Energy Dissipation on Flat-Sloped Stepped Spillways: Part 2. Downstream of the Inception Point

摘要

Many aging watershed dams require hazard classifications changes. As a result, these dams may no longer meet state and federal dam safety regulations because of inadequate spillway capacity and flood protection. Rehabilitation options for these embankments are often limited due to encroaching urban development. Roller compacted concrete (RCC) stepped spillways are a popular choice in these situations because the spillway capacity can be increased with little or no additional changes to the embankment dimensions. RCC stepped spillways are also selected because of the cost and time savings in the construction of these structures. Design engineers require more information about the inception point location and the approach velocity and energy dissipation in the spillway chute. These elements are important for properly dimensioning the spillway training walls and stilling basin. Research and more specifically design guidelines for RCC stepped spillways applied to small earthen embankments have been limited. A two-dimensional, physical model was constructed to evaluate the inception point, velocities, air concentrations, and energy dissipation in a 4(H):1(V) slope spillway chute having 38 mm (1.5 in.) high steps. Model unit discharges ranging from 0.11 m 3 s -1 m -1 (1.2 cfs ft -1 ) to 0.82 m 3 s -1 m -1 (8.8 cfs ft -1 ) were tested. The findings from this research show that a relationship developed by H. Chanson can be used to determine the inception point location on stepped chutes with Froude surface roughness (F _* ) ranging from 10 to 100 for slopes as flat as 4(H):1(V). Additionally, air concentrations near the inception point are approximately 0% and rapidly increase to 10% slightly downstream of the inception point. These air concentrations continue to increase gradually to a constant as the flow descends the chute. The study results show that energy losses increase from 30% when a normalized length (L/L _i and L/L _i* ) equals 1 to 73% when L/L _i and L/L _i* equals 3.5. A first attempt at providing an energy loss relationship at any point downstream of the inception point is provided. This research will assist engineers with the design of stepped spillways applied on relatively flat embankment dams