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Title: When air really matters: Flow depth relationships for stepped spillways

Author
item Hunt, Sherry
item Kadavy, Kem
item HANSON, GREGORY - Retired ARS Employee

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 8/1/2013
Publication Date: 9/10/2013
Citation: Hunt, S.L., Kadavy, K.C., Hanson, G.J. 2013. When air really matters: Flow depth relationships for stepped spillways. In: Dam Safety 2013. Proceedings of the Association of State Dam Safety Officials Annual Conference, September 8-12, 2013, Providence, RI. CDROM.

Interpretive Summary:

Technical Abstract: With changing demographics in the vicinity of aging embankment dams, hazard creep, a change in classification from low to significant or high, can become problematic and limit rehabilitation options. The most common deficiency for embankment dams due to hazard creep is inadequate spillway capacity. Stepped chutes are a growing trend in addressing this deficiency, and they may be used for the construction of new dams. Researchers at the USDA-ARS Hydraulic Engineering Research Unit (HERU) in Stillwater, OK, have made great strides in developing generalized design guidelines related to surface inception point (Li), flow depth (y), clear-water flow depth (ycw), average air concentration (Cavg), and energy dissipation for stepped chutes. Upstream of Li, the flow appears smooth and glassy before developing a minor undulating flow pattern. This flow pattern is attributed to turbulence observed at the water surface as well as entrapped air in the flow near Li. Entrained air begins to develop downstream of Li. Cavg is observed to increased rapidly from L/Li =1 to L/Li = 2. For L/Li > 2.0, the flow becomes fully developed air entrained flow with Cavg trending to a constant value for a give slope and step height. Data indicates that Cavg is a function of step height normalized by the critical depth, slope, and the normalized length from the crest, L/Li. Flow depth decreases rapidly from the crest to Li, and it becomes relatively constant downstream of Li for a given slope and step height. Data shows that the normalized flow depth downstream of Li is a function of slope and normalized step height. Upstream of Li, the normalized flow depth is a function of slope, the ratio of step height to critical depth, and the normalized length from the crest, L/Li. The objectives of this paper are to introduce 1) generalized air concentration relationships for stepped chutes downstream of Li and 2) flow depth relationships capable of predicting the flow depth at any location along the stepped chute.