February 2025 USSD Webinar: Physical Modeling to Aid Design of Raised Gross Dam, Colorado

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When completed in 2027, the stepped spillway for raised Gross Dam, Colorado will be the tallest and steepest stepped spillway in the world (slope 2V:1H, height = 456 ft or 139 m). The raising (heightening) of the dam to increase reservoir capacity led to the need to adjust the design of the spillway’s energy-dissipation basin from an existing standard design of basin. The new spillway was designed by AECOM aided by the Hydraulics Laboratory of Colorado State University.

The webinar will focus on the physical modeling used to assist the design of the new spillway and its energy-dissipation basin. The modeling used a 1:24 length-scale hydraulic model supported by a FLOW-3D numerical model, used mainly to check uniformity of flow at the spillway’s entrance. The effects of the spillway steepness and the spillway chute’s convergence (about 20% from top to base), and various site constraints, created a new form of energy-dissipation basin: a compact basin and fitted with a single row of baffle blocks that blended aspects of a slotted roller bucket and a baffled stilling basin. Pressures were measured in the basin to assist estimation of loads and related to flow through the basin.

Also discussed are insights regarding air entrainment by flow down the spillway, whose chute convergence resulted in non-uniform flow depth across the chute. Measurements showed that flow depth along the chute’s centerline was slightly less than flow depth at the sidewall but contained slightly less entrained air. The depth-averaged concentration (volumetric proportion of bulked flow) of entrained air near the end of the chute decreased with increasing water discharge and was less than the value suggested by the literature. For the spillway’s design discharge, the streamwise values of air concentration and flow depth were essentially constant across the chute’s end.

This webinar is given with the permission of Denver Water (owner of Gross Dam and Reservoir) and AECOM, and the webinar’s topics are among the contemporary developments in hydraulic engineering for dams.

Dr. Robert Ettema

Sr. Research Engineer

Colorado State University

Dr. Robert Ettema has extensive hydraulic-engineering expertise in design and performance of hydraulic structures (e.g., dams, bridges, and intakes), including ice effects. He attained the Ph.D. degree in civil engineering at Auckland University, New Zealand, in 1980. Presently, he is a Sr. Research Engineer at CSU’s Hydraulics Laboratory, prior to which he worked as a professor at Colorado State University (9 years at CSU) and the University of Wyoming (8 years, of which 6 years were as Engineering Dean). Prior to these positions, he was a faculty member and research engineer at the University of Iowa (27 years).

Dr. Ettema has served as Editor of the American Society of Civil Engineer’s (ASCE’s) Journal of Hydraulic Engineering (1998-2002). In 2002, he obtained ASCE’s Hunter Rouse Award in engineering hydraulics. He got ASCE’s 2011 Karl Emil Hilgard Hydraulic Prize for best paper in the Journal or Hydraulic Engineering, and in 2024 got an award for the journal’s Best Technical Note. Earlier, in 1991 he received the Gustave Willems Best Paper Award from the U.S. Section of Permanent Association for Navigation Congresses (PIANC). In 2015, he got ASCE’s Hans Albert Einstein Award for Sedimentation Engineering, and in 2019 gained ASCE’s Harold R. Peyton Award for Cold Regions Engineering. He has served (2019-2023) as a Vice-President (Americas) of the Intl. Assoc. for Hydro-Environment Engineering and Research (IAHR; formerly Intl. Assoc. for Hydraulic Research) and is a Fellow of ASCE and Engineers New Zealand. He served on the Investigative Panel of the Assoc. of State Dam Safety Officials (ASDSO) for the failure of Spencer Dam.

Yongqiang (Frank) Lan, Ph.D., P.E.

Principal Water Resources Engineer/Project Manager

AECOM

Frank (Yongqiang) Lan, Ph.D, PE. Dr. Lan is a Principal Engineer with AECOM with more than 30 years of successful consulting experience. He holds a PhD in Civil Engineering. His expertise includes H&H analysis and modeling, hydraulic structure design, river mechanics, sediment transport, physical modeling, dam breach and flood inundation modeling. He is an expert in CFD modeling to the design of dam and spillways, intakes, outlets, pump stations, and others and has applied CFD for more than 120 dam and spillway projects in the US and around the world.