Derwent Scour Hole project design and repair

Project overview

The Derwent Scour Hole project is an excellent example of collaborative working across organisations to deliver a critical project in a short window of opportunity. The Howden, Derwent and Ladybower Reservoirs are crucial infrastructure assets serving Derbyshire, South Yorkshire, Nottingham and Leicester, providing valuable water resources maintained by Severn Trent Water. Located in the Peak District National Park, Derwent Reservoir is an iconic tourist attraction close to the Dambusters memorial, which had to remain operational throughout the project.

A 210 m³ scour hole formed at the inlet of the Ashop intake on the side of Derwent Reservoir basin. If left, this would start to have a critical impact on the reservoir as a usable asset. Works carried out approximately 30 years ago to install a new inlet channel changed the flow of water into the reservoir, causing the scouring. This project, to fill the hole and create a wall to prevent further scouring and undermining, took place from August to October 2023.

Our role

Binnies undertook the lead designer role and employed a design philosophy to provide a robust solution that was sympathetic to the natural landscape, respecting the heritage of the area and using local materials and local companies for installation. The design had to accommodate challenging site details.

The natural landscape constrained the site size, with minimal working area for plant and storage. Three designs were considered, resulting in a nature-based stone solution chosen as the best sustainable option. The rockbags and gabions provide valuable habitats for invertebrates, as gaps within the stones mimic a natural environment; they also worked practically alongside the single access road and limited storage area.

The project design had to be modular so that it could adapt to the changing circumstances of the hole and site and provide options for different construction methods. Drone footage was captured in August 2022. The use of laser scanning enabled the scale of the hole to be established by the building of a 3D digital model. Various options were 3D modelled to find the optimum configuration of rockbags and gabions. The project did not start on-site until August 2023. Changes to the hole in that intervening 12 months were not completely known, so allowance for additional scour and need for extra rockbags was calculated and provided for. The assumption that the hole had grown bigger was proved to be correct.

The preferred option was to deliver the project in the drier summer period but had flexibility to use divers underwater if water levels were high. Summertime also provided the benefit of a naturally dropping water level due to the time of year. We responded to stakeholders’ feedback requesting extra gabion basket fixing. The amended design improved the durability of the gabions should there be any extra settlement or movement.

Works were required on the existing embankment to provide an area for the temporary works crane platform. A balance had to be found between being close enough to the edge of the dam embankment to ensure the lifting radius of the crane and providing sufficient distance to ensure no embankment failure. The crane platform could not impact the access road, leaving very limited space available.

The sustainable benefits of the repair were shown in December 2023 when the reservoir coped with heavy rainfall, which did not impact the local area and proved it was fit for purpose.