The Environment Agency’s radical solution to hold back water from the River Douglas, just a mile from the town centre, has provided the answer to reducing the flood risk to around 600 properties.
The £12million Flood Alleviation Scheme included the installation of two giant Hydro-Brake® Flow Control devices, the centrepiece of an 8 m high dam, 120 m wide and 120 m 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 a flood, the stored water would be slowly released back into the river until normal river levels are achieved.
Environment Agency Project Manager Eddie Goddard said: “Constructing temporary flood storage was the clear solution in such a narrow, steep-sided valley. The site was fortuitously close to the town centre and on undeveloped land.
“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.
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® 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 two cone-shaped Hydro-Brake® Flow Controls are 2 metres in diameter, weigh 15 tonnes each and 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® 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.
“Whilst not quite as big as the Hydro-Brake® Flow Controls installed as part of the Glasgow Flood Prevention Scheme, the Wigan scheme is still one of the largest, and certainly the most innovative flood storage projects to date.
“The beauty of Hydro-Brake® 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.
“Hydro’s design and modelling teams worked closely with the Environment Agency and Jacobs to provide a unique solution for Wigan, which could have major benefits for future flood prevention schemes,” he added.
“A completely new design of Hydro-Brake® Flow Control has resulted, incorporating in-built adjustability that can be used on any scheme from the biggest like Wigan to the more typical drainage and sewerage schemes”
The Wigan flood storage project was the second phase of the Environment Agency’s flood alleviation scheme at Wigan. Phase one, completed in 2008, involved the raising of defences along the River Douglas.
The completed dam and flood storage area will be landscaped and made attractive for visitors as part of a green corridor running from the town centre through to Haigh Hall Country Park.