Inception point for embankment dam stepped spillways

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/11/2012
Publication Date: 1/2/2013
Citation: Hunt, S.L., Kadavy, K.C. 2013. Inception point for embankment dam stepped spillways. Journal of Hydraulic Engineering. 139(1):60-64.

Interpretive Summary: Originally built to protect agricultural land, many embankment dams now provide flood protection for residential and commercial property, infrastructure (i.e. power lines, highways), and the general public; consequently, these dams need to be upgraded to meet the dam safety laws of today. A common design practice to upgrade these dams is to retrofit stepped spillways to the dam. As the water flows down stepped spillways the water surface changes from a smooth surface to a rough surface with visible white water. For many dams retrofitted with stepped spillways, the chute length is not of sufficient length to develop white water. Knowing where the white water begins for stepped spillways applied to embankment dams is important for the design of stepped spillways and stilling basins. New relationships are provided to determine where white water begins that improves previously developed relationships. These new relationships are applicable to stepped spillways retrofitted to embankment dams.

Technical Abstract: Stepped spillways applied to embankment dams have become a common design practice with the rehabilitation of aging watershed dams, especially those experiencing a hazard classification change from low to high hazard. Previous research on stepped spillways focused on gravity dams where aerated flow is often developed. The non-aerated flow region becomes critical to embankment dams retrofitted with stepped spillways. The inception point, the location where the turbulent boundary layer reaches the free surface, defines where the non-aerated flow ends and the aerated flow begins. The inception point is a key spillway design parameter used in energy dissipation, flow depth, and air entrainment prediction relationships. Chanson developed an inception point relationship for gravity (theta greater than or equal to 26.6 degrees) stepped spillways with an ogee crest control section. However, Chanson's relationship tends to overestimate the distance from the downstream edge of a broad-crested weir to the inception point for stepped spillways (theta less than or equal to 26.6 degrees) when the Froude surface roughness is less than 10. Meireles and Matos optimized Chanson's relationship for broad-crested weir stepped spillways retrofitted for embankment dams, but the relationship is only valid for 1.9 less than or equal to the Froude surface roughness less than or equal to 10. New relationships for predicting the inception point location for broad-crested stepped spillways (theta less than or equal to 26.6 degrees) typically designed for embankment dams for a wide range of flow conditions such that the F* less than or equal to 100 and Froude surface roughness greater than 100 are presented in this paper.