Located within the conservation area of Harrow on the Hill in Northwest London, and close to the prestigious Harrow School, Harrow Reservoir is a key asset within the local water network. Built around 1976, the covered reinforced concrete clean water service reservoir has a total storage capacity of 80,000 m³ and is divided into two operational cells, East and West.

The structure has an irregular plan, reflecting the constraints imposed by the surrounding site boundaries. Geological studies have also identified variations in ground conditions and foundation types across the site, which have contributed to differing structural behaviour over time.

Shortly after commissioning, leakage was identified and has persisted despite several historic repair attempts. Inspection records have consistently reported differential movement at the interface between the North wall joint and the floor slab, along with distress at construction and movement joints. Degradation of the existing overbanding systems has led to ongoing seepage along joint lines. The primary defect mechanism is believed to be differential settlement and rotational movement at the wall-to-slab interface, which has compromised the performance of the original waterbar system.

Following a competitive tender process, CRL were appointed to deliver enabling works and the full refurbishment of the East cell, valued at £900k. The works include installing stainless steel transition plates across movement joints to support the overbanding system and protect existing waterbars during rapid filling cycles and fluctuating hydraulic loading. Around 1,000 metres of defective overbanding is being removed and replaced with approved DWI-compliant materials. Additional works include re-bedding and sealing access hatches, replacing flap valves, carrying out localised concrete repairs, and internal cleaning to support inspection and maintain water quality standards.

Now approaching Week 15, the project is progressing in line with the programme and is scheduled for completion in early April, allowing the East cell to return to service ahead of peak seasonal demand.