2024 – High Labyrinth Weirs with High Heads – Wall pressures could be lower than you think – Prado Dam Spillway – A Case Study

Mike Phillips, Brian Crookston, Julie Allen, Yajayra Diaz

The U.S. Army Corps of Engineers has completed the design of a unique labyrinth weir spillway at Prado Dam, located in Los Angeles, California, USA. The proposed labyrinth weir consists of an arced configuration, 7-cycles, with varying weir wall height from 8.5 m to 12.5 m with upstream head over the weir cycles of up to 10 m. This hydraulic structure would provide critical flood protection with a spillway capacity for the revised Probable Maximum Flood estimates of up to 17,800 m3/s. Previous research has demonstrated that high head labyrinth weirs can experience significant fluctuating negative pressures at the weir wall resulting in hydrodynamic loads that should be considered during structural design. A Froude similitude model of one of the proposed labyrinth cycles was constructed at 1:14 and used at 1:20.25 geometric scales to measure static and dynamic pressures on the weir walls to calculate the potential hydrodynamic loads for select reservoir levels and dynamic pressure frequencies. The results of these physical model tests showed that the maximum hydrodynamic loads (negative pressures on downstream weir wall face) at the crest were approximately 2 to 3 times the upstream head, or -4.6 m to -5.5 m, and surging of the nappe that resulted in large pressure spikes. Additionally, the measured dominant pressure frequencies (0 to 6 Hz) along the weir wall and crest at various head and tailwaters were compared to the calculated structural fundamental frequencies of the weir wall (1.6 to 1.8 Hz) to ensure that the frequency of pressure fluctuations did not amplify the structural frequencies over design for structural stability This paper describes the physical model results including, the observed nappe behaviour and the static and dynamic pressure results, and compares the data to structural design requirements to assist in the structural design of labyrinth weirs.