Lowdown on the River Thames strategy14 April 2010
A new flood risk management strategy for the River Thames will help reduce the risk of flooding in one of the most populated areas of undefended floodplain in the UK
The River Thames has been the lifeblood of England for centuries, with tens of thousands of people living and working alongside the country’s longest waterway. Since the 1500s, London’s trade with the continent and the rest of the world steadily increased along the banks of the Thames, shipbuilding flourished, new wharves to handle their cargos were built, and London prospered, which had a positive knock-on effect up and down the river from source to mouth.
The River Thames was modified over time to keep up with the ever-changing face of society, and locks and weirs were installed as industry sought ways of transporting goods using the river. Communities living in and around the river grew but, as time went by, more and more goods were moved by road rather than river, making the river a less attractive transport option for industry. But the legacy of living in and around the river remains, and today there are thousands of properties lying within the Thames floodplain.
The Environment Agency has the responsibility of managing the flow of water along the Thames. Every day of the year, Environment Agency lock keepers manage the flow of water to control flooding, while also providing a safe and viable navigation along the non-tidal river for boaters. The volume and speed of water flowing down the river is managed by adjusting the gates at each of the 44 weirs.
However, control of the river can only be managed to a degree, and when heavy rainfall threatens flooding to properties, the weirs are fully drawn to allow the Thames to resort to operating as a natural river.
The onset of climate change simply increases the potential threat to homes and businesses in the Lower Thames area. The threat of flooding comes from the sea and the river catchment itself, and several major engineering solutions have been implemented with a view to reducing the risk of flooding from the Thames in the past 40 years.
Undoubtedly the most iconic is the Thames Barrier, which was officially opened by HRH the Queen in 1984. The barrier is closed several times a year to prevent flooding to London’s low-lying areas upstream from tidal surges. In the late 1990s, the 11km long Jubilee River was built on the non-tidal Thames, acting as a flood relief channel for the Thames around Maidenhead, Windsor and Eton, protecting more than 1000 homes from river flooding.
Despite the many thousands of homes that are protected, many properties are still at risk from the Thames with more than 15,000 homes and businesses now standing within the 1 in 100 year floodplain (equating to a 1% annual chance of occurrence) between Datchet, in Berkshire and Teddington, in Middlesex.
The Lower Thames area has already experienced two significant floods this century: in 2000 and 2003, and narrowly avoided flooding again in 2007. The worst floods in living memory were in February 1947, when rain falling on melting snow caused more than 100,000 properties to flood nationally.
The Lower Thames Flood Risk Management Strategy has been developed to reduce the risk of flooding to these properties between Datchet and Teddington, one of the most populated areas of undefended floodplain in the UK.
A combined strategy
The Environment Agency has proposed a combined strategy of both hard and soft engineering measures to manage flood risk along the Lower Thames corridor. This includes flood diversion channels between Datchet and Walton Bridge, widening of Desborough Cut and improvements to Sunbury, Molesey and Teddington weirs. These are backed up by a comprehensive package of floodplain management measures, including improved flood mapping, warning and evacuation procedures, and longer term policies such as discouraging future development on the floodplain.
“This is the best solution for managing the current flood risk in the area, and will help to mitigate the future impacts of climate change,” says Graham Piper, Lower Thames Strategy project leader.
Characteristics of the study area
This is the area around the Lower River Thames that broadly follows the course of the river from Datchet, in the northwest, to Teddington, in the east. The Environment Agency divided this area into two distinct parts, Reach 3 and Reach 4.
The study area of Reach 3 runs from Datchet to Walton Bridge, and it has a wide, open floodplain. Reach 4 runs from Walton Bridge to Teddington, and has a much narrower floodplain.
Reach 1 (Hurley to Cookham) and Reach 2 (Cookham to Windsor) do not form part of the strategy. Properties immediately upstream of the study area, in Reach 2, are protected by the Maidenhead, Windsor and Eton flood alleviation scheme. Major dredging works stopped over 10 years ago on this stretch of the River Thames because the impact it has on reducing flood risk is negligible. Since then the water quality of the River Thames has improved and is likely to continue to improve for both people and the environment.
The Environment Agency looked at many different options and locations for managing flood risk.
“We considered engineered solutions as well as measures such as making homes more resilient to flooding and improving flood warnings. We also looked at how we could influence planning policies in the area,” Piper says.
It was found that widespread alleviation of flooding in the Reach 3 area could only be achieved through large-scale flood diversion channel works.
“Any channels to divert the river from its natural course, to reduce the risk of flooding, would be a major undertaking, so we had to be realistic about where these could be built, assessing all of our options against a range of economic, environmental and social factors. Reach 4 is more built up, and it is hard to find enough space to fit the channels in,” Piper adds.
Discussions with various organisations showed there could be an advantage in considering the strategy as a package of channel works and non-engineered measures. These would be undertaken in partnership with local authorities to influence planning policies and to assist in effective communication with the public in advance of any flooding occurring.
However, it is possible that the government will require the Environment Agency to either phase the various parts of the strategy over a number of years, or only implement parts of the strategy.
The Environment Agency has also studied the effects that building flood diversion channels would have on flows in the rest of the river, downstream from where flood diversion Channel 3 returns to the Thames. This shows that other river and weir improvements would need to be made through Reach 4 to maintain flows at their current level to prevent any increase in flooding. These proposals would also reduce flood risk for most people in Reach 4, and include:
• Modifying weirs - This would involve increasing the capacity of Sunbury, Molesey and Teddington weirs to help convey water during a flood.
• Widening the Desborough Cut – This would involve widening the river by 3 to 4m and increasing its conveyance capacity. The Environment Agency would maintain public access along the river once the works are completed.
• Other local defences – these would protect localised areas such as those around Teddington Studios and on the river frontage at Kingston. However, the Environment Agency ruled out this approach in visually sensitive locations such as around Hampton Court Palace.
The floodplain management element of the Lower Thames strategy would follow government recommendations for managing flood risk according to national and local priorities. It would comprise measures such as land control, improved public awareness and community-based options.
The main areas of work would be:
• Increasing public awareness of flooding, encouraging the uptake of Floodline Warnings Direct, a free service that provides flood warnings via telephone, mobile, email and SMS text messages. The Environment Agency would also continue to work in partnership with local authorities and other public bodies to improve flood mapping, develop emergency plans, local flood action plans and apply the best means available to make individual properties resistant to floods.
• Working through policy and planning strategies for local
councils and encourage increased flood storage in upstream tributaries.
• Community based measures, which may include providing financial support for individual and community based flood prevention initiatives. These would include the use of demountable and temporary defences, and flood resistance schemes for individual and groups of properties.
• Floodplain management tools, which consist of interactive flood mapping tools, working with local planning authorities, new procedures to guide and promote sustainable development, and effective community evacuation plans.
• Working with local authorities to safeguard flood flow routes, which generally coincide with potential diversion channel routes. Other approaches would include continuing to reduce development in areas that are prone to flooding.
The next step
More than 1400 people have attended the public exhibitions held across the Lower Thames area since October 2009 to find out what the flood plans mean to them and to make comments that will help finalise the strategy.
The Environment Agency has been collating all responses before submitting its final business case for its strategy for Board approval, prior to seeking approval from the Department of Environment, Farming and Rural Affairs and The Treasury in 2010.
“We have had a fantastic response so far, and it is great to see a large number of local people taking an interest in their community and giving us such valuable feedback. All responses from the community are taken on board and will help to finalise the strategy,” Piper says.
“The reality, unfortunately, is that we cannot stop flooding from happening all together, but this strategy does set out a wide range of measures for helping to reduce the risk and impact of flooding. These include increasing flood resistance, making people more conscious of their risks and raising awareness about building in the floodplain. This, in conjunction with the proposed channels, can all play a major role in bringing a greater peace of mind to thousands of people who live with the risk of flooding.”