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London's Dulwich Park sees MGJV battle severe flood risk

 

The Grade II-listed Dulwich Park in south London is the focal point of an unlikely flood-alleviation project, which has seen public and private sectors working together with local community groups.

Project: Dulwich & Herne Hill Flood Alleviation Scheme
Client: Thames Water (in partnership with Southwark Council and Environment Agency)
Contract value: £4m
Region: London
Main contractor: MGJV
Plant hire: Hewden
Drainage subcontractor: TC James
Concrete subcontractor: D&J
Start date: March 2014
Completion date: December 2014

“Don’t touch our park!” was the typical reaction from local community groups in 2010 to the prospect of excavators crawling over the Grade II-listed Dulwich Park in south London, as part of a major flood relief plan, according to Southwark Council’s flood risk manager John Kissy.

But their view would change.

The various groups, including the Friends of Dulwich Park, agreed to work with the partnership driving the scheme - Southwark, the Environment Agency, Thames Water, and its framework contractor for south London MGJV (a Morrison-Galliford Try joint venture) - to find an outcome that satisfied all stakeholders.

Most importantly, it needed to alleviate the flooding problem that has affected this corner of south London for a century (see box).

According to Thames Water’s senior contracts manager David Donnelly, the £4m Dulwich & Herne Hill flood alleviation project, which began in March, will be “more sustainable and better aesthetically as a result of the feedback from the local community.”

“It is the kind of scheme the government is keen to foster for the ongoing management of flood risk, and is likely to feature in the AMP6 period”

Simon Tanner, Environment Agency

Thames Water is the client for MGJV, but has worked closely with Southwark, backed by the Environment Agency.

“In terms of the multi-agency co-operation, it is unique,” says the agency’s flood risk management advisor Simon Tanner.

“It is the kind of scheme the government is keen to foster for the ongoing management of flood risk, and is likely to feature in the AMP6 period.”

MGJV site manager and scheme designer Danyel Pullen says the original plan was a “heavy civils project”, with reinforced concrete storage tanks to deal with the flood waters and swales to mop up run-off in high risk areas.

Community agrees plan

Local community groups opposed this original plan. “They were worried about the appearance of the park, and didn’t want the disruption,” says Mr Kissy. “So the scheme has changed in response to their concerns.”

The agreed solution involves a variety of flood relief measures in three parks - Dulwich Park, Belair Park, and Dulwich Sports Ground - which have been designed to integrate or enhance,the parks’ appearance.

“This solution is cost effective, compared to reinforced concrete, and quicker to build”

Danyel Pullen, MGJV

There will also be new drainage channels installed in nearby Turney Road, which has been affected badly by flooding.

In Dulwich Park, site workers are constructing two geocellular storage tanks, each with dimensions of 34 m wide, by 35 m long, and 1 m deep (1,200 cu m capacity). They are formed of blocks comprising small, vertical hexagonal plastic tubes, packed together like honeycomb.

“This solution is cost effective, compared with reinforced concrete, and quicker to build,” says Mr Pullen. The tanks will eventually be covered and grassed to give a better aesthetic appearance.

Bunds will be constructed in high-risk areas as part of an ingenious earthworks programme - some 10,000 cu m will be moved in total - which will move material around the three parks.

“Dulwich is built mainly on London clay, and the material’s impermeable properties make it ideal for reuse in the bunds,” explains Mr Pullen.

The bunds measure 1 m to 1.7 m high. MGJV will install drainage pipes totalling 700 m in length, with 650 m of carrier drains, to channel surface water from the toe of the bunds into the tanks.

Screw pile bund support

A brick, flood-defence wall, with reinforced concrete core, will back the bund surrounding the children’s playground. Twnty-two screw piles are supporting this, each 6 m long with 600 mm bearing plates.

“We installed them using a mini excavator with piling attachment, as the hedge around the playground restricted access for a rig,” says Mr Pullen.

Following the earthworks, many areas of the park will be landscaped sensitively. The bunds around the playground have been designed to create a series of undulating hillocks, which will disguise the defence wall and enhance the play area. There are even slides and a ‘play tunnel’ for the area that Mr Pullen calls “Teletubbie land”.

In Belair Park, a wetland area will be created, as well as a detention basin in the north-east corner. “The community groups for Belair Park were interested chiefly in biodiversity, so we have created new nature habitats here,” says Mr Pullen.

Bunds in Belair Park will direct surface water to the lake, which will be reduced in depth by 150 mm to provide extra capacity for floodwater.

In Dulwich Sports Ground, another geocellular tank is being fitted, this one 45 m square and 1 m deep, with 2,025 cu m capacity. This will be surrounded by a flood-defence wall, which will help protect properties in Turney Road behind - whose drainage channels will run into this tank.

Logistics challenge

The big construction challenge for MGJV has been managing what is a huge and logistically challenging muckshift.

“All earthworks jobs are about planning the movement of material - which is easy if it’s one site - but we are spread over three different sites or parks,” says Mr Pullen.

The bunds require 7,500 cu m of clay, which he plans to use from within the three sites - the job will be cut-and-fill neutral. “But in the sports ground,” he explains, “what we are excavating is a sandy, gravel material, so we will reuse that as backfill around the tanks - another 2,500 cu m in total.”

This is another advantage of using geocellular tanks instead of reinforced concrete, says Mr Pullen: “It can be built in stages, as the backfill material becomes available, and the embankment construction around the tank progresses.”

MGJV has opted to manage the earthworks directly to assume close control over the project, while Hewden is providing the plant. “There is the potential for a lot of claims in a public-facing job like this, and we need certainty,” says Mr Pullen.

Normally, such a large volume of material would be moved around with a fleet of tipper lorries - but that would risk local unrest.

“Our plan is to keep the lorry movements to a minimum and ensure they are always loaded, whichever way they are travelling,” he says.

“So if a lorry is bringing sandy gravel into Dulwich Park from the sports ground, we will load it up with clay for the bunds on the way back.

“Additionally, we must have a gateman and banksman for each lorry movement and all deliveries. They arrive at the park gate and are escorted into the site.”

Disruption minimised

Generally, the works have progressed without the disruption that locals had feared. The annual Dulwich fair was cancelled, but otherwise, it has been almost business as usual for a park that attracts 10,000 visitors a day on summer weekends.

“We invited the children to pour water onto the model so that they could see how it collected in certain areas, and therefore why the flood alleviation scheme was so important”

Danyel Pullen, MGJV

Community relations have also been improved by the transparency of the contractor. “We created a scale model of the site to take into schools and explain the work we were doing,” says Mr Pullen.

“It showed the topography, and we invited the children to pour water onto the model so they could see how it collected in certain areas, to show why the flood alleviation scheme was so important.”

MGJV aims to complete the earthworks in Dulwich Park by the end of August and in the other parks by October. “Any later and the weather starts to get in the way - we’ve been very lucky so far,” he says.

After that, all that remains is the planting phase, including the grassing of the storage tanks. “It will complete in December and is on programme,” says Mr Pullen.

The scheme will provide a flood risk protection level of 1 in 75 years.

High hills and hidden rivers

Flooding in this part of south London dates back a century.

The cause is twofold: a lack of capacity in the former River Effra, one of London’s lost rivers that was culverted as a sewer in the 19th century; and the hilly topography of the area which leads to high volumes of run-off.

Impermeable London clay is prevalent in the area, which quickly gets saturated, exacerbating the problem.

The first serious flood in Dulwich was recorded in 1915. More recently, there have been major events in the 1984, 2004, and 2007. Dulwich Park, Belair Park, Dulwich Sports Ground, the Herne Hill Velodrome, and the centre of Herne Hill were often badly affected.

In 2013, a flood from a burst water main in Herne Hill, which overwhelmed the drainage network, caused £4m of damage. “That gives an idea of the cost of the problem,” says Southwark councillor with environment responsibility Darren Merrills.

After the 2007 floods, a government investigation identified the lack of responsibility for dealing with surface water flooding. “In the old days, this would have fallen to the drainage boards,” explains Southwark’s flood risk manager John Kissy.

This coincided with the EU Floods Directive of 2007, and the UK outcome was the Flood Management Act of 2010, which obliged all councils to draw up risk management plans.

“It gave us the power to involve other stakeholders where there is a high risk of flooding, for example, Thames Water,” says Mr Kissy. “We also had to draw flood hazard maps, and identify hot spots. The Dulwich area is the highest risk in Southwark.”

One spin-off of the new legislation was the Drain London Forum establishment, formed by flood-affected councils in the capital. It was here that the key partners in the Dulwich & Herne Hill scheme met, and following initial exploratory discussions, conceived the flood-alleviation project that is now taking shape.

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