Energy dissipation on a 4(H):1(V) stepped spillway

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 8/10/2008
Publication Date: 9/8/2008
Citation: Hunt, S., Kadavy, K.C. Energy dissipation on a 4(H):1(V) stepped spillway. In: Dam Safety 2008. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 7-11, 2008, Indian Wells, CA. CDROM.

Interpretive Summary: Many small earthen embankments are faced with inadequate spillway capacity due to sediment pools filled with sediment and sediment now filling flood pools and changes in hydrologic conditions resulting in an increase in runoff. As a result, state and federal dam safety regulations are no longer met. With increased urbanization surrounding these dams, a greater number of people are at jeopardy of a structure failing if these problems are not addressed. Roller compacted concrete (RCC) stepped spillways is becoming a popular choice in correcting the deficiency of the spillway capacity because it often doesn’t require additional alterations to the existing embankment dam. RCC stepped spillways are also selected because of the money and time saved in the construction of these structures. Design guidelines for RCC stepped spillways applied to small earthen embankments are limited, especially for chute slopes of 2(H):1(V) and flatter. Flow depths, velocities, and energy dissipation is required to properly design the spillway chute walls and the stilling basin located at the toe of the spillway. A two-dimensional, physical model was constructed to evaluate the increase in flow depth due to air development in the flow, velocities, and energy dissipation in a 4(H):1(V) slope spillway chute having 38 mm (1.5 inch) high steps. Water surfaces, bed surfaces, velocities, and air concentrations in the flow were collected during the tests. The findings from this research show that a relationship developed by Hubert Chanson can be used to determine the point (Li*) where the flow will develop a white water appearance within a spillway chute with a slope of 4(H):1(V). Velocity profiles appeared to show similar trends for flow rates tested. Air concentrations near the point of white water development are approximately 0% and increased rapidly to 10%. Energy losses increase from 30% near the point where white water develops to 73% for a normalized length (Li*/L) of 3.5. The incoming velocity and energy dissipation into the stilling basin will allow design engineers to size the dimensions of the stilling basin. This research is expected to assist engineers worldwide with the design of stepped spillways applied to small embankment dams.

Technical Abstract: Many small earthen embankments are faced with inadequate spillway capacity due to filled sediment pools filled with sediment and sediment now filling flood pools. Additionally, hydrologic conditions have changed as a result of urbanization; thereby causing changes in the hazard classification of the structure. Unobtainable land rights around the embankment limit the rehabilitation options for design engineers to bring these structures into compliance with state and federal dam safety regulations. Roller compacted concrete (RCC) stepped spillways is becoming a popular choice in these rehabilitation situations because the spillway capacity can be increased with few or no additional changes to the embankment dimensions needed. RCC stepped spillways are also selected because of the money and time saved in the construction of these structures. Yet, design guidelines for RCC stepped spillways applied to small earthen embankments are limited, especially for chute slopes of 2(H):1(V) and smaller. Velocities and energy dissipation upstream of the inception point on stepped spillways applied to these scenarios are important for determining the size of stilling basin because in many instances air entrainment is not fully developed prior to entering the basin. A two-dimensional, physical model was constructed to evaluate the inception point, velocities, and energy dissipation in a 4(H):1(V) slope spillway chute having 38 mm (1.5 inch) high steps. Model unit discharges ranging from 0.11 m2/s (1.2 cfs/ft) to 0.82 m2/s (8.9 cfs/ft) were tested. Water surfaces, bed surfaces, velocities, and void fractions in the flow were collected during the tests. The findings from this research show that relationships developed by Hubert Chanson can be used to determine the inception point for slopes as flat as 4(H):1(V). Additionally, the velocity profiles transition from uniform at the crest to approaching a one-sixth power law distribution at the inception point. Air concentrations near the inception point are approximately 0% and rapidly increased to 10% slightly downstream of the inception point. Energy losses increase in a logarithmic fashion from 30% when the normalized length (Li*/L) is one to 73% for a normalized length (Li*/L) of 3.5. Knowing the energy dissipation in the spillway chute and the incoming velocity into the stilling basin will allow engineers to properly size stilling basins for non-converging stepped spillways. This research is expected to assist engineers with the design of stepped spillways applied on relatively flat embankment dams.