Pontoon poised for action
Ashley Sansom reports on the Bewl to Darwell water transfer scheme
Visitors to Bewl Water in Lamberhurst, Kent, are attracted by a host of attractions during the summer months. The reservoir, owned by Southern Water, is a popular location for water sports, walking and angling. Despite so many activities, there is another curiosity currently drawing much of the attention. A mobile pontoon, part of the Bewl to Darwell water transfer scheme, is making its way slowly across the reservoir.
Southern Water and South East Water are installing a 29km, cross-country pipeline to safeguard water supplies in the Hastings and Bexhill areas. Insufficient local resources mean they are vulnerable during times of drought. Water will be moved, via an existing pipeline, from the Medway area to Yalding and on to Bewl. It will then flow to Darwell Reservoir before treatment at Hazard's Green WTW. An existing transfer pipe from Bewl to Darwell has a capacity of 10Ml/d and will be supplemented by a further 25Ml/d when the new pipeline is operational in September. In total, 2,500 pipes are being laid, following a route through Ticehurst, Etchingham and Robertsbidge. Southern has contracted Morrison, Brown & Root (who are working on 50-60 projects for the company throughout AMP3) and its sub-contractors to install 17km of pipeline. At Bewl, Land & Water is laying 3km of high-performance polyethelene (HPPE) pipe along the bed of the reservoir from Tinker's Marsh, along the Bewl straight to Chingley Deep.
To lay the pipeline, a temporary pontoon has been constructed. Tug boats push the pontoon to the desired location. Three, 25m-long legs are then dropped onto the reservoir bed to fix it in place. Pipe lengths of 18m are delivered to Bewl by truck and welded into 230m lengths on shore. As required, these are towed out to the pontoon where a crane lifts it up to be welded on to the last pipe section. Concrete collars are then fitted at 5m spacing, before a tug pushes the pontoon forward. As the platform moves, the pipe is lifted by crane, letting it slip over the edge. The concrete weights help to sink it to the bottom of the reservoir, some 20m deep in places. Divers regularly ensure the pipe is positioned correctly. On average, work is progressing at around 100m/d.
Elsewhere in the project ductile iron pipes are being laid at between 100-150m/d, depending on soil quality and weather. When the pipe exits the reservoir at Tinker's Marsh, 450mm-diameter pipe is being routed as far as Robertsbridge, then 650mm-diameter to Darwell. The project consists of various pipe-laying techniques. Open-cut is being employed across farmland but the pipeline must transverse many features of a typical modern and rural landscape.
"We have two micro tunnels, which are 1,200mm-diameter tunnelling machines, to go underneath railway lines, and directional drilling techniques are used to go under rivers," explained project manager Brian Mackay. Such obstacles may have caused delays to the project, which is on a fast-track programme - but Southern has made mitigation plans. For instance, the railways' approval procedure can be a lengthy affair. In order to get around this, the water company has arranged an alternative set-up for the pipeline.
"We chose to go through an existing 27-inch pipe at Robertsbridge to direct flows on a temporary basis," said Mackay. Thus, the transfer scheme can continue while approval is sought. Southern has also made provisions to minimise disruption to the environment. At the time of its first planning application in 2002, the company held a series of public consultations. Subsequently, some changes were required to reduce the impact on ancient woodland, golf courses and sensitive wildlife, for example. The length of the construction programme was also shortened. In some areas, hedges were removed and trees trimmed in February and March, before construction work even began.
This reduced the risk of disturbance to nesting birds. Fencing to shield great crested newts has also been erected along 14km of the route and at almost 30 sites work must take into account badger sets and dormice. A second planning application took place in 2003.
This was a year affected by drought and there became more urgency to get the project under way. Southern has made every effort to speed up the process, which is currently running to schedule