Putting the brakes on flooding22 June 2012
Innovative vortex control engineering helped ‘fine-tune’ the Environment Agency’s £12M 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 £12M solution six years in the planning.
The natural Douglas valley, still undeveloped less than a mile away from the centre of town provided an obvious solution to the town’s flooding troubles and the protection of around 600 homes and 170 commercial properties.
The Environment Agency’s £12M Flood Alleviation solution was to build a dam to hold back the river during a major flood and create up to 370,000m3 of temporary storage along a 1km stretch of the valley.
Two 2m diameter Hydro-Brake Flow Controls are the centrepiece of a 8m high dam, 120m wide and 120m long constructed by main contractor Galliford Try in an 80-week contract. With the scheme complete, 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.”
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.
“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.
“This means that a great deal more river water can flow through the dam before the need to start attenuation. As a result, significantly less storage is required.”
To precisely engineer the flood storage area, teams from Jacobs and the Environment Agency worked closely with Hydro over many months. A physical model was built and subject to extensive testing as well as applying Computation Fluid Dynamics techniques to model the flows precisely confirming the performance of the dam under varying flow parameters.
A new design of Hydro-Brake Flow Control with an adjustable intake was also developed specifically for the project by Hydro in close co-operation with Jacobs and Environment Agency teams. The addition of specially designed restrictor plates on the Hydro-Brake® Flow Controls’ intakes will enable the median 10,000 litres per second flow rate from the outfall to be adjusted by plus or minus 20% in future.
The new design will enable the flow of the river downstream to be adjusted in future to take account of climate change, or to alter the outflow based on ongoing experience. The Environment Agency is continuing to monitor river flows closely.
The cone-shaped Hydro-Brake® Flow Controls weighing 15 tonnes each were installed into the dam using a 200-tonne crane.
Main contractor Galliford Try moved some 70,000 tonnes of earth to create the dam and flood storage area, including a stilling basin immediately upstream of the dam. Construction challenges included the specialist removal of tonnes of Japanese Knotweed and the investigation and ground stabilisation of nearby coal mining sites.
The inlet to the dam is protected by a series of debris screens and a specially-designed debris screen capable of withstanding loads of 50 kilonewtons.
Extensive landscaping works and the creation of an access road and car park has helped to restore the public amenity value of the valley area for local residents. The spillway of the dam was covered in Grasscrete to encourage rapid ‘greening’ of the site.
For more information about Hydro-Brake® Flow Controls contact any of Hydro’s sustainable solutions for stormwater control and treatment, tel: 01275 337977, email: [email protected] or visit www.hydro-international.biz.