Taking pressure off the Thames4 July 2002
Were small hydro developments overlooked when designing a new flood relief channel off the river Thames in the UK?
THE MAIDENHEAD, Windsor and Eton flood alleviation scheme is a new 11.6km manmade channel (now known as Jubilee river), plus 5km of flood banks, designed to protect people, property, roads and vital services from flooding in southern England.
On average, flooding from the Thames occurs in Maidenhead every five to seven years. In March 1947 Maidenhead flooded so severely that the Thames was over 1km wide in places. Large areas of Eton were underwater and almost a third of Windsor's population was evacuated. Floodwater entered over 2000 homes. Statistics show that a flood on this scale has a 1 in 56 chance of happening in any one year. A second flood, in 1990, was minor in comparison, but still severely affected an area of the Thames at Maidenhead near Boulter's Lock. The flood persisted for 14 days.
There are flooding problems along the whole length of the river Thames, but when the Water Authorities undertook flooding and drainage surveys in 1973, in order to develop strategies for possible solutions, they identified the Maidenhead area as a priority, because the money spent would produce the greatest return in damage avoided.
Consultants appointed by the Thames Water Authority investigated the problem and undertook detailed studies of the complex engineering and environmental considerations of various flood alleviation schemes. Local authorities and other interested organisations were involved in the process to ensure that the most appropriate method was taken. All methods of flood relief were considered including storage, dredging and embankments before it was decided to construct a flood relief channel. The channel was chosen because it was the only solution that would provide minimum protection for the area of a one in 65 years flood event standard.
After a public inquiry, the scheme received approval from the Secretary of State in 1995. The Environment Agency succeeded the National River Authority, and inherited the construction of the scheme.
The channel, now called Jubilee river, runs along the east side of the river Thames, leaving it at Boulter's Weir in North Maidenhead and rejoining at Black Potts Viaduct, just downstream of Windsor. It has a trapezoidal cross-section that is unlined with a bottom width of around 30m.
The river Thames in this area can accommodate a flow of 293m3/sec of water. Together, with the Jubilee river and existing channels in Maidenhead, 515m3/sec can be carried before flooding will occur. This is greater than the flood flow of 1947.
The Jubilee river will work by taking excess water along its route. When high flows are experienced on the Thames, the channel intake weir will be opened to allow more water to enter the channel, and will have running water all year round. The water will be supplied naturally with a small flow of about 10m3/sec taken from the Thames. Two control structures and three weirs regulate water levels within the channel and these have been set to ensure that groundwater levels in the area are not adversely affected.
Ossie Goring of Water Power Engineering points out that the flow rate on the Jubilee river is at least 10m3/sec, passing over newly-constructed weirs which dissipate the energy of water falling 7m. Goring calculates that if the water was passed through a turbine it could produce 560kW in summer and 3.36MW at times of higher flow.
The EA makes much of the fact that it purchases increasing amounts of its electricity from renewable energy sources. So why was small hydro not considered when the Maidenhead project was designed?' asks Goring.
'The main aim of this scheme was to reduce the risk of flooding to the residents of Maidenhead,' Graham Cowell, flood defence and water resources manager for the Thames region of the EA, said. 'Our remit for this scheme was primarily to reduce flooding.'
Although Roger Powling, project manager for the Jubilee river, was not involved at the design stage, he believes that the scheme was designed when 'water power was in its infancy in the UK' - planning permission was sought for the scheme in January 1989.
'There are also practical restrictions with regards to any small hydro development,' Powling explains. 'At the largest drop there are space restrictions on the width of the river channel between a paper mill and across the road to a bore hole. It would have been too difficult to incorporate anything here.'
Furthermore, Cowell admits that it is not the EA's remit to promote small hydro development in the UK. 'Rightly or wrongly it was not considered in this scheme,' he said. 'But I will go a long way to say that the EA strongly supports the government's renewable energy targets. Small scale developments such as small hydro can have an accumulative contribution to reducing greenhouse gas emissions, and the EA is keen to support and bring the best designs forward.'
Powling is sure that if small hydro developments on the Jubilee river had been a worthwhile investment they would have happened. 'We don't rule things out on cost per se,' he said, 'but any such development would have had to have shown economic returns and have been practical to carry out. There were proposals to develop hydro on the river Thames itself several years ago,' he added, 'but these never got off the drawing board.'