Hampstead Heath swimmers Christmas morning, 1919

Dam floods and ponds

From long-awaited flood defences in Oxfordshire to Hampstead’s famous swimming ponds, we plunge into the choppy waters surrounding flood risk and what is being done about it.

Located in the natural flood plain of the River Cherwell, the market town of Banbury, Oxfordshire, has a devastating recent history of flooding. In 1998, floods closed the railway station, shut local roads and caused more than £12.5m of damage. A repeat performance nine years later made action more urgent still. Flash forward to today, and Banbury is riding high after new flood defences were unveiled in 2012, protecting 500 properties from flooding and also expediting the redevelopment of a neglected stretch of heritage canalside in the town centre. The award-winning scheme stores the floodwater in a reservoir, controlling its release using passive flow control structures rather than an energy-hungry powered gate and pump system.

“What’s new about this approach is the double baffle design of the flow control structures,” explains Emma Booth, project manager at engineering company Black & Veatch. “Ten years ago we might have put in a series of gates with a hydraulic system, needing electrical and mechanical services and maintenance.” Within the passive flow control devices, specially shaped fixed orifices throttle the outflow as the reservoir fills up, reducing it to a manageable 38m3 per second.

“The whole-life cost of this scheme is lower than many alternative engineering options – and even the capital cost can be lower too,” says Booth. It will last up to 100 years, while gates would likely need replacing in a quarter of that time.

Towns and cities up and down the UK may look to Banbury’s experience as they seek to cope with the risk of flooding. “We need to get used to the idea of rainstorms and floods,” says Dr Hannah Cloke, a hydrologist at the University of Reading. “We know that climate change is likely to make some cases of flooding more probable – for example summer rainstorms are likely to be more intense. Our urban and rural areas should be planned to be sustainable under floods and droughts.”

Flood protection

But engineering is not the only solution to flood risk, as Dr Cloke points out. “Simple land use changes can slow the pace of storm water – from including upstream ponding to more meandering waterways in the lowlands,” she says. “We need better ways of infiltrating rainwater into the ground, both in rural and urban settings.”

In the search for a solution to Banbury’s problem, Black and Veatch examined various options from upstream storage to forestation in the upstream catchment, as well as flood storage. The Environment Agency then narrowed down the most realistic proposals.

Booth says: “In Banbury we were working on a flood plain. The footprint of the flood storage is not hugely different to the area outside of the town that used to flood.” A 2.9km-long embankment now runs parallel to the M40 motorway, with two control structures, one on each branch of the river, to restrict flow. The scheme is designed to hold back about three million cubic metres of water during a one-in-200-year flood.

How is that risk level calculated? Booth explains: “There was a long study by the hydrologist who looked at rain and flood events in records and reports, back as far as the late 1800s, getting a feel for the flooding and producing a statistical analysis.”

An economic analysis was also done, looking at the cost of protecting against a one-in-100-year flood or a more serious one-in-200-year flood. “If you can afford it, more protection is worth it,” says Booth.

Welcome though it was, the Banbury project was almost a washout. In 2007, Defra withdrew funds as it refocused its priorities. Having come so far, the Black & Veatch team sought other sources for the £17m required. “There was no more from Defra, but the Environment Agency’s regional flood defence committees came in. Also there were contributions from Cherwell District Council and others,” Booth recalls.

Heath swimming

The calculation of flood risk, as well as the question of how to combat it, is certainly not always plain sailing. Storm clouds are gathering over London’s Hampstead Heath amid controversial plans to build dams around the historic swimming ponds to combat the risk of potential flooding.

The action is proposed by the City of London Corporation, which has responsibility for Hampstead Heath. “We have taken expert advice and had this checked independently,” says a Corporation spokeperson, “and all agree that in the event of a huge storm, the existing 300-year-old structures might not be able to cope with the overtopping caused by the current inadequate spillway capacity, which could have catastrophic consequences to the communities who live downstream.”

The largest dam, 18ft (5.5m) high, is proposed for the Hampstead Mixed Bathing Pond. Existing dams on other Highgate and Hampstead ponds will be heavily built up, and large overflow spillways constructed.

“The City Corporation believes that the Heath’s existing pond dams need work to ensure their safety. The current carefully designed proposals were drawn up in consultation with user groups and residents associations and will meet the standards that those living downstream have a right to expect, and which are common to other similar dams in England,” the spokeperson adds.

But campaigners say the plans are an over-reaction – and would unnecessarily risk a long tradition of leisurely swimming. The ponds have their origins 300 years ago, when the Heath springs were leased to the Hampstead Water Works Company who built a series of reservoirs to provide water for London. By Victorian times, two of the ponds were favoured informal bathing spots and Heath managers improved the provision by clearing mud, adding fences and building changing rooms. Today, lifeguards maintain the safety of swimmers and are the only known in the country to be contractually obliged to swim several times a week to acclimatise themselves to the chilly water when its temperature drops below 12°C.

The life-enhancing properties of the ponds cannot be overstated, according to Rachel Douglas, chair of one of the ponds’ user groups. “I started swimming regularly when I was going through depression following a bereavement,” she says. “I found it an enormous aid to achieving peace of mind - and have been swimming here for 22 years.”

Douglas and other water-lovers clock up more than 250,000 visits to the swimming ponds each year. In every season, the ladies’ and men’s ponds offer an oasis for contemplation, enjoyed by a diverse range of Londoners – not to mention kingfishers, cormorants, mandarin ducks and dragonflies. The mixed pond is busy with groups of friends and family in the summer. “To be able to swim so near the centre of one of the world’s great cities is as magical a privilege as it is unique,” says Douglas.

Source of the floods

The City’s proposals to change the ponds came out of the blue, according to Jeremy Wright, a civil engineer and dams expert who has represented the Heath and Hampstead Society on the consultative Ponds Project Stakeholders Group. His evidence is that local areas experiencing floods are nothing to do with water from the Heath. “Flooding downstream of the ponds is due to sewers being inadequate to cope with heavy rain falling on the streets, rather than Heath run-off or dam failure,” he says.

However, the City’s advisors say it is legally obliged to virtually eliminate any risk of dam collapse in the future. Wright and other stakeholders feel this is based on misunderstandings. “The proposed dams are designed for a probable maximum flood, which has a one-in-400,000 probability of occurring in any year,” says Wright. “To justify this highly cautious design, they have produced a Quantified Risk Assessment that predicts there could be a likely loss of 1,400 lives if these works are not carried out.”

Some local residents are understandably in favour of protecting their homes from floods, however unlikely. But at a tempestuous forum meeting in June 2014, City representatives and pond campaigners clashed over the dam plans. According to Wright, the City’s risk assessment assumes that the probable maximum flood would cover the entire Heath uniformly, that all the dams fail simultaneously, and that no warning could be given to the downstream community – even though the dams would not, by the City’s own estimation, collapse for six hours. “Couldn’t someone just go and ask people in their basements to come upstairs?” was one plaintive suggestion that provoked laughter.

Today, the City has not changed its position, but has now agreed to have the matter of liability resolved by a judicial review of their final proposals, which the Heath and Hampstead Society are now raising £100,000 to fund.

Protecting worst-hit areas

Whatever the final decision in Hampstead, debates about flood risk and action in the UK aren’t going to go away. However, the UK’s flood science is strong, according to hydrologist Dr Cloke, and the Environment Agency and Defra commission work to put the science into action. “Operational flood-risk management is benefitting from the best scientific practice,” Dr Cloke believes, although counselling that in times of flood crisis “it is very important that the science is not squashed by a rush to please the loudest voices”.

Wright concurs, pinpointing examples of where flood prevention appears to have been carried on an emotional tide. “There are curious anomalies,” he says. “On the Somerset Levels, where only a handful of houses from two villages were affected, because of publicity the government overruled the Environment Agency and pledged £20m to alleviate the problem. Yet last December, 700 properties in Boston were flooded by a river breaking its banks, it had far less publicity, and it appears that only a small amount of funding has been made available for repairs, rather than for a long-term solution.”

Banbury is fortunate to be able to plan for the future with its flood defences safely in place, while other town’s hopes may be dashed as funding recedes.

Huge investments wouldn’t be needed as often if we saw the bigger picture. “Small measures could really make a difference to how the river catchment functions – only then can we make our homes and livelihoods more resilient to flood,” says Dr Cloke.

Back at the ponds, as people while away a hot afternoon in the lovely green lagoons, their minds are probably elsewhere. But they’d be bound to agree that small and well-judged measures would help preserve this special place for generations to come. 

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