Vann Lake is a 55,000m3 reservoir retained by an 8m high earth dam in rural Surrey. The reservoir dates from around 1750 and is now part of a site of Special Scientific Interest (SSSI), designated for a number of features water and woodland features.
The project was implemented following recommendations in the interests of safety from a statutory inspection under Section 10 of the Reservoirs Act 1975. The works were required to provide a spillway which would safely pass peak flows from a 1 in 150 flood event with sufficient freeboard.
The main scheme works comprised raising the dam crest with clay fill and constructing a reinforced concrete spillway channel to replace a trapezoidal channel; the original channel was lined with interlocking precast concrete blocks which had been washed out during regular spillway flows. Ancillary works included modifying the spillway weir crest level, constructing a new gabion headwall to replace a deficient structure built about 12 years earlier and fitting a modern trash screen on the existing piped overflow. The dam crest carries a public footpath and the raising included installation of a concrete kerb along the crest; this provides a defined crest level so that there would be an even spread of water across the whole dam in the very rare event of dam overtopping.
Construction work was preceded by environmental mitigation works, such as tree felling, grass strimming and reptile exclusion fencing undertaken by Surrey Wildlife Trust with support from Stillwater.