Modifying the downstream hydrograph with staged labyrinth weirs

Labyrinth and piano key weirs are hydraulically more efficient than linear weirs of common width. As the spillway discharge efficiency increases, the required reservoir detention volume reserved for flood routing reduces and the maximum base-flow operating reservoir pool elevation can subsequently be increased (additional water storage) without a reduction in dam safety. Increased spillway discharge efficiency also causes the reservoir outflow hydrograph to compress temporally and increases the peak outflow discharge, potentially increasing downstream flooding impacts. The influence of linear, labyrinth, and staged labyrinth weir (i.e. cycles with different crest elevations) head-discharge characteristics on the outflow hydrograph behavior were evaluated by numerically routing various flood discharges through a reservoir; peak outflow discharges, the maximum water surface elevation, and the required detention volumes were quantified for each weir alternative. In addition to the benefit of isolating base flows to a subset of the labyrinth weir, the staged labyrinth weir proved to be an effective alternative for modifying (decreasing) the spillway discharge efficiency to limit downstream flooding impact for higher-frequency return-period storm events.