Spillway Approach Hydraulics

---

“Conveyance features located immediately upstream and downstream of a control structure include approach channels, inlet structures, chutes, conduits, and/or tunnels. These conveyance features pass flow from the reservoir to the control structure, as well as pass flow from the control structure to the terminal structure. The conveyance feature (such as an approach channel and/or the inlet structure) located immediately upstream of the control structure generally has a different level of concern in terms of significant loading conditions that could lead to damage or failure of this feature resulting in an uncontrolled release of the reservoir. However, it should be noted that (hydraulic) head losses associated with the approach channel and/or the inlet structure should be accounted for in the computation of the discharge capacity of a spillway. It is also important that the approach channel configuration not be vulnerable to stability issues (such as slope failure during saturated conditions) so that head losses through the approach channel will not increase during flood operations."

“Spillway approach configuration will influence the abutment contraction coefficient, the nappe profile, and possibly the flow characteristics throughout the spillway chute and stilling basin. There are three general configurations for the spillway approach, each of which requires a different treatment at the abutments in order to provide acceptable spillway characteristics”. These generally consist of spillways with a deep approach, shallow approach, or confined approach.

“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.”

“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.

Best Practices Resources

Design Standards No. 14: Appurtenant Structures for Dams (Ch. 3: General Spillway Design Considerations), USBR

National Engineering Handbook: Chapter 50 - Earth Spillway Design, NRCS

Hydraulic Design of Spillways (EM 1110-2-1603), USACE

Citations:

Revision ID: 7460
Revision Date: 07/25/2023