Hydraulic Performance of Spillways

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[Page Summary]

Reservoir Routing

“This procedure derives the outflow hydrograph from a reservoir from the inflow hydrograph into the reservoir with consideration of elevation, storage, and discharge characteristics of the reservoir and spillways. The conservation of mass equation is solved with the assumption that outflow discharge and volume of storage are directly related.” “For reservoirs, the relation of surface area and release capacity to storage content must be described. Characteristics of the control gates on the outlets and spillway must be known in order to determine constraints on operation. The top-of-dam elevation must be specified and the ability of the structure to withstand overtopping must be assessed” (EM 1110-2-1420 Hydrologic Engineering Requirements for Reservoirs, USACE, 1997).

Spillway Approach Hydraulics

“Spillway approach configuration will influence the abutment contraction coefficient, the nappe profile, and possibly the flow characteristics throughout the spillway chute and stilling basin” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

“Crest piers, abutments, and approach configurations of a variety of shapes and sizes have been used in conjunction with spillways... Not all of the designs have produced the intended results. Improper designs have led to cavitation damage, drastic reduction in the discharge capacity, unacceptable waves in the spillway chute, and harmonic surges in the spillway bays upstream from the gates. Maintaining the high efficiency of a spillway requires careful design of the spillway crest, the approach configuration, and the piers and abutments. For this reason, when design considerations require departure from established design data, model studies of the spillway system should be accomplished” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

“Another factor influencing the discharge coefficient of a spillway crest is the depth in the approach channel relative to the design head... As the depth of the approach channel … decreases relative to the design head, the effect of approach velocity becomes more significant. The slope of the upstream spillway face also influences the coefficient of discharge… the flatter upstream face slopes tend to produce an increase in the discharge coefficient (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

Spillway Control Structures

“The value of an uncontrolled fixed crest spillway in providing an extremely reliable operation and a very low-cost maintenance facility is undeniable. Topographical, geological, economical, and political considerations at many dam sites may restrict the use of an uncontrolled fixed crest spillway. The solution to these problems is usually the inclusion of crest gates; however, the uncontrolled fixed crest spillway should be used regardless of these considerations when the time of concentration of the basin runoff into the reservoir is less than 12 hours. When the time of concentration is between 12 and 24 hours, an uncontrolled fixed crest spillway should be given preference over a gated spillway. Basically, the inclusion of crest gates allows the spillway crest to be placed significantly below the maximum operating reservoir level, in turn, permitting the entire reservoir to be used for normal operating purposes; and results in a much narrower spillway facility, avoiding the problems associated with high unit discharge/high-velocity flow and increased operation and maintenance costs. A gated spillway must include, as a minimum, two or preferably three spillway gates in order to satisfy safety concerns” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

Spillway Chute Hydraulics

“The chute is that portion of the spillway which connects the crest curve to the terminal structure. The term chute when used in conjunction with a spillway implies that the velocity is supercritical; thus, the Froude number is greater than one. When the spillway is an integral part of a concrete gravity monolith, the chute is usually very steep. Chutes as steep as 1.0 vertical on 0.7 horizontal are not uncommon. The steepness thus minimizes the chute length. Chutes used in conjunction with embankment dams often must be long with a slop slightly steeper than the critical slope. This long, prominent structure is termed a chute spillway. The designs for long spillway chutes and steep chutes on concrete dam monoliths involve many of the same geometric and hydraulic considerations. Due to the extreme slope and short length of a steep chute, many of the hydraulic characteristics that become prominent in spillway chutes have insufficient time to develop prior to reaching the terminal structure” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

“Hydraulic characteristics that must be considered in the design of a chute are the velocity and depth of flow, air entrainment of the flow, pier and abutment waves, floor and wall pressures, cavitation indices, superelevation of the flow surface at curves, and standing waves due to the geometry of the chute. Obtaining acceptable hydraulic characteristics is dependent upon developing proper geometric conditions that include chute floor slope changes, horizontal alignment changes (curves), and sidewall convergence… A model study is recommended to confirm any design that involves complex geometric considerations and/or large discharges and velocities” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

Spillway chutes do not have to be designed with parallel sidewalls. Chutes commonly are designed and constructed with either diverging or converging sidewalls for a variety of site-specific reasons. “The height of a chute sidewall should be designed to contain the flow of the spillway design flood… The computed profile may require adjustment to account for the effects of pier end waves, slug flow or roll waves, and air entrainment. Sidewall freeboard is added above the adjusted profile; as a minimum, two feet of freeboard is recommended” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

Spillway Terminal Structure Hydraulics

“The design of the energy dissipator probably includes more options than any other phase of spillway design. The selection of the type and design details of the dissipator is largely dependent upon the pertinent characteristics of the site, the magnitude of energy to be dissipated, and to a lesser extent upon the duration and frequency of spillway use. Good judgement is imperative to assure that all requirements of the particular project are met. Regardless of the type of dissipator selected, any spillway energy dissipator must operate safety at high discharges for extended periods of time without having to be shut down for emergency repairs. An emergency shutdown of the spillway facility during a large flood could cause overtopping of the dam and/or create unacceptable upstream flooding” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

“Except in some unusual conditions, an exit channel is required to transition between the stilling basin and the main channel of the river. Since dissipation of the entire spillway discharge energy within the stilling basin is not normally accomplished, enlarging the channel width immediately downstream from the (stilling) basin will assist in dissipating the residual energy. Due to the erosive nature of the highly turbulent flow exiting from a stilling basin, protection of the exit channel bed and side slopes is usually required to prevent channel scout and potential undermining of the stilling basin” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).

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Revision ID: 2706
Revision Date: 09/09/2022