Chute Spillways
Types of Chute Spillways
Baffled Apron Spillway
This type of structure is suited for service, auxiliary, and emergency spillways. Baffled apron spillways provide crest control, conveyance, and energy dissipation in one structure. Primary considerations are associated with low to moderate hydraulic heads to ensure the baffles function properly and crest lengths are limited (i.e., significant cost increases as crest length increases). Additionally, this spillway may be practical in areas where there is limited space for a terminal structure such as a hydraulic jump stilling basin. Also, effectiveness (energy dissipation and discharge capacity) of baffled apron spillways can be adversely impacted by debris. An example of a baffled apron spillway is the service spillway at Reclamation’s Conconully Dam (embankment).[1]
Grade Control Sill Spillway
This type of control structure is primarily suited for auxiliary and emergency spillways; however, in some cases, it can function as a service spillway. A grade control sill is a less robust, minimal spillway typically limited to a vertical reinforced concrete wall (sill) that is placed in a trench through an excavated trapezoidal channel. The grade control sill can be constructed in both rock and soil foundations. The grade control sill spillway tends to have limited reserve discharge capacity, given channel armorment could be damaged or fail due to discharges that exceed design levels. Therefore, an important consideration is to limit potential erosion during spillway operation. In summary, this type of spillway should only be used on low head situations where the hydraulic drop (vertical dimension between the reservoir water surface and the downstream river or stream) can be effectively controlled, including limiting erosion potential. Examples of grade control sill spillways include the service spillway at Reclamation’s Crane Prairie Dam (embankment) and the emergency spillway at Reclamation’s Davis Creek Dam (embankment).[1]
Stepped Chute Spillway
This type of structure is suited for service and auxiliary spillways. Stepped spillways refer to the stepped chute portion of the spillway and have primarily been used with RCC dams, which take advantage of the RCC lift construction methods, resulting in offsets on the downstream dam face, creating the spillway steps. These steps can be formed or unformed RCC or capped with conventional concrete. A chute-type crest (flat), along with straight or curved ogee crests, are typically used in combination with stepped chutes. Other applications have involved incorporating both smooth flow surfaces and steps into a reinforced concrete spillway chute and RCC overtopping protection for an earth dam. A consideration for a stepped spillway involves the potential kinetic energy dissipation via the steps, which can reduce the size and type of the terminal structure. However, the kinetic energy dissipation potential may be reduced as flow depth (relative to the step size) increases (due to skimming flows). Also, consideration should be given to evaluating the cavitation potential. Examples of stepped spillways include the service spillway at Maricopa Water District’s Camp Dyer Diversion Dam (concrete), the auxiliary spillway (Joint Federal Project) at Reclamation’s Folsom Dam (composite), and the service spillway at Reclamation’s Upper Stillwater Dam (concrete). [1]
Various Shaped Weirs
This type of structure is suitable for service, auxiliary, and emergency spillways. The hydraulic control is established by various shaped weirs ranging from broad-crested to sharp-crested weirs to no weir (flat bottom or sloping channel). Various shaped weir control structures are not as efficient as an ogee crest control structure, but they still tend to have sizable reserve discharge capacity (i.e., increased discharge due to elevated reservoir water surface). Also, various shaped weir control structures are relatively free of operation and maintenance issues. This type of spillway is applicable to both concrete and embankment dams and can be gated or ungated. Examples of various shaped weir spillways include the auxiliary spillway at Reclamation’s Deerfield Dam (embankment) and the service spillway at Reclamation’s Lost Lake Dam (embankment).[1]
Best Practices Resources
Citations:
Revision ID: 6680
Revision Date: 03/16/2023