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Herne Hill flood alleviation scheme

Herne Hill flood alleviation

Herne Hill flood alleviation scheme has just won the BCI Award for sustainability for being a hidden project whose innovation lies in its simplicity.

Herne Hill has a long history of flooding and is particularly at risk along the route of one of London’s lost rivers, the River Effra, which was culverted in the 1880s.

In April 2004, 60mm of rainfall fell over two hours, flooding over 200 properties in the densely populated area of south London. The storm caused an estimated £1M of damage to council infrastructure alone.

Temporary flood mitigation measures were put in place, and more permanent solutions such as enlarging pipes and sewer systems and building large concrete holding tanks for excess water were mooted. But all were too expensive or too disruptive to build and ruled out. In 2013 a new scheme was proposed.

Herne Hill flood alleviation

Retention tank: Underground tanks will delay the release of water intro the sewage system

The new scheme was simple. It involved building earth bunds to intercept and temporarily hold up to 51,000m3 of water in three l0cal parks: Belair Park, Dulwich Sports Trust and Dulwich Park, during severe storms.

The stored water is then released into the sewer network after the storm has peaked.

“The concept is very simple,” says Southwark Council flood risk manager John Kissi. “We hold water on large open fields and release that water when the storm is over. We’re sitting on clay, so there’s very little penetration into the ground. The idea of the bunds is to hold the water on the surface. It then goes through the drains and into the sewer,” he adds.

Three underground geocellular tanks connected to the sewer network provide water storage for normal conditions. Filter drains at the toe of the bunds then direct water to the storage tanks helping to improve general drainage within the parks.

Herne Hill flood alleviation

Park: Used for storing stormwater

The bunds are built seamlessly into the children’s play areas. Gentle slopes, which are easy to maintain, provide the children with new features to explore. Tunnels have been built through them to add to the interest.

He says that although parts of the play areas are taken out of action during the worst rainfall, the system is designed to dissipate the water within 48 hours and that in normal rainfall conditions, drainage in the parks has been significantly improved.

“The water stays in the fields for a maximum of 48 hours,” says Kissi. “But in the day to day situation, the area is improved because of the French drains and the underground tank we’ve installed.”

Herne Hill flood alleviation

Excavation: For geocellular tanks

A large part of the success of the project was due to the intensive consultation between the council and surrounding residents. Meetings were held to ensure that the scheme received the full backing of the local community and these paid off, as when there were no objections when planning permission for the project was sought.

Around 2,700m2 of new wetland and 4,500m2 of wildflower meadows have been planted around the existing lakes in the parks. The lakes also have a new water source as run-off has been channelled in to them.

“Some of the water that’s collected feeds an existing lake in both parks,” says Kissi. “Sometimes when it’s quite dry they have to buy in water, so this is providing a new water source.”

The system has also been designed to minimise future maintenance.

“Where we have the underground storage, we have to de-silt the storage tank once a year and the pipe network, the velocities and the gradients of the pipes have been designed so that we don’t have to do anything,” says Kissi.

The project has cost the council £4.2M as a whole-life cost over its 100-year design life. It is estimated that the project will prevent £12M of flood related damage and disruption.

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