Stepped chute design: Does step shape really matter?

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: State Dam Safety Officials Association Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 9/1/2019
Publication Date: 9/11/2019
Citation: Hunt, S.L., Kadavy, K.C. 2019. Stepped chute design: Does step shape really matter?. In: Dam Safety 2019. Proc. of the Assoc. of State Dam Safety Officials Annual Conf., 09/08-11/19, Orlando, FL. 3 p.

Interpretive Summary:

Technical Abstract: More than half of the dams constructed under the USDA Small Watershed Program authority have reached the end of their planned service life. These aging dams have experienced changes in demographics (e.g. residential and industry); in turn, altering the hydrology for which they were originally designed. Sediment pools in the upstream reservoirs have filled with sediment now depositing within the flood pool, and structure deterioration has occurred. Subsequently, the risk potential for dam overtopping and the consequences associated with dam failure have increased. These issues combined with financial constraints can limit dam rehabilitation options. Scientists at the USDA-Agricultural Research Service (ARS) Hydraulic Engineering Research Unit (HERU) in Stillwater, Oklahoma have developed stepped spillway design guidance as an overtopping protection measure for embankment dams. While the design guidance is comprehensive, new questions arise as new and/or improved construction techniques are used to place roller compacted concrete, the construction material of choice for stepped spillways. For instance, does step shape really matter regarding the hydraulic performance of stepped chutes? To address this question, HERU scientists expanded their research to address the performance of stepped spillways when the step shape is altered. Scientists conducted tests in a large-scale stepped spillway research facility to examine the affect beveled face steps have on the developing free-surface air entrainment inception point as compared to vertical face steps placed in the chute. Steps approximately 152-mm high with a 1(H):1(V) beveled face were tested for a range of unit discharges (e.g. 0.15 less than or equal to q less than or equal to 1.83 m3/(s·m)). Steps with beveled faces created a rougher chute surface, causing the free-surface air entrainment inception point to develop upstream of the free-surface air entrainment inception point created by vertical face steps of similar heights and tested under similar discharge conditions. This research provides practicing engineers with improved knowledge and understanding on the affect the step shape has on the performance of stepped chutes especially for stepped chutes where the steps may be exposed to cyclic freeze/thaw conditions that may alter their shape.