Case study on hydraulic performance of Brent Reservoir siphon spillway

The Brent Reservoir was constructed in the mid-1830s and its siphon spillways were completed in 1936 to protect the dam from overtopping in the event of an extreme flood. Since completion. there have been problems with the hydraulic performance of the siphons. some of which primed simultaneously, causing flooding downstream. A physical hydraulic model study has been conducted to investigate the hydraulic performance of the siphons in order to establish reliable stage discharge relationships. The existing bellmouth siphon system was found to be unsuitable, causing the siphons to prime suddenly at discharges of around 3 m(3)/s. This was due to the sudden removal of an air pocket from the siphon's crown. The model tests were carried out in two stages. In the first stage, the existing geometry was examined. Based on the results from stage I of the experiments, it was concluded that the air inlet requires redesign and various options to improve the air regulation should be considered. In the second stage, various options to regulate the inlet of air and establish stable siphon performance over the entire range of discharges were considered. It was found that the most stable conditions are provided by an air slot being cut into the spillway hood at an appropriate level. This geometry provides excellent air-regulated stability, unimpaired spillway capacity, and is insensitive to tail water level and wave conditions in the reservoir.