Rivers in the UK are on a very much smaller scale than those draining continental land masses, such as the Rhine, Mississippi and Danube. Nevertheless serious flood events still occur and these may cause a disproportionate amount of damage and disruption because of the high population density in many catchments. The River Thames flood of March 1947 is still remembered for the inundation of 2,000 homes in Maidenhead, Windsor and Eton and a lesser flood in 1990 affected around 500 homes. 

The 1947 River Thames Flood

If the 1947 flood was repeated today it would affect 4,800 domestic properties and 700 commercial units, bringing misery and loss to at least 12,500 people (Martin, 1999). In addition there would be a major impact on the important transport corridor into London, with the M4 motorway at risk of closure as well as interruptions to rail, electricity, telephone and other services. Clearly the disorder this would cause would have an adverse affect on the strong local economy, which would, in turn, affect the performance of the national economy. The estimated cost of such a flood re-occurring today is over £40 million in addition to the sentimental value of lost possessions and the trauma caused by the stress and strain to all those involved.

The Chief Engineer stated in his report after the 1947 flood that it would take two, or even three, rivers to contain the volume of flood water rushing down the catchment after abnormal rain, especially when the land is supersaturated ( as in the autumn of 2000) or, as in 1947, rendered impervious by deep frost and aggravated by substantial snow drifts and a sudden thaw. The Deputy Ranger of Windsor Great Park, Sir Eric Savill, said in 1947 that the water ran off the Park as off a slate roof. This resulted in local flooding from streams such as the Bourne Brook, when the stream burst its bank and flowed through houses in its path two or three days before the main flood reached Windsor. No warning was given. Some residents remembered the disastrous flood of 1894 which produced flood levels 100 mm than those of 1947, with one old lady reported as saying she had been flooded twice before and felt a bit old for a third inundation.

The River Thames was badly affected by the 1947 flood. Along the river bank upstream, trees were in danger of being uprooted and swept away, thus putting Windsor Bridge, with its vital services, at risk, so the trees were blown up by a party of Scots Guards, who were also briefed to blow up any large boats that broke away from their moorings and swept downstream. The roar and vibration from the mass of water passing under Windsor Bridge was alarming. Afterwards it was found that large round boulders had rolled downstream, swept along by the raging torrent, and a mass of sand and gravel had collected at Romney Weir (Cullingham)

Steam trains evacuated the pupils from Eton College and soldiers built clay dams in the Princess Avenue area to protect the Eton Waterworks wells and a vital electric power transformer. Water levels rose to within 12 mm of the danger level but the electricity and water supply to Windosr and Eton was maintained - just. However water mains and service pipes tended to break under the softened roads and footpaths, sometimes because heavy vehicles ran along the higher footpaths, out of the more deeply flooded roads. Food deliveries became difficult, although one baker specialised in throwing loaves through open bedroom windows from his boat, his aim eventually becoming quite good (Cullingham).

Sustainable Solutions

It is vital for society to seek protection from these natural events, and historically it has been the responsibility of engineers to devise solutions to these problems by designing and constructing flood defences. It is now seen as vital that this engineering work, whilst essential to ensure minimum disruption from flooding, is carried out in a manner which is sensitive to the environment and even achieves enhancement to natural habitats whilst providing the levels of protection demanded by the public. Modern engineering schemes aim at satisfying the objectives of flood relief whilst also improving the landscape and protecting flora and fauna. A good example of this integrated approach is the Maidenhead, Windsor and Eton Flood Alleviation Scheme which was constructed between 1996 and 2001, over 50 years after the infamous 1947 floods.

After 1947 there was a lot of public pressure on the relevant authorities to stop a similar flood happening, and it was recommended to the Thames Conservancy Board that planned zones should be established along the length of the river in which new development should be prohibited and existing development be demolished when it came for sale. Some minor engineering works were carried out following another flood in Maidenhead in 1954 but it was not until a survey in the 1970's by the then Thames Water Authority, were strategies developed for possible solutions to the wider flooding problems in the area. There are flooding problems along the whole length of the river Thames, but it was considered impossible to solve the problems along the whole length in one go. The Maidenhead area was identified as needing priority action because of its history of repeated flooding and where money spent would produce the greatest return in damage avoided, and in 1989 this was extended to include Windsor and Eton. Today this cost-benefit approach would likely to be supplemented with a formal Environmental Assessment of the impacts of the alternative schemes being conducted.

Alternatives considered

The unique feature of the Flood Relief Scheme is that the whole concept was environmentally led. Four methods of flood alleviation were considered. The first involved deepening and widening the existing river channel but this approach was rejected because of the significant environmental effects this would cause, including the removal of several islands. Floodbanks up to 2 m high were considered but it was realised landscaping would be a major problem and raised water levels would lead to increased groundwater levels leading to further flooding. It was estimated the additional volume of storage needed to protect the area from a repeat of the 1947 event, which produced a flood peak in the river of 500 m³/s, would be 720,000,000 m³ which with land availability and value in the catchment at a premium would simple be impractical, especially as it would require an area the size of Oxfordshire to accommodate all the water from a large flood. It was decided therefore that the only realistic solution to the flooding problems would be the construction of flood relief channels. It was decided to study improvements on both the East and West Banks of the river and in total 29 alternative scheme elements wee considered in 492 different combinations before the selected option was identified. (Fryer G., 1999).

Chosen Solution

The channel solution was selected on the basis of providing a minimum protection for the area of 1 in 65 years flood event standard, with the main channel running along the east side of the River Thames, leaving the main river in North Maidenhead and rejoining 11.8 kilometres away just downstream of Windsor. The channel and the river together will be able to accommodate flow of about 515 m³/s with the main Flood Relief Channel carrying 215 m³/s. Whilst it has a trapezoidal cross section with an unlined bottom width of about 30 m, side slopes of 1 in 15 and an average depth of 5m the channel has been carefully designed to look and function as a natural living river. It has been designed to contain water all the year round and is sensitively landscaped to enhance the environment and create new habitats in what was previously a relatively ecologically uninteresting area. This includes restoring some habitats which have been lost from the River Thames itself over the years, as a result of river-side developments.

The area through which the Flood Relief Channel was built was very sensitive in planning terms as the land outside the towns was designated as green belt, which carries a presumption against development except in very special circumstances. The main channel also was built extensively in gravel bearing land which was particularly contentious as commercial gravel extraction is very unpopular with local people and there was a fear that the scheme was simply a backdoor means of allowing gravel digging ( Clear Hill, 1994).

Care was taken to assess the impact on the rest of the catchment both upstream and downstream. A major concern was that the removal of the floodplain in the urban areas of Maidenhead could have significant effects on the areas downstream of Eton. It was found however that the effects on peak flow of removing this floodplain were negligible downstream because the River Thames floods cover a long period. In 1947 the floodplain took 5 days to fill and the average flow into storage for these 11 days was 11 m³/s, with the flow intro storage at the peak being only 2-3 m³/s (or 0.5% of the peak flow). Using river modelling techniques it was found that peak flows downstream could actually be reduced as a result of the Flood Relief Scheme, only modest siltation would be expected in the relief channel, and the likely effects on the River Thames itself were quantified (Fryer, 1999).

Following a Public Inquiry, approval for the scheme was granted in 1995 and the Inquiry Assessor concluded that " The Scheme …. is a good technical solution…..and would be a uniquely attractive addition to the landscape between Maidenhead and Windsor". This was achieved by recognising the need for a multidisciplinary approach to developing a solution which involved consultations between engineers, conservation, fisheries and recreation experts and discussions with local councils, residents and special interest and local pressure groups.

Habitat Enhancement

The Scheme has provided huge opportunities for genuine habitat enhancement, making it one of the most significant wetland creation schemes ever seen in the U. K. (Driver, 1999) . Shady pools and nest boxes have been provided for Mandarin ducks, and nesting cliffs introduced for sand martins and kingfishers. It is hoped the reed beds will attract bitterns, reed warblers and water rails whilst muddy scrapes have been designed for passage waders and dabbling ducks. Shingle islands in the channel and beaches will attract common terns and little ringed plovers whilst nest boxes will provide breeding sites for barn owls, kestrels and tree sparrows. Bats are being encouraged with roosting, breeding and hibernation boxes and even stick pile holts for otters and steeper banks with marginal vegetation bases for water voles are part of the overall scheme. Fish will be provided with sheltered shallow margins and ponds will be created for breeding amphibians and dragonflies. Alongside the channel care has been taken to seed wildflower rich grasslands for butterflies and other insects, and log piles will help introduce fungi, invertebrates and amphibians. The net habitat creation will include 31 ha of woodland, 23.5 ha of species rich grassland, 37 ha of open water, 24 ha of marginal wetland and 6.7 km of hedges.

Another highly innovative aspect of the scheme has been to ensure that local provenance plant material was used throughout. The majority of the 150,000 trees and shrubs were grown from native seed specially collected locally. On completion the scheme will have created a unique wildlife corridor that will attract species to the area that have been lost through development on the River Thames.

The engineering objectives of protecting life and property from flooding have been achieved whilst creating the tranquility of a new river, which is visually attractive and an asset for all to enjoy. By operating in day to day contact and ensuring a consistency of approach and regular feedback of information between all the many disciplines and discussion forum, the engineering and environmental co-ordinators were able to achieve a highly innovative scheme which includes landscape improvements, wildlife habitat creation and recreation opportunity (Clear Hill, 1994). This highlights a key aspect of Integrated Catchment Management which requires engineering and environmental disciplines to work together in solving problems and using the opportunity to achieve environmental enhancement and sustainable outcomes.