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Flooding: Capital projects

147 one shot one ride hampstead heath swimming pond

Hampstead Heath’s picturesque ponds have undergone some major work to make them resistant to flood damage

All year round, London’s Hampstead Heath teems with people out for walks, flying kites and even swimming. The 320ha park, located just 6km north of central London, attracts over 10M visitors a year, including around 200 a day who swim in its famous bathing ponds – even in the depths of winter.

These ponds – the Highgate Men’s Pond, Kenwood Ladies’ Pond and Mixed Bathing Pond – are just three of the 30 ponds on the Heath, all fed by natural springs. Some of the ponds are natural, but there are two sets of interlinked ponds – the Hampstead and Highgate chains – that are artificial, and were originally dug in the 18th Century to provide drinking water for London (see box).

model boating vantage1 proposed

One of the largest ponds is Model Boating, which was the focus for a large proportion of the work associated with the entire £15M project

The City of London Corporation manages the Heath, and is responsible for maintaining six ponds in each of the two chains, of which three – Hampstead No. 1 Pond in the Hampstead chain and Model Boating Pond and Men’s Bathing Pond in the Highgate chain – are large enough to classify as reservoirs under existing legislation. A statutory 10-year inspection of these three ponds in 2007 found that the overflow capacity on all three was inadequate, and recommended that an assessment should be carried out to look at the downstream impact and risk of a breach.

In 2009 the City commissioned a study by its then supervising engineer, Cares, which concluded that two of the ponds fell into the highest risk category. This in turn led the City to commission Haycocks Associates to undertake a review of the hydrology and hydraulics of all the dams within Hampstead Heath.

And it was not just the three major reservoirs that the City was concerned about. “Three are statutory, but we’ve always taken the pragmatic view that if it’s a dam, we should treat it as statutory,” says City of London Corporation assistant director engineering Paul Monaghan.

He also points out that the 2010 Flood and Water Management Act gave government powers to bring smaller reservoirs – over 10,000m3, rather than the current 25,000m3 – under statutory regulation, as well as saying that the combined volume of water in a cascade could be counted as a large reservoir. If this comes into force, all the ponds in the Highgate and Hampstead chains would have to confirm to reservoir legislation and standards, and it is likely they would be assessed as high-risk because of their impact on the downstream community.

Haycocks’ report said the ponds did not have adequate spillways, which means there is little capacity for controlled overflow in heavy flows.

“The study identified that many of the dams were at risk of overtopping in a significant rain storm,” says Monaghan.

“We needed to do something,” he adds.

Haycocks came up with some concept ideas to make the reservoirs and ponds on both the Hampstead and Highgate chains comply with legislation, and in 2012 the City of London Corporation appointed Atkins to develop these into detailed proposals.

This was followed two years later by the appointment of Bam Nuttall on an early contractor involvement (ECI) basis. Atkins’ modelling showed that most of the dams would be overtopped in very much lower return period floods than the Probable Maximum Flood (PMF) – some from as low as a 1 in 5-year return period event. Its initial design report states: “Any size flood event, whether 1 in 20, 1 in 1,000 or the PMF, could theoretically happen tomorrow.”

It adds: “The capacities of the existing overflow pipes at each pond are too small, and the storage capacities of each pond, between the overflow level and the dam crest level, are not sufficient to deal with the floods without floodwater flowing over the dam crests onto the downstream faces.”

In the past, some of the dams have overtopped – most notably during severe storms in 1975, and more recently in 2010. Overtopping weakens the dam, increasing the chance of breaches in the future, so one key function of the project was to create adequate spillways to ensure that any excess floodwater flows safely around the dams rather than over the top.

“The design standard for reservoirs is almost absolute,” says Monaghan. “If there’s a risk of injury or loss of life in the downstream community, then you have to make it safe. We’ve come up with a design that does that.”

The project was taken to Judicial Review by a group of local residents who objected to the work, but Bam Nuttall eventually got started on site in early 2015 – just in time to do works to trees ahead of the bird nesting season.

Work has been carried out on all 12 of the ponds in the two chains, mostly to restore the dam crests and to create spillways around the sides. A few had an extra 1m or 1.25m of height added to the dams – either with site-won earth or sheet piles and capping beams – and additional storage capacity has also been added in the middle of each chain.

“The design is very much landscape-led rather than engineering led,” says Monaghan. “It’s soft and natural. Sheet piling and concrete is minimal.”

One of the largest ponds is Model Boating, which was the focus for a large proportion of the work associated with the entire £15M project. Its location in the middle of the Highgate Chain makes it ideal for storing water in the event of high flow, while its position between two hills creates a natural bowl that can hold water in the case of an extreme flooding event.

The entire pond was drained, excavated and re-profiled to give it a more natural shape, and a new clay dam was built at the southern end of the pond, upstream of the existing dam and 2.5m higher. A new spillway has been created in the south-west corner of the pond using reinforced grass turf, enabling water to flow safely to the next pond in the chain – Men’s Bathing Pond – in the event of a flood.

“We installed temporary steel sheet piles, then drained the pond down, and dug out the silt so that we could build the new dam,” explains Bam Nuttall project manager Rob Stapley.

The new dam was built using material excavated from a borrow pit on the hillside alongside the pond, one of two opened up in the park during the construction phase. Silt removed from the ponds was put into lagoons to dry out and then used to fill the borrow pits.

“We won all the material on the Heath, and we tried not to take anything off the Heath,” says Stapley. “A total of 21,000m3 of soil was taken out of that hillside [next to Model Boating

Pond], but if you walk up there now, you will just see the hillside hydroseeded and topsoiled.”

Monaghan adds: “The original dams are made from the material on the Heath, so we knew the material here is good.”

Another new dam was built in the area known as “Catchpit”, between the Viaduct and Mixed Bathing ponds in the Hampstead chain. This dam is designed specifically to introduce some flood storage into the Hampstead chain, and is 5.6m high, with an open grass spillway along the whole crest of the dam. Again, the 8,500m3 of material needed for the dam construction was won from a borrow pit on the adjacent hillside.

“Catchpit is a brand new dam across the valley,” explains Stapley. “We have created a holding dam so there is lower impact on the section below. In certain storm conditions the water backs up behind the dam.”

Amid all the pond work are two new structures: a new changing room and lifeguards’ facility at the Kenwood Ladies’ Bathing Pond in the Highgate chain. The original changing rooms sat on the dam and had to be removed so the dam could be raised. Because of its poor condition, the old building could not be repositioned, so the City opted to commission a prefabricated replacement, which was lifted in using a crane brought through the park along the public footpaths.

“One of our biggest challenges was the logistics,” explains Stapley.

“Although we were using site won clay material, we were still bringing concrete in and sheet piles in – as well as the modular building and crane.

“We had 12 sites around the Heath, and we were escorting everything in and through what is effectively live footpaths.

“We couldn’t split people and plant by fences, so we had to manage it through escorts and banksmen,” he adds. “Something we did at the ECI phase was programming the logic for the job. We had 12 ponds, which meant 12 items of work simultaneously, all fitting into an 18-month programme in which we had to consider environmental aspects like the nesting season and the fish spawning season.

We also had the bathing ponds: we always had two out of three open, and in the summer we had to have all three.

“Everything we did – the designers, the City, the Heath management and ourselves – was about finding the best solution for the heath,” he adds.

Most the work was completed in October 2016, with the last two months of the year spent completing the planting.

 

Historic water supply

Hampstead Heath has around 30 ponds fed by natural springs, including the Hampstead and Highgate chains.

The chains are fed by two major streams, separated by Parliament Hill. The stream to the west has a source near the Vale of Health and supplies water to the Hampstead chain, while the eastern stream has a source near Kenwood and supplies the Highgate chain.

The two streams meet north of Camden to form the Fleet river, which flows through London to join the Thames at Blackfriars.

In 1544 the London Conduit Act allowed the City to make use of “dyvers great and plentiful sprynges at Hampstead Heath” to supply fresh water; and 150 years later the City leased the springs on Hampstead Heath to the Hampstead Water Company.

This company dug the Hampstead Ponds for use as fresh water reservoirs in the early part of the 18th Century, adding the Vale of Health Pond in 1777.

Around the same time, another series of six ponds (the Highgate Chain) was made by damming the eastern stream.

The Hampstead Water Company supplied water to a large part of north London through wooden pipes made of bored elm trunks.

The ponds are no longer used to supply drinking water, and are predominantly used for leisure activities including fishing, model boating, bird watching and swimming.

The oldest of the bathing ponds is the Mixed Bathing Pond in the Hampstead chain, which has been used for swimming for over 200 years. The Highgate Men’s Bathing Pond was opened in 1893, and the Kenwood Ladies’ Bathing Pond in 1926. At the time of writing, the water temperature in both was 5°C.

 

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