Energy dissipation for flat-sloped stepped spillways using new inception point relationship

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/1/2009
Publication Date: 9/29/2009
Citation: Hunt, S., Kadavy, K.C. 2009. Energy dissipation for flat-sloped stepped spillways using new inception point relationship. In: Dam Safety 2009. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 27-Oct. 2, 2009, Ft. Lauderdale, FL (CDROM).

Interpretive Summary: Many earthen embankments once protected rural farmland, but years of development has led these embankments to now protect residential communities and infrastructure As a result, many dam designs no longer comply with state and federal dam safety regulations, so rehabilitation of these dams are now required. RCC stepped spillways are one method for increasing spillway capacity. Yet, little design guidelines are available for stepped spillways expected to have slopes flatter than 2(H):1(V). A generalized two-dimensional, physical model of a 4(H):1(V) stepped spillway was constructed to evaluate the air entrainment inception point location and energy losses. The inception point is the location where "white water" is observed at the flow surface. The inception point location for three different step heights were compared for the different flow settings. Hubert Chanson developed an equation for determining the inception point for stepped spillways with slopes primarily greater than 2(H):1(V). The research findings show that Chanson’s equation is applicable for this particular design under certain circumstances, so a new equation similar to Chanson’s relationship was developed and optimized for flat-sloped (2(H):1(V) or flatter) stepped spillways with a broad-crested weir crest section. The new inception point equation provides a more accurate location of the inception point. The inception point is important because it serves as a reference point for determining the energy losses along the length of the chute and identifies the location of significant "white water." The results of this study are applicable for scales 10:1 or larger, and are 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.

Technical Abstract: Transforming from a rural to an urban landscape has created a change in hazard classification for many earthen embankments. As a result, these facilities provide inadequate spillway capacity for the upgraded hazard rating. To bring these dams into compliance with state and federal dam safety regulations, rehabilitation of these dams are required. RCC stepped spillways are one method for increasing spillway capacity. Yet, little design guidelines are available for stepped spillways expected to have slopes flatter than 2(H):1(V). A generalized two-dimensional, physical model was constructed to evaluate the air entrainment inception point location and energy dissipation in a 4(H):1(V) stepped spillway. Step heights of 38 mm (1.5 inches), 76 mm (3.0 inches), and 152 mm (6.0 inches) were evaluated. Model unit discharges ranging from 0.11 m3/(s·m) to 0.82 m3/(s·m) were tested. The inception point location for the different step heights were compared for the different flow settings. Hubert Chanson developed a relationship for determining the inception point for ogee-crested stepped spillways for slopes primarily greater than 2(H):1(V). The research findings show that Chanson's relationship can be used to determine the inception point for slopes as flat as 4(H):1(V) when the Froude surface roughness, F*, is greater than 10 which is reflective in step heights of 38 mm (1.5 inches) and 76 mm (3.0 inches). When increasing the step height to 152 mm (6.0 inches), the corresponding Froude surface roughness F* for the range of flows tested was less than 10. As a result, the observed inception point location deviated from the predicted location when applying Chanson's relationship. A new relationship similar to Chanson’s relationship was developed and optimized for flat-sloped (2(H):1(V) or flatter) stepped spillways with a broad-crested weir crest section. The new inception point relationship provides a more accurate location of the inception point and is valid for F* ranging from 1 to 100. The inception point is important because it serves as a reference point for determining the energy dissipation along the length of the chute and it identifies the location of significant air entrainment. The results of this study are applicable for scales 10:1 or larger, and are 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.