Environment Agency unveils plans for £300M Thames flood defence scheme

Plans for £300M of flood diversion channels, weir improvements and river widening to safeguard homes and businesses in west London have been unveiled by the Environment Agency.

The scheme, the biggest of its kind since the Thames Barrier was completed in 1984, includes three new flood diversion channels between Datchet and Shepperton to the west of London, increasing the capacity of the Sunbury, Molesey and Teddington weirs and widening Desborough Cut, near Walton-on-Thames.

The Agency hopes the measures, coupled with efforts to raise public awareness, improve the mapping of the flood plain and encourage the storage of water upstream, will protect more than 15,000 properties in the Lower Thames region.

Environment Agency area manager Innes Jones said: “The reality is that flooding can’t ever be entirely prevented, but this strategy does set out a wide range of measures for helping to reduce the risk and impact of flooding.

“The proposed channels will play a major role in bring more peace of mind to thousands of people who live with the real risk of flooding every day.”

In January 2003, heavy rain brought flooding to many areas of the River Thames downstream of Datchet.

In response, the Agency has produced a draft Flood Risk Management (FRM) strategy for the Lower Thames from Datchet to Teddington. It sets out its preferred option for managing the risk of flooding in the area in the future. The plans were first outlined in 2004, to a mixed reaction.

The Agency has found that widespread alleviation of flooding in the Datchet to Walton Bridge area can only be achieved through large-scale flood diversion channel works.

It also proposes to improve weirs at Sunbury, Molesey and Teddington and widen Desborough Cut to accept higher flood flows created by the increased conveyance capacity upstream. It is hoped that these improvements, and construction of localised flood walls, will even reduce the flood risk to many properties between Walton Bridge and Teddington.

The diversion channels are similar to the Jubilee River scheme, which protects Maidenhead, Windsor and Eton and opened in 2002. Residents flooded in 2003 maintain that poor management of increased flows from the Jubliee River were to blame. A subsequent investigation by Atkins revealed that the £110M Jubilee River was under-designed and could convey only two-thirds of its design capacity.

The plans in detail

Reach 3: Datchet to Walton Bridge

In the Agency’s strategy there would be three separate but consecutive new flood diversion channels between Datchet and Shepperton. These would link up some of the lakes in the area and reconnect with the River Thames. The idea is to bypass the existing weir structures and improve the passage of water downstream.

The Agency would aim to make the flood diversion channels look, as much as possible, like a natural watercourse with gently sloping banks. New vegetation would help them blend into the existing landscape. Each flood diversion channel would be a similar width to the River Thames, between 50m and 60m wide, within a broader corridor of up to 100m. Some might have a footpath or cyclepath running alongside them, and contain attractive features such as reedbeds.

Water in the flood diversion channels would be about 3.5m deep for most of the year, rising to 4m in times of flood. The Agency would not plan to have a continuous flow along them. Instead, the water would be still, like the water in many of the gravel pits in the area.

A number of other bridges, culverts and weirs would also be required to cross existing roads and railway lines, and to keep the water at the correct level. The flood diversion channels would skirt around some areas of historic landfill sites. It is likely that the Agency would need to construct heavily engineered channels in these areas, and possibly to line them.

Reach 4: Walton Bridge to Teddington

The Agency has studied the effects that building flood diversion channels would have on flows in the rest of the river, downstream from where the third and final flood diversion channel returns to the Thames. This shows that works are needed to the river downstream of Walton Bridge to maintain the flows at their current level to prevent any increase in flooding.

However, the area between Shepperton and Teddington is too built up for engineering works such as diversion channels to take place. Instead, weirs at Sunbury, Molesey and Teddington will have their capacity to convey water during a flood increased, Desborough Cut will be widened by 3m to 4m on the southern bank, and local defences would protect areas such as those around Teddington Studios and on the river frontage at Kingston.

However, such defences have been ruled out in visually sensitive locations such as around Hampton Court Palace.

The benefits

The Agency believes that building these flood diversion channels and associated works would bring major long-term benefits to the Lower Thames area. It plans to substantially reduce flood risk to 5,100 residential properties (housing about 12,800 people) experiencing the most frequent flooding.

The scheme should enable some 8,200 residential properties (housing about 20,500 people) to qualify for flood insurance. This would involve moving them out of the insurance industry’s significant flood risk band, for properties with more than 1.33% chance of flooding in any one year.

It expects that 7,200 residential properties (about 18,000 people) would be taken out of the flood risk envelope for the 1 in 100 year, or 1% annual chance of occurrence. Flood risk would also be reduced for about 450 commercial properties, besides some public buildings and critical infrastructure within the Lower Thames area.

The plans have been put out to public consultation until 4 December.