The Effect of Step Height on Energy Dissipation in Stepped Spillways

A two-dimensional, physical model was constructed to evaluate the energy dissipation on a 4(H):1(V) slope spillway chute. Step heights of 38 mm (1.5 inches) and 76 mm (3.0 inches) were evaluated, and energy losses created by these steps were compared. Model unit discharges ranging from 0.11 m³/(s⋅m) (1.2 cfs/ft) to 0.82 m³/(s⋅m( (8.9 cfs/ft) were tested. Water surfaces, bed surfaces, velocities, and void fractions in the flow were measured during the tests. The findings from this research show that a relationship developed by Hubert Chanson can be used to determine the inception point for slopes as flat as 4(H):1(V). With increasing step height, the energy losses at similar locations within the spillway chute also increased. The results for energy losses for the 38 mm (1.5 inches) and 76 mm (3.0 inches) steps showed similar linear trends upstream of the inception point, with near zero percent energy loss at the spillway crest to approximately 30% at the inception point. Downstream of the inception point for the two smaller step heights, the results showed a more logarithmic trend from 30% at the inception point or when a normalized length, L/L_i*, equals one to 73% when L/L_i* equals 3.5. This research is expected to assist engineers with the design of stepped spillways applied on relatively flat embankment dams and the associated stilling basin located at the toe of the spillway.