In 2019, prolonged heavy rain caused serious damage to the dam auxiliary spillway at Toddbrook Reservoir and there were concerns that water from the reservoir may flood the town of Whaley Bridge below, endangering the community. As a result, Arup was appointed to work alongside the Canal & River Trust to restore the reservoir in Derbyshire.

A rapid emergency response – which included evacuating 1,500 residents, stabilising the dam and draining the reservoir – prevented a disaster. As lead designer for the permanent repair, Arup is ensuring the dam and its replacement spillway are safe and secure, protecting the town in the event of extreme weather. Working closely with our client, we helped to manage the public consultation about the plans for the reservoir and developed the design for the public park that sits below it. As part of the works we have also designed a new sailing club.

Because the reservoir area is a SSSI (Site of Special Scientific Interest), the Environmental Impact Assessment was an important part of the process for our client. By repairing the dam we will be securing the future of this important haven for the rare plants that thrive here and also preserving the water supply to the other habitats downstream that depend on it.

Improving resilience to extreme weather events

In 2019, Toddbrook Reservoir – just upstream from Whaley Bridge – experienced severe rainfall and part of its auxiliary spillway collapsed. Subsequent independent reviews revealed that, although the dam complied with legislation, the auxiliary spillway had not been well designed and had deteriorated over its life.

Arup’s extensive experience on dam safety projects, such as Ulley Reservoir in Yorkshire, helped us win a competitive tender to secure the long-term safety of the dam for the Canal & River Trust.

Expert collaboration across disciplines

We brought together a team with industry-leading expertise in dam safety, supported by our specialists in seismic, hydraulic, geotechnical, civil and structural engineering. To assess how well the dam would withstand seismic activity, our seismic engineers reviewed the risk of earthquakes and the region’s history of mining. Collaborating with a range of environmental specialists, planners and visual communications professionals was integral to the success of the project. Additionally, our hydraulic, geotechnical, civil and structural engineers enabled the design of a new, 200m-long reinforced concrete spillway to cope with projected extreme rainfall patterns.

The new system comprises a weir at the overflow and a tumble bay to still the flow before it enters a sloping channel. This then takes the water away from the dam before it enters a stilling basin which removes any remaining energy from the water before entering a channel in the park downstream. To withstand the force of the water, this is typically made entirely of reinforced concrete. Instead, our solution uses a grid of piles underneath the stilling basin that work in tension to keep the basin securely in the ground. We also incorporated the temporary stabilising works into the permanent structures. Our innovations aim to save time, money and materials as well as cutting carbon emissions and construction risk. They will also reduce vehicle movements through the town.

We designed the new emergency drawdown to give the Trust more control over volumes leaving the reservoir, should they want to empty it. Monitoring systems within the dam embankment will provide early warning of any changes in performance. 

Arup’s support to the Toddbrook Reservoir restoration project has been invaluable to the Trust. Their multi-disciplinary team has provided expertise across a very broad scope, from resolving the technical complexities of the engineering design, to the essential environmental and landscape considerations for the adjacent community park-space.

Tom Greenwood

Principal Project Manager, Canal & River Trust

Maximising participation in the public consultation

Everyone in Whaley Bridge was affected by the events of 2019, and it was important to hear their views. We held in-person consultation events when Covid-19 lockdown rules permitted. We also deployed our Virtual Engage system which enabled over 1,300 people to visit a virtual consultation space, viewing information boards and giving their feedback. This was then collated and analysed quickly and easily.

The public park below the dam is an important recreation space. The initial spillway design featured a 2m high wall at the end and we were concerned that people might climb over this and either become trapped or misuse the space. Guided by risk assessments considering public safety, we revised the plans to create a more open design, to reduce the likelihood of misuse, and placed safe escape points throughout.

Preserving Toddbrook’s unique environment

Our landscape architects, planners and sub-contractors supported the Trust to ensure the engineering solution would complement the sensitive parkland environment. The reservoir outflow will run through the park, with a new terraced seating area at the water’s edge and a pedestrian bridge.  Toddbrook’s sailing club was moved to accommodate the new spillway, so our buildings team are designing new facilities for them at the site.

The area’s SSSI status meant our Environmental Impact Assessment was critical to the planning process, particularly as Toddbrook is home to rare dwarf bladder moss. Our assessment showed the project is on track to exceed the biodiversity net gain target of 10%.

The incident at Toddbrook has also led to reviews which will influence ongoing changes to UK reservoir safety legislation. As part of this, Arup’s experts are helping to develop a new hazard classification system for reservoirs in England and Wales – creating a safer future for communities across the country.