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www.managingwater.co.uk Wigan Flood Alleviation Scheme


Flood Defence


Innovative vortex control engineering helped ‘fine-tune’ the Environment Agency’s £12 million flood


alleviation scheme in Wigan. The result is one of the North West’s largest and most innovative flood alleviation schemes to date.


Centuries of history as a Lancashire mill town and coal mining centre have shaped Wigan’s development in the flood plain of the River Douglas. But the River that powered its industrial revolution also left a


destructive legacy for homes and businesses at its mercy during peak storm events.


As it heads from the picturesque Haigh Hall Country Park, the Douglas is narrow and sometimes fast flowing, travelling through a wooded and steep-sided natural valley on its way downstream towards the town centre.


Homes and businesses in the low-lying southern areas of the town, including the town’s main bus depot, as well as major supermarkets, have been impacted by flooding. The Environment Agency’s determination to reduce the risk and provide much-needed stimulus for urban regeneration has resulted in a £12million solution six years in the planning.


Natural Valley


The natural Douglas valley, still undeveloped less than a mile away from the centre of town provided an obvious solution


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to the town’s flooding troubles and the protection of around 600 homes and 170 commercial properties.


The Environment Agency’s £12million Flood Alleviation solution was to build a dam to hold back the river during a major flood and create up to 370,000 cubic metres of temporary storage along a one kilometre stretch of the valley.


Two 2 m diameter Hydro- Brake® Flow Controls are the centrepiece of a 8 m high dam, 120 m wide and 120 m long constructed by main contractor Galliford Try in an 80-week contract. Once the scheme is completed in Autumn 2011, there will be just a 1% chance of flooding from the River Douglas in any given year.


Environment Agency Project Manager Eddie Goddard explains: “The first phase of the solution was to raise flood defence walls along the River Douglas in the town centre which we completed in 2008.


“Constructing temporary flood storage was the obvious next step. With such a narrow, steep-sided valley so close to the town centre and on undeveloped land, it was the perfect opportunity.”


Attenuation Solution


“The dam’s design needed to provide a means of attenuation only during severe flooding, and allow the river to flow naturally through the valley at other times.”


Alan Brown from consulting engineers Jacobs adds: “The initial solution for the dam was to install gates with real-time controls. However this would have been operationally a lot more onerous with significant additional risk management implications. The gates would require power and back-up power, regular maintenance and operator intervention.


“By changing the dam design to use Hydro-Brake® Flow Controls, neither power nor


ongoing maintenance would be necessary.”


Eddie added: “The use of Hydro-Brake® Flow Controls was vital because it has enabled the dam to be fine- tuned. We were able to reduce the reservoir footprint significantly, compared to a conventional flood gate solution.


Fine-tuning


“With the Hydro-Brake® Flow Controls, the river water can flow unimpeded through the dam until it reaches a pre- designed level. At this point the device is engineered to trigger a vortex which throttles back the flow, releasing it at a strictly pre-determined rate.


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