Clear Lake, Wisley
Case study

Clear Lake, Wisley

We were commissioned by the Royal Horticultural Society (RHS) to undertake a Feasibility Study for Clear Lake in June 2020. The proposal was for Clear Lake to collect and store rainwater from the new Hilltop development, and then re-use it for irrigation throughout the gardens. This would help the RHS achieve their longer-term water neutrality targets.

Transport Planning
Infrastructure Design
Environmental Assessment


We undertook the preliminary hydraulic and earthworks designs, and successfully demonstrated that the RHS’s vision for Clear Lake was feasible, and it could be built to meet requirements and brief set out. Subsequently the RHS decided to proceed with the project, and Mayer Brown progressed the design and produced the design drawings and Flood Risk Assessment to accompany the Full Planning Application, which was submitted in December 2021, with Planning Permission granted in April 2022.

We commenced the detailed design of Clear Lake immediately, and Construction work began in June 2022, with the project reaching Practical Completion in November 2022.


A few key points about Clear Lake are provided below:

  • The Lake is 2m deep. The bottom 1.54m are used for irrigation, the next 0.16m is for the attenuation of rainwater, with the remaining 0.3m for freeboard.
    • Total Storage Capacity = 9,164m3
    • Irrigation Storage Capacity = 7,098m3
    • Attenuation Storage Capacity = 897m3

  • Based on the RHS Wisley rainfall data and the catchment size, Clear Lake should capture and reuse 6,600m3 of rainfall in an average year.

  • The earthworks for the Lake were designed using Civil 3D and ensured no import or export of material was required. This approach helped minimise the project’s impact on the environment, whilst also keeping the costs down.

  • When the Lake is full, it overflows into an infiltration basin at 1l/s, and if the infiltration basin is ever full, excess water will overflow into the local ditch network.

  • The water is pumped from Clear Lake to the existing Battleston Hill tank, where it is distributed through the gardens.

  • There are 5no aerators in the Lake to help create aerobic conditions to naturally break down nutrients.

All Stakeholders agree that the Lake achieves the objectives of the brief, and it is collecting large volumes of rainwater, which demonstrates the amenity value and the environmental benefits including rainwater reuse, biodiversity habitat creation, rainwater treatment and attenuation will be met. 

The scheme was completed on budget, on time and to the satisfaction of the project team and hopefully the future visitors of the RHS Gardens at Wisley!

Clear Lake is now open to the public!

Clear Lake was opened to the public on 31st May 2023, and has filled up nicely over the winter and spring months following a very wet spring. The RHS are now abstracting from the lake and using it for irrigation throughout the summer months. 

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