Inception point relationship for flat-sloped stepped spillways

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/8/2010
Publication Date: 1/18/2011
Citation: Hunt, S., Kadavy, K.C. 2011. Inception point relationship for flat-sloped stepped spillways. Journal of Hydraulic Engineering. 137(2):262-266.

Interpretive Summary: The USDA-Natural Resources Conservation Service provided both technical and financial assistance for the construction of more than 11,000 small earthen dams. Many of these were originally constructed to protect agricultural land. Today, homes, businesses, roads, bridges, and other infrastructure have been built in the vicinity of these dams, and flood protection has become ever so more important due to public safety. Changes in the environment surrounding these dams can have a direct impact on the functionality of the dam. In some cases, these dams no longer meet federal and state dam safety regulations. Consequently, physical changes to the dam may be required. One of the more common requirements for these dams is increased spillway capacity, and the use of roller compacted concrete (RCC) stepped spillways are becoming a popular choice for addressing this issue. Yet, design guidelines for RCC stepped spillways applied to small earthen dams are limited, leaving engineers with questions regarding the design of these structures. To answer some of these questions, engineers with the USDA-ARS Hydraulic Engineering Research Unit have been leading research on stepped spillways applied to embankment dams. Changes in the surface roughness of a stepped spillway alter the location where white water appears in the flow surface. Knowing the location where the white water begins is important because downstream of this point, the flow depth is expected to be greater than the flow depth upstream of this point. This greater flow depth downstream is likely to require taller walls to contain the flow in the spillway. The location where white water begins also factors into determining the energy dissipation in the spillway chute. An equation developed by Hubert Chanson may be used to determine the location of where white water begins if the surface roughness is relatively small. A new equation was developed to encompass a broader range of surface roughnesses. This new relationship is similar to Chanson’s, but it is optimized for flatter-sloped stepped spillways. This research is expected to assist engineers with the design of stepped spillways applied to embankment dams.

Technical Abstract: Many small earthen embankments are faced with hazard classification changes due to urban encroachment. As a result, some embankments have inadequate spillway capacity. To bring the dam into compliance with state and federal dam safety laws, rehabilitation of the dam is often required. RCC stepped spillways are becoming a popular choice for addressing these issues. However, design guidelines for RCC stepped spillways applied to small earthen dams are scarce, especially for spillways with slopes flatter than 2(H):1(V). A two-dimensional, physical model was constructed to evaluate the air entrainment inception point location in a 4(H):1(V) stepped spillway. Step heights of 38 mm, 76 mm, and 152 mm were evaluated. The physical model was constructed with a broad-crested weir, and model unit discharges ranging from 0.11 m3/(s·m) to 0.82 m3/(s·m) were tested. The research findings show that Chanson’s relationship effectively predicts the location of the inception point for slopes as flat as 4(H):1(V) for Froude surface roughness values (F*) greater than 10, which in this study corresponds to model step heights of 38 mm and 76 mm. Chanson’s relationship did not adequately predict the location of the inception point for F* less than 10 which for this study corresponds to a model step height of 152 mm. A new relationship for predicting the location of the inception point was developed from this data set, and it is applicable for flat sloped stepped spillways with F* ranging from 1 to 100. This relationship is similar to Chanson’s, but it is optimized for flat-sloped stepped spillways with a broad-crested weir crest section. This research is expected to assist engineers with the design of stepped spillways applied on relatively flat embankment dams.