Cleaning a critical pressure-relief system in protected national landscape

Innovative, Chemical-free Reservoir Cleaning of Cow Green Reservoir

Cow Green Reservoir is a strategically important water supply asset and an iconic landmark within the North Pennines National Landscape. Located within a designated Site of Special Scientific Interest and a National Nature Reserve, its remote, high-altitude setting presents significant logistical, environmental and safety challenges.

Under statutory safety regulations, the reservoir undergoes a Section 10 inspection every ten years. The most recent inspection confirmed work was needed to remove hardened deposits of calcium carbonate and other salts blocking sections of the pressure-relief drainage system by the end of 2026. The system comprises 92 relief drains, 90 pressure relief holes and around 445 metres (1,460 feet) of interconnecting pipework, all critical to safely relieving pressure within the dam.

Access was particularly challenging. The relief holes run horizontally before turning vertically through a 90‑degree bend and are accessed via an inspection gallery 23 metres (75 feet) below water level in a medium-risk confined space. Acid etching was ruled out due to the confined space risks and environmental sensitivity, while high-pressuring jetting proved ineffective against the rock-hard deposits.

Esh‑Stantec, Northumbrian Water Group (NWG) and Triton worked together to explore several alternative methods. A bespoke designed drill attachment was trialed that removed the deposits mechanically without chemicals. The method was highly effective at accessing and removing the hardened deposits safely and efficiently.

Environmental controls captured debris throughout the process, preventing contamination of the River Tees. Within five weeks, 90–95% of the build‑up was removed—11 months ahead of the compliance deadline.

Northumbrian Water plans to standardise the approach for use across all its reservoirs due to its simplicity, replicability and cost effectiveness.