Labyrinth weir outlet ramps: managing flow instability and discharge efficiency

Abstract

The installation of a labyrinth weir as the crest of a stepped chute significantly influences the downstream chute inflow conditions relative to published design standards for more traditional crest shapes. A current study at the Utah Water Research Laboratory (Utah State University) evaluated a 10.67° trapezoidal labyrinth weir crest discharging into a 0.8H:1.0V stepped chute. To better direct flows into the chute and mitigate flow instabilities (i.e. flow surging), sloped ramps were placed in the labyrinth weir outlet cycles. The influence of six different ramp geometries are reported here relative to the weir discharge efficiency, flow surging and chute wall height requirements based on splash and spray depths. The labyrinth weir increased the spillway wall height required for flow containment by 200–300% relative to linear weir wall height predictive methods. The addition of ramps provided better flow transition into the stepped chute and reduced the required chute wall heights by up to 15%. Furthermore, ramp heights taller than half the weir wall height effectively reduced flow surging but increased local submergence, which decreased the discharge efficiency by up to 9%. Thus, an outlet cycle ramp geometry selection should be optimised by discharge efficiency, flow surging and chute wall height requirements.