Beyond resilience: How evidence-led design strengthened Guildford’s water supply

Galliford Try appointed GHD as design partner for the Thames Water scheme, a major AMP7 investment to secure supply for approximately 9,000 homes in an outage-prone area of Surrey. The work includes a new pumping station and an 8km pipeline from Pewley Reservoir to Netley Mill Water Treatment Plant.

Netley Mill had operated as an island zone. It relied on its own boreholes and treatment plant, leaving customers exposed if there was an issue at the site or with local supply. The new connection provides an alternative water source from Pewley Reservoir, improving resilience now and helping the network prepare for future changes to borehole abstraction licences.

The challenge was to deliver this infrastructure through an Area of Outstanding Natural Beauty (AONB), where environmental, stakeholder and construction considerations all had to be addressed together. The team needed to understand ecological constraints, land ownership, heritage, access, constructability, carbon and cost before settling on a route.

GHD brought these considerations into a shared GIS-based decision framework. The approach combined constraint mapping, digital modelling, ecological evidence and multi-criteria assessment, allowing the team to test route options against technical, environmental and stakeholder objectives.

This mattered because the route could not be assessed through one constraint at a time. Moving away from one environmental risk could create another. Bringing the information together made it possible to see those trade-offs and select an alignment with the lowest overall impact.

The team challenged the original brief and identified Pewley Reservoir as an alternative supply point to Shalford Water Treatment Works. The revised route reduced the pipeline length by around 1km, lowering construction disturbance, cost and programme pressure. It also avoided a crossing of the River Tillingbourne, sensitive habitat and other environmental receptors.

Digital visualisation helped turn the evidence into a working tool for collaboration. Thames Water, Galliford Try, regulators, landowners and project teams could see proposed changes and their implications in a shared environment. Rather than taking issues for separate review, teams could consider options together and decide more quickly. The process supported more than 150 collaborative workshops.

The scheme’s resilience benefits also extend beyond the new pipeline. GHD developed an intelligent control philosophy for water transfers between Pewley Reservoir and Netley Mill, with three operating modes that respond to changing demand, outages and abstraction constraints. This allows Thames Water to use available capacity at Netley Mill while reducing reliance on the constrained Pewley supply, and to maintain alternative supply capability during an outage.

The result is a project that brings engineering, environmental planning and network operation into the same decision process. It is expected to deliver an estimated £2–3 million capital saving, more than 12 months of programme acceleration and around 400 tCO₂e of carbon savings.

Alongside the community benefit for the residents of Guildford, the project provides transferable lesson for constrained water programmes: make the key trade-offs early, test them with the right evidence, and keep the people affected by those decisions in the conversation.