Effects of a Labyrinth Weir with Outlet Ramps on Downstream Steep-Stepped Chute Sidewall Height Requirements

Labyrinth weirs are commonly used to increase spillway discharge capacity. Stepped chutes represent a common spillway element used to help dissipate kinetic energy, thus reducing the size requirement and often the cost of the downstream energy dissipation basin. When combined, the complex, highly turbulent, aerated flow patterns generated by the labyrinth weir create different aeration inception point behaviors relative to traditional stepped chute applications (e.g., linear weir upstream). To reduce the risk of erosion and other damage near the spillway, stepped chute sidewalls are typically designed with sufficient height to contain the majority, if not all, of the bulked air-water spillway flow and surface waves. This study evaluated some of the hydraulic impacts of coupling a labyrinth weir with a relatively steep-stepped chute (08H:1V) at the laboratory scale in an effort to provide some useful guidance related to the sizing stepped chute wall heights. Test results showed that required chute wall heights for bulked flow containments were higher at the chute entrance than the traditional stepped chute (i.e., no labyrinth weir) design predictions. Placing ramped floors in the labyrinth weir downstream cycles facilitates the transition from the labyrinth outlet cycles to the steep chute and, in some cases, reduced maximum chute water levels. To better understand this phenomenon and provide guidance toward the use of ramped floors to reduce chute wall height requirements, the hydraulic behavior of various ramped floor configurations was systematically evaluated. The results for the geometries tested ranged from no effect to a 15% reduction in maximum chute flow depth.