Structural flood defences
Across Greater Manchester, flooding is the most pressing current climate risk.
Some areas are already at quite significant risk of river (fluvial) flooding. In Manchester, 9,629 properties are located in Flood Warning Areas.
These risks will only increase with climate change, a risk that will be exacerbated through an intensification of urbanisation, construction on green space, and as sewerage and drainage systems become overwhelmed.
There already are structural (engineered) flood defences across the city-region and the city of Manchester itself.
Flooding at Northenden, 21st January, 2021.
Nigel Lawson. Reproduced with permission.
Storm Christoph, January 2021
In mid-January 2021 flood warnings were issued for Greater Manchester with particular concern regarding the threat of flooding around Didsbury and Northenden given the anticipated levels of the River Mersey.
An evacuation took place prioritising vulnerable people, and those requiring assistance to evacuate, an operation that had to deal with the added complexity of Covid restrictions. A Reception Centre was opened at Wythenshawe Forum attended by over 80 citizens. Flood waters peaked at approximately 5am on Thursday 21st January. They did not breach the flood basin, however, it was reported that they did come within millimetres of doing so.
Flood defences
Many areas have been protected from the risk of flooding through heavy engineered flood defences placed along rivers designed to contain rising river levels, or to provide flood storage (attenuation).
For instance, the River Mersey has two water retention reservoirs that can be used to manage river levels and prevent flooding. One of these, the Didsbury basin has a capacity of 778,000cm3, working in tandem with the Sale Ees which has a capacity of 1,585,000cm3. Excessive flood water is sluiced into the reservoirs to maintain the river at optimum capacity.
However, these defences, though still functioning, require regular inspection and maintenance. They have come close to being overtopped in recent years.
Flood basin at Withington Golf Club, 21st January, 2021.
Nigel Lawson. Reproduced with permission.
Didsbury Flood Storage Reservoir
The reservoir is located off Millgate Lane, Didsbury. The sluice station has one large sluice and is operated at 150mm intervals. Excessive water is sluiced out of the River Mersey, flowing down an inlet channel and onto Withington Golf Course.
When the River Mersey reaches 6.9m the Sluice Gate is operated at 150mm intervals, balancing the river at 6.85m. A Flood Gate is situated at the end of Stenner Lane which has to be closed prior to opening the Sluice to prevent flooding to the cottages and block pedestrian and vehicle access.
The reservoir is emptied through sluices back into the Mersey and Fielden Park brook. The Flood basins working together balance the river reducing flooding to nearby areas and enabling the river to flow more easily through Barfoot Bridge.
