A Laboratory Study on the Hydraulics of Stepped Spillways: Effects of Step Geometry and a Labyrinth Crest

Observed trends in extreme storms are affecting the safety of several embankment dams. Spillways are critical for safely conveying excess water in the reservoir over the dam and into the downstream river, yet many dams that were built over 40 years ago have undersized spillways to handle modern flood estimates requiring repairs and upgrades. Consequently, a unique spillway crest, the labyrinth weir, has been used as a cost-effective improvement for numerous projects to enhance discharge capacity of existing weirs. Another cost-effective option for overtopping protection due is to place a less expensive concrete, Roller-Compacted Concrete (RCC), over the dam to prevent erosion during floods. RCC is placed in layers creating flat steps that typically have a 45-degree angle instead of a vertical face down to the next step. Both the beveled step and labyrinth weir aerate the flow passing down a stepped spillway and modify the characteristics and behavior of the water including the flow depths, velocities and energy dissipation capacity, but little information is available to engineers and scientists to support the design of these stepped spillways. Therefore, this Ph.D. study investigated how labyrinth weirs and beveled steps affect flood flows through a stepped spillway in the laboratory with a physical and modern instrumentation. Results show that beveled steps increase flow aeration, resulting in greater flow depths. They also influenced the splash and spray from the water surface, requiring higher chute walls to contain the flow. Beveled steps had slightly higher energy dissipation compared to vertical steps. Similarly, replacing a broad-crested weir with a labyrinth weir significantly modifies flow properties at the upstream end. Enhanced aeration due to labyrinth might have further positive consequences such as greater reoxygenation of water in oxygen depleted waters. Overall, these findings demonstrate that altering step geometry or the upstream entrance conditions can substantially affect stepped spillway hydraulics, underscoring the importance of factoring such changes into stepped spillway design. Skimming flow over stepped spillway results in a highly dynamic complex multiphase structure of the air-water interface posing a challenge for precise and reliable measurements of free surface topography using conventional methods. Hence, this dissertation also investigated the feasibility of a low cost RGB-D camera technique to scan in 3D the water surface through a stepped spillway. The results confirmed its viability for studying the complicated water surface when the flow is white from entrained air.