Stanford Reservoir – Design and Construction of the Upgrade Works to Increase the Overflow Capacity

Stanford reservoir, with a capacity of 1.5Mm³, is impounded by a 10m high, 265m long embankment constructed in 1928 for water supply purposes. The main overflow consists of a 76.6m long side weir discharging into a concrete spillway channel. The full length of the crest of the dam and downstream slope are protected by cellular concrete blocks to allow overtopping during an extreme flood event.

Historic model testing determined that the main spillway could only accommodate the 1 in 100 year flood event before the dam was overtopped. The upgrade works to increase the main overflow capacity to safely pass the design flood were recently completed. This paper describes the design and construction stages of the spillway works, which consisted of the lowering of the invert and the widening of the channel from 6.1m to 11.5m to achieve a discharge capacity of 80m³/s. This solution was chosen in order to retain various existing upstream weir structures, penstock gates and the right side spillway wall. The development of the detailed design also took the contractor’s construction staging into consideration and the requirement for the spillway to be able to ‘operate’ during a flood at all times during construction.