To create the lake at Bowood, Capability Brown, landscape designer (1716-1783), dammed two streams and moved a small hamlet called Mannings Hill to the nearby village of Sandy Lane. It was a vast undertaking, and the construction took five years.
When completed in 1766 the lake held a capacity of 400,000m3 of water. (1m3 is equal to 1,000 litres and the average showerhead uses 12 litres of water per minute).
To manage water levels during periods of heavy rain and for maintenance, four spillways, including an auxiliary, were built into a dam at the north end of the lake. The Bowood waterfall is supplied by these spillways, which are checked and maintained on a regular basis.
Anticipating climate change and adapting to recommendations by the Environment Agency, in 2015 a decision was taken to upgrade and enlarge the auxiliary spillway. Work started in October 2017 under the supervision of Geoff Partridge, Head Groundsman, and was completed in March 2018. Sitting on solid clay topped with grass maintained to a length of 150mm, the spillway is 15 metres wide and slopes at a five percent gradient. Flowing water is discharged into a small stream that lies parallel with the lake dam and from which it passes into the watercourse.
Since completion the auxiliary has been active five times, three of which were in January and February this year. Between October 2023 and February 2024, the lake saw 637mm of rainfall. Successfully keeping the dam intact, the auxiliary itself has suffered some erosion where it discharges into the stream.
