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Make that a double

Water Dams

Upgrade designs for a couple of Victorian dams in picturesque North Yorkshire are not as stress free as the location suggests. David Hayward reports from Wharfedale.

Reservoir Cottage in the remote North Yorkshire hamlet of Lindley Wood appears to the passing walker unchanged for a century. Nestling close alongside a large Victorian earthfill dam, the cottage and Water Board tenant would still seem to act as custodian to the surrounding reservoir.

But the doors and windows are false. Ground floor rooms are filled solid with 3m high concrete. The main bedroom boasts a telemetry outstation and the only occupants - some 80 brown long-eared and pipistrelle bats - live in the roof.

The ground floor has become a structural extension of the raised dam crest. A purposeful dip in new earthfill by the front door is profiled to ease the flightpath of the roof's nocturnal residents. And Reservoir Cottage is now officially renamed the Bat House.

Apart from this bizarre conversion, the current 3m raising of Lindley Wood Dam following a design upgrade, and a similar operation at Swinsty Dam 6km upstream on the same Washburn River, appear routine.

Yet if any of those walkers were also water engineers they may question why two adjacent impounding dams were being raised, and their reservoirs drawn down at exactly the same time. Surely prudent engineers would avoid simultaneously decommissioning two large reservoirs for fear that flash flooding from the upper could risk inundation of unprotected construction works in the lower.

That Swinsty is a supply reservoir, and Lindley Wood only for river compensation, minimises the risk. The real reason though is that client Yorkshire Water did indeed plan the upgrade works for consecutive years. But, when it lost a year in delayed designs, the client reprogrammed the original two contracts into one to recover lost time.

Questioned on the reasons for the delay, the water company's answer is as convoluted in design terms as Reservoir Cottage is in its reconstruction.

'A serious source of confusion' is how Yorkshire Water's solutions manager Jim Claydon sums up the various flood design recommendations used to re evaluate the two 127 year old reservoirs. ' Our upgrade design changed when new guidelines suggested we base our analysis on a totally impractical one in 10,000 year flood return period. ' The result was a more stringent design and a year's delay doing further flood studies.

In 1986, when both dams were graded by a Panel One dam engineer as category B, the then relevant flood design codes suggested no upgrade was needed.

But a decade later, inspection by a different engineer raised their classification to top category A.

This virtually doubled their design needs and local flood studies by Mott MacDonald concluded that both dams needed raising. But during the analysis new government guidelines emerged which increased the standards further and involved flood return extrapolations based on one in 10,000 years.

'These latest guidelines were really intended for river analysis not dams and we think it is wrong to extrapolate so far upwards' claims Claydon, himself an experienced 'All Reservoirs Panel' inspecting engineer.

'But until long overdue clarifying advice is published, we must cater for these increases and both our dams can be raised further if necessary.' (see box).

The result is that Swinsty Dam is being raised 1.2m with a combination of extra fill and new higher wave wall. And at the smaller Lindley Wood Dam, reprofiling with increased fill will leave it 3m higher.

Both dams can, if needed, be further raised between 500mm and 1m when the design debate is finally resolved.

Such indecision however is not part of Mike Mann's worry list. As Morrison Construction's project manager for the £3.5M contract for both dams, he is more concerned about reductions than increases.

Alternative designs by the contractor for sections of the work have led to a reduction in the volume of both imported fill and new stone for wave walls.

This means fewer lorry trips through narrow Yorkshire lanes, plus a significant cost reduction for the project.

At Swinsty, the new 457m long wave wall was to be keyed into the dam's puddle clay core directly beneath by a vertical 3m deep concrete cut off. By replacing this cut off wall with sheet piling, and reducing required excavation, Mann reckons he has saved 1500m 3of imported fill and some 400 lorry journeys. He is also cladding the new concrete wave wall with reused limestone from the dam's now demolished masonry original.

Similar value engineering at Lindley Wood means a changed profile for the higher embankment. Instead of raising the whole 25m wide dam by the full 3m, only the downstream half is increased and the service road across the structure rerouted along the lower height, upstream side. Saving 6,000t of imported fill means another 1,000 fewer lorry journeys on the steep access lane to the site.

Mann estimates that total environmental savings, including reuse in raised embankments of haul road aggregate, will mean nearly 2,400 fewer lorry journeys, and result in £200,000 overall saving to the project, most of it identified at tender stage.

The Bat House is another environmental saving as this attractive cottage lay in the path of the new dam wall extension and was earmarked for demolition. The discovery of protected bats in the roof, and a realisation that it could be economically infilled to form part of the new wall itself, has offered the cottage a new if unusual use.

Wanted: reliable guide

Upgrade designs for the two dams have been caught up in what Yorkshire Water's Jim Claydon claims is 'nationwide confusion among dam owners over the relevance of latest guidelines'. Until three years ago analysis was based on the government's 1975 Flood Studies Report and the ICE's 1996 Floods & Reservoirs Safety guidelines. When both dams were classified category B in 1986, the requirement for them to accommodate 50% of the probable maximum flood (PMF) could be met with no upgrade.

But when in 1996 the next ten yearly examination raised their status to category A - acknowledging the importance and size of downstream conurbations within the dams' failure zones - design upgrades needed increasing to 100% PMF. During the subsequent flood analysis the rules changed again with the publication in 1999 of the Flood Estimation Handbook.

These guidelines, Claydon argues, were originally intended for river flood planning and their use in dam analysis resulted in extrapolation to a one in 10,000 year flood. 'We have at most only a few hundred years of accurate flood records, so such extrapolation is wrong and could indicate higher standards, ' claims Claydon.

Follow-on interim guidelines to clarify this anomaly have still not fully resolved the issue, Claydon alleges. 'They suggest we either delay improvements altogether, redesign them for the worst possible scenario or phase the work so further dam raising can be carried out later, ' he explains. 'We were expecting final government guidelines last year but are still waiting.'

The only good news is that Claydon reckons new and separate studies by consultant Brown & Root, based on site specific overall risk analysis, may eclipse all previous guidelines. 'These will concentrate on the cost effectiveness of upgrades based on a portfolio of risk assessment and seem a very sensible way forward, ' he says.

'And by combining the two dam raising contracts into one we have saved on overall construction costs.'

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