Free Overfall Spillways
Free overfall spillways (including ogee crests, various shaped weirs, and straight drop control structures): These types of spillways are structures where the flow drops freely from the crest. These types of control structures are suited for service, auxiliary, and emergency spillways.[1]
Ogee Crest Control Structures and Various Shaped Weirs Control Structures
This type of control structure is suited to a concrete arch dam or to a crest that has a steep downstream face. This spillway can be gated or ungated. Flows may be free discharging (as is the case with a sharp-crested weir), or the flows may be supported along a narrow section of crest (such as an ogee crest that immediately terminates at a lip or flip that directs the free jet downstream). For free overfall spillways, the flow undernappe should be ventilated sufficiently to prevent a pulsating, fluctuating jet. Of note, where no artificial protection (such as an armored plunge pool) is provided, scour of the streambeds may occur and form (erode) a plunge pool. Where erosion cannot be tolerated or needs to be controlled, an artificial pool can be constructed. Examples of free overfall (ogee crest) spillways are the service spillways at Reclamation’s Crystal Dam (concrete) and Pueblo Dam (composite). Example of free overflow (various shaped weir) are the modified dam crests of Reclamation’s Buffalo Bill Dam (concrete) and Gibson Dam (concrete), which serve as auxiliary spillways.[1]
Straight Ogee Control Structures
This type of control structure tends to have considerable reserve discharge capacity (i.e., increased discharge due to elevated RWS). Also, straight ogee control structures are relatively free of operation and maintenance issues. This type of spillway is applicable to concrete, embankment, and composite dams, and it can be gated or ungated. Examples of straight ogee spillways include the service spillways at Reclamation’s Scofield Dam (embankment) and Sugar Pine Dam (embankment).[1]
Curved Ogee Control Structures
This type of control structure is influenced by similar considerations as the straight ogee control structure. In addition, a curved control structure lends itself to rapid narrowing of the downstream conveyance feature, which helps to minimize excavation or allow transition to a tunnel conveyance feature. These types of spillways are applicable to concrete, embankment, and composite dams, and they can be gated or ungated. Examples of curved ogee spillways include the service spillways at Reclamation’s Casitas Dam (embankment) and Meeks Cabin Dam (embankment).[1]
Straight Drop Control Structures
This type of control structure can be very effective over a wide range of tailwater depths and is applicable for low embankment dams. It consists principally of a straight wall (sharp crested) weir set at the upper end of a rectangular chute section, with an apron placed below streambed, and includes floor blocks and an end sill. This type of spillway is not applicable to high drops (large hydraulic head) on unstable foundations. Ordinarily, this spillway type should be limited to no more than a hydraulic head drop of 20 feet (distance between the reservoir and the tailwater surfaces). Examples of the free-flow (straight drop) spillway are the emergency spillway at Reclamation’s Trial Lake Dam (embankment) and the service spillway at the National Park Service’s PEEC’s Dam (embankment).[1]
Labyrinth Weir Spillways
These types of control structures are suited for service and auxiliary spillways. The spillway provides added crest length for a given total crest width, so less hydraulic head (than a straight weir) is needed to pass a given discharge. The additional crest length is obtained by a series of trapezoidal, rectangular, or triangular walls within the total width. These walls are thin and cantilevered, vertical on the upstream face and steeply sloped on the downstream slope. Labyrinth weir control structures can be considered where space is limited, large discharge associated with small hydraulic head is needed, and there is adequate foundation (typically rock). However, larger hydraulic head than design levels can result in reduced discharge efficiencies (i.e., acting more like a broad-crested weir with reduced effective crest length rather than a sharp-crested weir with extended crest length). These types of control structures are used with chute conveyance features. An example of a labyrinth weir spillway is the service spillway at the New Mexico Interstate Stream Commission’s Ute Dam (embankment).[1]
Orifice Headwall Spillways
These types of control structures are suited for service, auxiliary, and emergency spillways. A variation of the orifice spillway, which has been successfully used in modifying existing straight ogee spillways, involves the construction of a headwall above the ogee crest or various shaped weirs. The opening between the bottom of the headwall and the crest creates orifice control during elevated reservoir water surfaces. This type of modification has been very effective in limiting maximum spillway discharges to no more than the original design discharge capacity even with reservoir water surfaces greater than the original design maximum reservoir water surface. Also, these types of spillways are applicable to concrete, embankment, and composite dams. An example includes the service spillway at Reclamation’s Glendo Dam.[1]
Overtopping Protection Structures
These types of structures are suited for auxiliary and emergency spillways. Overtopping protection should only be considered if there are no other technically viable and cost-effective options to safely pass flood events. Overtopping protection can apply to concrete, embankment, and composite dams. Overtopping protection generally applies when there is some combination of remote chance of operation, physical or environmental constraints of constructing other alternatives, and/or prohibitive cost of other alternatives. Overtopping protection applications could include the following:[1]
- "For embankment dams or embankment portion of composite dams: Overtopping protection is placed over the embankment and at the downstream toe of the dam to limit erosion during overtopping. Overtopping protection materials include RCC, conventional or mass concrete, precast concrete blocks, gabions, riprap, turf reinforcement mats, vegetative cover, flow-through rockfill, reinforced rockfill, geomembranes, geocells, and fabric-formed concrete. [1]
- "For concrete dams or concrete portion of composite dams: Overtopping protection is typically placed on the abutments and at the downstream toe of the dam where erosion might compromise the dam foundation. Overtopping protection materials include RCC, conventional or mass concrete, foundation and abutment reinforcement, abutment, and plunge pool erosion protection.[1]
Tunnel Inlet Spillways
These types of control structures are suited for service and auxiliary spillways. The tunnel inlet spillways are applicable to situations where there is a small amount of space to locate the control structure, there is adequate rock foundation, and the conveyance feature will be a tunnel. These types of spillways are applicable to concrete, embankment, and composite dams, and they can be gated or ungated. The control structure will include a geometry transition from a crest structure to a circular tunnel section. The tunnel inlet control structures have included ogee crests, side-channel and bathtub features, and various shaped weirs. Also, this spillway has the potential of moderate to large discharge capacity. Larger discharges than design levels can result in hydraulic control shifts (crest to orifice and/or orifice to pipe control) that could result in adverse hydraulics in the downstream tunnel (slug and/or pressure flow). Examples of tunnel inlet spillways include the service spillways at Reclamation’s Kortes Dam (concrete) and Twitchell Dam (embankment).[1]
Culvert Spillways
These types of control structures are suited for service, auxiliary, and emergency spillways. The culvert spillways are most applicable as appurtenant structures for low head dams (i.e., hydraulic head is 25 feet or less). Although there is simplicity and economy of construction, there are some significant potential concerns that must be fully addressed. These concerns include: (1) under certain conditions, the culvert may operate as a siphon, which can lead to adverse hydraulics (sudden surges and stoppages of flow, outflow exceeds inflow if operation shifts from inlet control to exit control, and significant vibrations that could damage the culvert and its foundation); (2) culverts on steep slopes flowing full can lead to negative pressures along the boundaries of the culvert, resulting in potential cavitation issues; and (3) if there are cracks or joints in low pressure areas, there is a possibility of drawing in soils surrounding the culvert. A culvert spillway does not have sizable reserve discharge capacity (i.e., increased discharge due to elevated reservoir water surface). Also, a culvert spillway is more susceptible to debris blockage. Examples of culvert spillways include the service spillway at Reclamation’s Martinez Dam (embankment) and the emergency spillway at Reclamation’s Weber Basin Comber Dam (embankment)”.[1]
Best Practices Resources
Citations:
Revision ID: 7127
Revision Date: 07/11/2023