a series of contingency plans were put in place for flooding during the construction, limiting the damage to both the site and town below.

Eller beck2

Eller beck2

Workers construct the concrete slab weir section, where overflow from Eller Beck will run.

Waller Hill Beck Dam

At Waller Hill Beck dam, about 2km west of Skipton, the hills are smaller and the valley narrower. It’s easy to see how, in flood, this area would be less severely hit.

Accordingly, the dam is about a quarter of the overall size as Eller Beck – 9m high, 60m wide, 75m long and only about 35,000t of fill is required. Unlike the Eller Beck dam, this structure has no spillway, but relies instead in “controlled overtopping”.

“Because it’s such a tight valley, we have no room for a spillway. The way the contours are designed it’s a controlled overtopping.”

The controlled spill is assisted by a lowered central section on the crest over the dam, which is lined with grasscrete, to prevent the dam surface from washing away; and concrete slabs to dissipate the water’s energy.

“This one’s slightly different in the way it’s controlled, instead of having an actuated penstock, we’ve got an orifice weir, there’s a 400mm2 hole in the plate, bolted on to the culvert, so there is no active control, the amount of pass-forward water is down to water levels.”

Underneath Waller Hill Beck is about a 3m depth of fractured limestone, so more underground work was needed.

“There was a concern there might be high permeability so there’s a 6m deep grout curtain designed. There ended up being 100 drill holes, they’ve been grouted in two stages, to get a coefficient of 5x10-7m/s was the sort of permeability we were trying to achieve,” says Arup scheme design consultant Sue Thompson.

Waller Hill Beck dam, the smaller of the two dams.

Waller Hill

Waller Hill Beck dam, the smaller of the two dams.

The result

The two dams take care of 95% of the flooding issue, Cooke says, with minor floodwall works in Skipton finishing the job. About 300m of walls are to be raised in four areas of town.

“It would have been great if we could have done everything upstream, but there were a couple of low spots, needing some town works.

Work has been going on since June 2015, and is expected to run until September, with the full scheme comissioned by the end of the year.

The flood scheme has been certified to a 1-in-100 flood protection. But Arup has qualified this by saying “1-in-50 protection beyond 2080”, with climate change likely to bring worse floods.

Says Cooke: “Our climate change scenarios, certainly in terms of recent floods we’ve been experiencing, are calibrated against events that have already happened – if you have a rare occurrence, that could change everything and you have to recalibrate. So we say 1 in 100, but really, it could be 1 in 200, depending on what’s coming in future, you just don’t know.

“It will significantly reduce flood risk, I think that’s the key message.”

How to: find flood funding

Environment Agency project manager Jenny Cooke says, overall, the project has been a great result for all involved. Especially considering the scarcity of flood funding.

“It’s a really interesting scheme to work on. And they’re increasingly harder to fund, just with pressures on funding from the government. But [the scheme has] been so long coming, it’s great for the town.”

The majority of the funding for the £13.95M scheme was secured from the Environment Agency’s Flood and Coastal Erosion Risk Management (FCRM) grant in aid budget.

But the government also called on local stakeholders to chip in for the infrastructure.

“The way the funding is done for the scheme, there are calculations on how many properties protected, how much the scheme is going to cost, then it turns out how much funding can be given,” Cooke says.

It took a long time to secure the funding.

Jenny Cooke

“If it falls short, then you fall on partnership funding from local councils and local businesses, that would benefit from the scheme.

“This system enables more flood defence capital projects to go ahead and give communities more responsibility and choice about what is done to protect them from flooding.”

Contributions were needed from the Department for Environment Food and Rural Affairs growth fund, North Yorkshire County Council, Craven District Council, Yorkshire Regional Flood and Coastal Committee and Yorkshire Water.

“It took a long time to secure the funding: all the councils and Yorkshire Water had to go through their own business cases for providing the funds,” Cooke says.

Then, when local business Morrisons Supermarkets refused to stump up some of the funding in 2014, the project ground to an indefinite halt. Culverts run directly beneath the Morrisons site, and it is one of the major victims of flooding in Skipton.

But £1.2M then arrived through North Yorkshire Local Enterprise Partnership to secure the necessary funding from their Local Growth Deal fund.

The “Local Growth” part of the deal involved the wider development of the South Skipton area. The area encompasses 24.5ha of Greenfield land with the capacity to accommodate 178 houses and 18.6ha of land for employment supporting 1,155 jobs. The scheme has been hailed by the UK government as a “foundation for new investment within the town and surrounding area”.

Skipton Flood Alleviation Scheme is one of nine schemes nationally which received Growth Funding from the UK Government in addition to accelerated FCRM Grant in Aid.

The government says the dams and flood walls have safeguarded homes, businesses, enabling growth and development within the town “with the potential to create over 500 jobs”.

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