Shearwater Reservoir New Auxiliary Spillway

Scope of work

• Detailed design services for crest raising works and a new auxiliary overflow spillway.
• Acted as Principal Designer for the scheme. Provided PCI documents, including DRA and buildability statement.
• Tender and construction phase support to Longleat Estate’s appointed Project Manager. 

Project description

In mid-2024, Stillwater were appointed by the Longleat Estate to prepare the detailed design of reservoir safety works at Shearwater Lake.  These works included raising the crest of the dam, constructing a new auxiliary overflow over the embankment crest and the sealing of two abandoned low-level culverts passing through the embankment.  The full tender pack was delivered towards the end of 2024 and works on site commenced in early March 2025.  In addition to providing technical engineering support during the construction phase to Longleat’s appointed Project Manager, Haycock Environmental Consultants, we fulfilled the role of Principal Designer throughout the project.  A particularly challenging aspect of these works was the laying of bitumen-impregnated geomats on the downstream face of the auxiliary overflow, which required careful coordination and risk management.  The successful delivery of these works in early August 2025 has significantly reduced the risk of failure of this Category A reservoir.