In Mansfield, Nottinghamshire, a pilot project is spearheading an exciting new approach to urban water management.
The Mansfield Sustainable Flood Resilience project is tackling the increasing challenge of surface water flooding – a risk intensified by climate change and urban development – through the widespread use of nature-based solutions such as rain gardens, bioswales and detention basins.
Hundreds of sustainable drainage systems (SuDS) are being installed throughout the town, making it the largest retrofit scheme of its kind in the UK.
When complete, the network of blue-green infrastructure will capture millions of litres of water in the event of a severe storm, protecting homes and businesses from flooding.
Why do we need sustainable drainage systems?
In England, 325,000 properties are in areas at risk of surface water flooding, according to the National Infrastructure Commission. The Met Office has warned that extreme rainfall events could be four times as frequent by 2080 compared to 1980s, causing storm overflows to be utilised, particularly in urban areas and places with ageing drainage infrastructure. Flood resilience and improved capacity of the network is therefore fast becoming a priority.
The town of Mansfield was identified by Severn Trent Water as one of the most at-risk communities in Nottinghamshire once the impact of climate change was considered, with almost 1,000 people living in homes classed as ‘high risk’ from flooding.
Working alongside Mansfield District Council and Nottinghamshire County Council, Severn Trent is investing £76 million in the Mansfield Sustainable Flood Resilience project to manage the excess water from extreme rainfall events. The retrofit project will provide additional storage capacity for surface water, creating resilience in the network to meet the catchment’s future estimated needs.
“Not only will [these] measures … be of significant benefit to residents and businesses in Mansfield in reducing the risk of flooding and its devastating impacts, but they will also mean that residents have more green, open spaces to enjoy.”
Councillor Neil Clarke MBE
How does blue-green infrastructure combat surface water flooding?
Plans and strategies to manage surface water differ depending on location. For instance, the Thames Tideway super sewer in London manages the problem in a city with a very high population density. The Mansfield scheme, by comparison, moves away from a traditional engineering approach and uses blue-green infrastructure instead to slow the flow and store the water, such as:
- Detention basins: like bioswales but usually in larger green areas such as parks.
- Bioswales: sunken channels adjacent to roads to capture surface runoff.
- Rain gardens: small gardens, often on the corner of a pavement and road, with layers of permeable materials.
- Tree pits: grids below the roots that capture and slow the rainwater.
- Permeable paving: used on hard surfaces with gaps that allow water to filter through.
A greener, more resilient town
A further benefit of these interventions is the introduction of new public realm and green spaces filled with vibrant feature planting.
For example, three large raingardens now sit within the historic town centre marketplace, transforming a pedestrianised area into an attractive, nature-based space.
Councillor Neil Clarke MBE said of the scheme: “Not only will [these] measures … be of significant benefit to residents and businesses in Mansfield in reducing the risk of flooding and its devastating impacts, but they will also mean that residents have more green, open spaces to enjoy.”
Mansfield serves as a fantastic example of how blue-green infrastructure can be used at scale to increase urban resilience in the face of a rapidly changing climate.
A blueprint for urban climate resilience
The interventions completed in the first phase of construction were put to the test during storms Babet, Gerrit and Ciaran in 2023/4, where they demonstrated significant hydraulic benefit, alleviating flooding hotspots.
Such is the success of the Mansfield pilot that Severn Trent is considering taking these lessons forward into the next asset management period (AMP8) and creating four ‘urban catchments of the future’, which use a combination of artificial intelligence, nature-based, traditional and community-focused means to manage surface water.
With the UK government’s recent announcement to mandate SuDS on all new housing developments in England from 2024, Mansfield serves as a fantastic example of how blue-green infrastructure can be used at scale to increase urban resilience in the face of a rapidly changing climate.
What is AECOM’s role on the Mansfield Sustainable Flood Resilience project?
All our team members have been proud to work on this scheme which stands out for its ambition, scale, and the speed at which it had to be delivered.
As lead design partner and strategic advisors to Severn Trent, we developed the catchment strategy and used hydraulic modelling to select sites for the SuDS and inform both outline and detailed design. Our modelling strategy is multifaceted, requiring us to work at a range of scales and detail through different phases of feasibility, design and construction.
Delivery of the first phase is near completion and is split across the town. Phase 2 of the project is well underway. AECOM and Galliford Try are delivering the project in the east and Arup and Kier in the west, with CPC Civils building the detention basins. Both the speed and scale of this project demanded effective collaboration between teams to ensure timely delivery.
Consideration for the community was also given when planning and sequencing works, so that disruption to local traffic was minimised.
Innovative approaches
The pace and scale of this programme demanded innovation and excellence at every stage. It forced us to rethink our approach to modelling as traditional methods would have taken too long.
For example, we collaborated with Severn Trent and Arup to develop a SuDS calculation tool which determines the m³ benefit from each intervention ensuring that they could be built quickly. We also developed template designs with approval from partners and RoSPA.
Other innovations included:
- Design: collaboration with suppliers to optimise flow control chambers, crucial to hydraulic performance.
- Monitoring: installing water level monitors to assess raingarden performance. This data-driven evaluation will inform our design approach for future phases.
- Modelling: developing the hydraulic model to include interventions and monitoring data and experimenting with innovative SuDS modelling methodologies.