North West England
Homes and businesses in the low-lying southern areas of Wigan, 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.
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 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.
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."
Hydro International supplied 2 No. 2 m diameter Hydro-Brake Optimum® Flow Controls which are the centrepiece of the 8m high dam, 120m wide and 120m long constructed by main contractor Galliford Try in an 80-week contract.
The dam has created a potential 370,000 cubic metres of flood storage extending along a kilometre of the Douglas valley. After six years of planning, the scheme now provides increased protection with just a 1% chance of flooding from the river in any given year.
In the event of a major flood, water would be held behind the dam to reduce the chance of flood defences in Wigan being overtopped. The excess water would then be stored in the valley upstream from the dam. After the event, the stored water would be slowly released back into the river until normal river levels are achieved.
The two cone-shaped Hydro-Brake Optimum® Flow Controls, weighing 15 tonnes each were lowered into the dam with a 200-tonne crane.
A new design of Hydro-Brake® Flow Control with an adjustable intake was 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 Optimum® 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 completed dam and flood storage area has been landscaped and made attractive for visitors as part of a green corridor running from the town centre through to Haigh Hall Country Park.
Hydro-Brake Optimum® benefits:
Consultant engineers Jacobs and the Environment Agency worked closely with Hydro over many months using both physical and computer-aided modelling to ensure the Hydro-Brake Optimum® Flow Controls were precision engineered to optimise the storage capacity.
"The initial solution for the dam was to install gates with real-time controls," explains Jacobs’ consultant engineer Alan Brown. "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 comparison the Hydro-Brake® Flow Controls require neither power nor ongoing maintenance."
"The beauty of Hydro-Brake Optimum® Flow Control design is that water flows through the device unimpeded until it reaches a pre-determined head," said Alex Stephenson Hydro’s UK Director of Stormwater. “At this point a self-activating vortex is triggered which throttles back the flow and releases it at a strictly controlled flow rate. The result is that less flood storage capacity is required, compared to conventional solutions.