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Floods: trial of reservoir 'drawdown' to begin shortly


Thursday, 21 December 2017

A Yorkshire Water official has also expressed caution on expecting to notice massive change within a very short period of time: "Reservoirs can only be allowed - by increasing outflows - at a rate which is limited by several factors, including reservoir safety, and pipe and valve capacity. The rate at which the reservoirs drop will also of course be dictated by weather conditions, so in the present wet weather it may be that no drop in levels is observed as the flows coming into the reservoirs may well be greater than those which can be safely released. In that scenario, what will happen is that the reservoir levels will rise, but this would be more slowly than would otherwise have been the case (if normal flows were only being released). 

"There's also a need to work to balance the drawdown across different reservoirs - so, for example, if Wallshaw Dean Upper is full, you can't simply release some water into the river downstream of the three Wallshaw Dean reservoirs. Wallshaw Upper water has to flow into Wallshaw Middle and then into Wallshaw Lower before it gets to the downstream river - so this might cause some temporary increases in level in Middle and Lower, but that could be as a consequence of lowering Upper."

Floods: trial of reservoir 'drawdown' to begin shortly

Thursday, 9 November 2017

Following the 21st October heavy rainfall event, when floodgates were being hastily mounted across the town centre, before a break in the weather removed the risk of actual flood, there’s now some very good news which may reduce the ‘short-term flood risk’ that continues to hang over the town, until such time as the main flood alleviation scheme, combined with natural flood management in the uplands, is able to lessen the long-term risk. But that is still a number of years away.

In the summer, the Environment Agency modelling presented for public consultation on the Hebden Bridge main scheme indicated that the ‘assessment has shown that the majority of benefits are gained when the reservoirs are 10% below full prior to an event’, with reduced flows of around 20% as a result.

Now on Monday 6th November the Calderdale Flood Recovery and Resilience Programme Board heard that -  following a significant amount of work by Yorkshire Water and the Environment Agency to understand the impact that altering reservoir operation and levels may have on flood risk, and also what that might mean for other critical issues such as security of water supply - Yorkshire Water have agreed to trial the drawdown of some of the reservoirs above Hebden Bridge over autumn/winter 2017. This trial is now possible because:

  • evidence provided by the Environment Agency has shown the potential benefit of drawing down reservoirs above Hebden Bridge on flood risk in the town.
  • Improved understanding at Yorkshire Water of the potential impacts on water resources from its long term regional modelling.
  • Specific modelling undertaken by Yorkshire Water for this autumn/winter shows reduced water resource risk of this period due to the wet summer that has been experienced.

The report presented to the board provides more information about aspects of the trial: for example, that it will be limited to just three out of the six reservoirs on the Hebden Water catchment, where the modelling has demonstrated the greatest flood risk benefit; and that the drawdown will be to 90% full by volume (but not by level). Yorkshire Water will have to advertise the trial for a minimum period of 28 days before it can be formally commenced, but it is hoped to begin it before Christmas.

At the Board meeting, Calderdale Council deputy leader Cllr Barry Collins said that, following the 21st October rain event, the leader of the council Cllr Tim Swift and its chief executive had spoken to Yorkshire Water stressing the urgency of proceeding with a reservoir drawdown trial. That seems to have done the trick.

See also

Natural Flood Management in the Calder Valley

Report presented to the Calderdale Flood Recovery and Resilience Programme Board

HebWeb News: Pioneering plans for reducing flood risk in the Calder Valley are launched (28 Oct 2017)

HebWeb News: After Saturday's near-flood (24 Oct 2017)

HebWeb News: Flood Walls on Hebden Water: Express your views(30 June 2017)

HebWeb Feature: the Boxing Day Flood

Photos of the Boxing Day Flood