Spillway repair

We were engaged to design and construct a robust, environmentally satisfactory solution. With our client, we decided to replace the gabions with a more durable concrete wall, using a design that would resist water erosion and blend aesthetically with the surroundings. The project included the replacement of an existing gabion mattress at the toe of the wall with a concrete base slab specially designed with a plunge pool to dissipate the energy from weiring water to reduce extreme flows and erosion to the riverbed.

Our approach took into consideration a solution that resisted extreme water erosion, was low maintenance -given the isolated location – and suitably designed to blend in with the natural surroundings of a South Pennine site of special scientific interest (SSSI). Faced with difficult access and the need to maintain access to popular walking trails, Stonbury and United Utilities worked together to address the constraints, minimise impacts, and ensure continued water supply.

Before works commenced, we conducted an ecology survey. Overgrown vegetation was removed prior to bird nesting season, to prevent any delay.

We dismantled the spillway’s degraded gabion baskets and some others from the sides to create the space needed for casting a more durable concrete support structure. The stone was set aside for re-use. After installing a ramp and scaffolding access into the upper and lower sections of the spillway, we installed a Portadam system with a central relief pipe to release flows from the spillway onto the other side of the works. Coir silt traps were installed on the lower side to catch debris and prevent it from entering the watercourse.

The area in front of the spillway was excavated and levelled with new stone before a concrete base and kicker were laid. Steel reinforcements were affixed on the concrete face and shuttering, and formwork was installed in which to cast the new concrete support wall and wingwalls.

Once the concrete was cured and the shuttering and formwork had been removed, we installed new gabion baskets – flush with the exiting baskets and the newly cast wall. The new baskets were filled with reused stone from the previous gabions and the access ramp so that no waste was removed from site.

To further help reduce the programme’s carbon emissions, we used an eco-welfare cabin, which ran solely on solar power, and used hydrotreated vegetable oil (HVO) fuel in plant.

Throughout the programme, we engaged with the council and local community to ensure walkers were not disturbed by the works.