Types I, II, III, and IV stilling basin performance for stepped chutes applied to embankment dams

Authors: Hunt, Sherry; Kadavy, Kem

Submitted to: American Society of Civil Engineers Journal of Hydraulic Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/18/2020
Publication Date: 6/1/2021
Citation: Hunt, S.L., Kadavy, K.C. 2021. Types I, II, III, and IV stilling basin performance for stepped chutes applied to embankment dams. American Society of Civil Engineers Journal of Hydraulic Engineering. 147(6). Article 06021004. https://doi.org/10.1061/(ASCE)HY.1943-7900.0001877.
DOI: https://doi.org/10.1061/(ASCE)HY.1943-7900.0001877

Interpretive Summary: Scientists with the United States Department of Agriculture, Agricultural Research Service conducted research on stepped concrete structures used for protecting the downstream slope of embankment dams when excess water from the reservoir spills over the dam. This research is important to address the nearly 6,000 USDA-constructed dams within the USDA Small Watershed Program authority that have surpassed their planned service life. Research was extended to include the concrete outlet basin that is used to calm the excess water before it returns to the downstream channel. Scientists found that previously developed design guidelines for the outlet basin attached to smooth concrete structures used for protecting the downstream embankment dam slope also applies those attached to stepped concrete structures serving the same purpose. This research is intended to help design engineers appropriately size the outlet basin for stepped concrete structures.

Technical Abstract: Scientists at the USDA Agricultural Research Service (ARS) Hydraulic Engineering Research Unit (HERU) conducted research on physical models of stilling basins (i.e., Types I, II, III, and IV) associated with stepped chutes to evaluate performance. Tests were conducted with naturally developing unit discharges (0.236 m2/s = q = 0.930 m2/s) descending a 3(H):1(V) stepped chute flume designed with 0.15-m step heights at near prototype scale and Froude numbers near the stepped chute toe ranging from 4.1 = F = 4.6. Data and visual observations indicated that dissipation features like floor blocks, the end sill, or a dentated end sill effectively reduced wave oscillations and lessened the impact of propagating waves downstream. The study data appear to agree with other published data for stilling basins associated with smooth chutes; thus, design engineers should have confidence in applying the US Bureau of Reclamation (USBR) stilling basin design criteria to basins associated with stepped chutes. Pressure profiles indicated positive values relative to stilling basin floor elevation along the length of all stilling basins. Additionally, pressure peaked near the basin entrance, the floor blocks, and the end sill.