Repairs to the cascade
The eighteenth century ornamental lake in West Wycombe Park is man-made and retained by an earth embankment (the dam and cascade). Containing 42,000m³ of water, it is categorised as a reservoir under the Reservoirs Act 1975 and is subject to regular inspection by appointed engineers.
A recent inspection identified a risk of failure of the earth embankment, and possible damage to the historic flint faced cascade, in the event of an extreme flood. The solution involved structural strengthening of part of the earth dam, and the banks to the River Wye immediately adjacent the historic cascade and sluice to the ornamental lake. A number of solutions were considered, from simple earth bunding to more complex engineered solutions. Aside from the safety implications, the design priority was conservation of the views and vistas across the historic landscape.
Following extensive consultation and geological surveys, the detailed design was agreed by our specialist engineers and the Environment Agency to meet the long-term safety requirements, whilst protecting the historic landscape and vistas. The solution required construction of new ‘king post’ retaining walls to both banks of the river to the east of the embankment, along with reinforcement of the grassed banks behind the new walls and additional scour protection at their feet.
The walls are formed in oak sleepers set between steel posts, which are set within 600mm diameter reinforced concrete piles that extend to the underlying chalk strata. A concrete walkway (just below water level) will enable safe access for future maintenance. Gabions (rock filled wire mesh containers designed to provide a robust and attractive solution for erosion) have been incorporated at the base of the walls to prevent future erosion, and additional planting of the gabions and banks will help improve the immediate environment. In addition, we have taken the opportunity to install monitoring pins along the length of the grassed bank. These are datum points from which regular measurements are taken, which will enable early identification of any future movement of the structure.
The works were carried out during winter 2017/2018 whilst the park was closed to visitors. The project has been a great success, delivered on time and within budget. The structure will protect the lake and cascade from failure in the event of a significant flood, and mitigate the scale and impact of possible future flooding.