Collaborating to deliver Loch Katrine emergency spillway repairs

Safeguarding a vital water supply through rapid response

Loch Katrine is the principal water supply reservoir serving Glasgow and surrounding communities, providing 500M litres of water each day for 1.3M people. The reservoir has supplied clean drinking water to Glasgow and surrounding areas since 1859 and remains central to a largely gravity-fed system, making continuity of supply a critical consideration during the emergency response. The masonry dam impounding the reservoir was raised in the 1900s and it was the spillway on the dam that was the focus of the emergency response.

The spillway features a free discharge section across a third of the dam with sluice gates to control water levels and a fish pass spanning the remaining two thirds of the dam. At some point in the past the masonry apron of the spillway had been covered by concrete slabs and it was the waterflow over this area following the extreme rainfall that first alerted Scottish Water to the potential problem.

"Someone from Scottish Water noticed the flow looked disturbed," explains Mott MacDonald civil engineer Matthew Craig. "Normally the flow would be consistent across the spillway, but they could see water behaving differently and that was the first sign that something wasn't right."

Subsequent inspections revealed that the concrete slabs had been displaced by uplift pressures. “The concern was that there had been further masonry damage and a risk of unravelling, especially if there was further heavy rainfall,” Matthew adds.

Image credit: George Leslie Ltd

Mobilising at speed

Mott MacDonald was first involved in the response through technical director for dams and reservoirs, Martin Hewitt, who is one of only three All Reservoir Panel Engineers in Scotland. Martin visited site shortly after the issue was identified and was able to assess that the damage did not put the dam at immediate risk of failure. However, his investigation pointed to the need for urgent remediation.

Mott MacDonald and George Leslie were engaged through existing Scottish Water frameworks that included provision for emergency support.

Daily coordination meetings brought together Scottish Water, Mott MacDonald, George Leslie and other stakeholders, including the Scottish Environment Protection Agency and the Scottish Government, to assess risks, agree priorities and implement a response strategy. Working closely with Scottish Water's capital investment and water operations teams was also critical to the project's success, ensuring Glasgow's water supply remained unaffected throughout the emergency response and subsequent repair works. According to Mott MacDonald senior associate civil engineer David Steven, the project's success was rooted in the team's ability to mobilise rapidly and work collectively from the outset.

"The thing we're probably most proud of is how quickly we were able to respond," he said. "Existing relationships with George Leslie and Scottish Water, having the expertise in-house and a base in Glasgow helped our team do that at pace."

The immediate objective for the team was straightforward. Based on Martin’s advice, water levels in the reservoir were lowered to reduce the risk of further damage while a temporary engineering solution that could be delivered safely and quickly was developed.

Designing an emergency solution

The emergency measures included diverting flow away from the damaged area and stabilising the spillway using interlocking concrete blocks that resemble large concrete Lego blocks that lock together and sandbags. Rock anchors were used to create a tie down for the replacement concrete slabs with pressure relief pipes installed to reduce the risk of hydraulic uplift pressures causing similar damage in the future.

"The temporary design was turned around in about a week," says Matthew Craig. “The focus was on creating a robust, buildable solution that the contractor could implement quickly and safely."

Investigation, design and construction progressed in parallel, with the design continually refined as site conditions became clearer.

Design of the temporary solution was not just about identifying the ideal engineering solution was – it was also about working around the site logistics too. The main site access was a single track road which limited the construction equipment that could be brought in to complete the work. This access challenge also called for George Leslie to work closely with nearby residents to keep them informed about the work and how it might affect them day to day.

Another consideration was the need to work in live conditions with water continuing to flow through the site. Although the water level had been lowered, it could not be dropped enough to create a dry site due to the large surface area and catchment of the reservoir and the need to maintain water supply to Glasgow.

Environmental controls were also central to the emergency works, with silt mitigation and slurry containment measures used to protect the reservoir and downstream watercourses while repairs were carried out in live conditions.

The construction of the temporary solution was completed in late November 2023 with measures in place ahead of winter and the risk of further storms bringing more extreme rainfall. The completion of this initial phase also gave the project team time to review what was needed to create a permanent repair.

Image credit: George Leslie Ltd

Planning for permanent repair

Following the successful completion of the emergency works, George Leslie was appointed to deliver the design and build contract and brought in Mott MacDonald as its design partner.

Lessons from the emergency phase, including the drilling methodology and approach to variable ground conditions, also helped inform later phases of the work.

The permanent repairs were undertaken in two phases over the summers of 2024 and 2025 and used rock anchors, reinforcement and improved detailing around the slab edges to prevent water ingress beneath the repaired section. Although relatively simple in concept, the solution needed to withstand challenging hydraulic conditions while remaining practical to construct in a restricted timeframe.

“Although only part of the concrete apron had failed, the permanent repair focused on replacing the whole slab to reduce the risk of future failure,” explains Matthew.

Having worked on part of the concrete apron during the temporary works, it was expected that the ground conditions under the rest of the slab would be similar. However, once work got underway, it was discovered that the depth to the bedrock beneath the undamaged part of the apron was much greater than anticipated.

“This called for a rapid redesign and an alteration to the rock anchoring design and foundation details,” says Matthew. “The regular meetings we’d undertaken during the temporary repair meant that we were able to work in close collaboration to quickly overcome the issue and work on site was not delayed by the unexpected findings.”

That collaborative approach extended across the wider supply chain, where accelerated and out-of-hours deliveries helped maintain progress despite the restricted access and emergency programme.

The success of this project came from everyone working as a single team from day one.

One-team approach

Throughout both the emergency response and subsequent permanent works, collaboration of this kind was central to decision making. Teams from Scottish Water, Mott MacDonald and George Leslie worked side by side to assess conditions, agree priorities and adapt designs as new information emerged.

As Matthew describes it, the project became a genuine demonstration of integrated delivery. "It was a real one-team approach," he says. "Everyone was focused on finding the solution rather than who they worked for. The collaboration between Scottish Water, George Leslie’s team and ours was a huge part of why we were able to move so quickly."

George Leslie project manager Allan Ralston adds: "The success of this project came from everyone working as a single team from day one. Faced with challenging site conditions and an evolving scope, we were able to make decisions quickly, adapt as new information emerged during construction and safely deliver both the emergency repairs and permanent works. That collaborative approach was essential to achieving the outcome."

Lessons for the future

For the Mott MacDonald project team, Loch Katrine demonstrated the value of combining specialist expertise with trusted relationships and established delivery frameworks. The emergency response validated contingency planning processes while highlighting the importance of having All Panel Reservoir Engineers in the Mott MacDonald team when critical decisions needed to be made quickly.

"I think the key lesson is the benefit of building good working relationships on day-to-day projects so that you are better able to work together in an emergency situation," said Matthew. "A project that would normally take months to establish was mobilised almost immediately because everyone understood the importance of the situation and knew how to work together."

Scottish Water general manager for capital investment Mark McEwen adds: "When the issue was identified, our priority was protecting the reservoir and maintaining the resilience of a vital water supply asset. The speed with which Mott MacDonald and George Leslie came together with our teams allowed us to assess the risk, implement emergency measures and develop a long-term solution with confidence. The collaboration demonstrated exactly how framework partnerships should work during an incident response."

The emergency response has had a wider benefit for Scottish Water in creating a stronger basis for future permanent improvements, giving the project team a clearer understanding of the asset, site constraints and practical repair options.