A laboratory investigation on residual energy of nonlinear weirs

Abstract

The labyrinth weir and piano key weir (PK weir) are a type of low head regulation structure capable of passing large amounts of flow. These nonlinear weirs are often considered for new construction and rehabilitation projects since they are a passive-control and (often) economic structure that may be adapted to a wide variety of site conditions. They are also relatively simple to construct and require minimal maintenance. However, few studies and design information are available to practitioners on the topic of energy dissipation, a critical parameter when designing basins and protection measures downstream. Therefore, an investigation was performed for headwater ratios between 0.2 to 0.8 using six laboratory-scale nonlinear weirs to explore the energy dissipation characteristics of these hydraulic structures. The models included PK weirs of two different scales and with different width ratios, and two labyrinth weirs; a 10-degree labyrinth weir and a rectangular labyrinth weir with and without ramp inserts. Energy upstream and downstream of the weirs was measured with point gage and ultrasonic sensors, with the downstream measurement location carefully selected based upon the downstream flow field. The data collected was compared to the few published studies available, with favorable agreement. Energy dissipation is most efficient at lower flows and decreases with increasing unit discharge, in part to the collision of the nappes and turbulence in the outlet cycles.