Controlled Spillways
“A controlled crest is one that includes gates which are used to control the flow; the uncontrolled crest is one unencumbered by gates” (EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992).
“Controlled spillways include crest gates that serve as a movable damming surface allowing the spillway crest to be located below the normal operating level of a reservoir or channel”.[1]
Types of Controlled Spillways
“Controlled spillways that are not integral with a concrete dam (i.e., located away from the dam on or through abutments, or on or through reservoir rim), existing topography must provide adequate space without excessive excavation. Also, the existing topography must allow appropriate orientation (alignment of the spillway between the reservoir and downstream river or stream) of the conveyance feature and terminal structure. Additionally, economics will come into play for all controlled spillways”.[2]
“Fuseplug Spillways – These types of control structures are associated with staged releases and suited for auxiliary and emergency spillways. A fuseplug control structure may include one or multiple bays containing the fuseplug embankments. The zoned embankments will be placed to a specified height and include a pilot channel through each embankment crest set to a specified elevation associated with a given flood event. The pilot channel(s) and the number of fuseplug embankments are set to limit the discharge for given ranges of reservoir water surfaces. Fuseplug spillway should only operate during remote flood events, where more frequent flood events are accommodated by reservoir flood surcharge and discharge from service spillways and/or outlet works. An important consideration is the very large discharge capacity associated with operation from a small increase in reservoir water surface. The large amount of flow that could be released from an operating fuseplug must be carefully evaluated in terms of downstream impacts. Other considerations include evaluating and mitigating erosion potential both upstream (due to reservoir wave actions) and downstream of the fuseplug that could lead to headcutting and undermining the fuseplug control structure. Also, careful and thorough design and construction of the embankments are needed to ensure proper operation of the fuseplug spillway. Additionally, note that once a fuseplug spillway operates, the reservoir cannot be maintained above the control structure crest (base of the control structure containing the fuseplug embankments). This could result in the loss of reservoir storage until the fuseplug embankment(s) has been reconstructed. Finally, this type of spillway is applicable to concrete, embankment, and composite dams”.[2]
“Fusegate Spillways – These types of control structures are associated with staged releases and suited for auxiliary and emergency spillways. Fusegate spillways are a proprietary system, and its inclusion in this design standard should not be viewed as an endorsement by Reclamation. This spillway type provides a means of passing more frequent smaller flood events by overtopping the fusegates (via labyrinth or straight weir) and more remote larger flood events by tipping and displacing sections of the fusegate spillway. This type of control structure provides the ability to increase (maximize) reservoir storage and/or discharge capacity. An important consideration is the potentially large discharge capacity associated with operation from a small increase in reservoir water surface (particularly during remote large flood events that result in tipping and displacing sections of the fusegate spillway). The amount of flow that could be released from an operating fusegate must be carefully evaluated in terms of downstream impacts. Another consideration includes evaluating and mitigating erosion potential downstream of the fusegate that could lead to headcutting and undermining the fusegate control structure. Also, as was noted for the fuseplug spillway, note that once a fusegate spillway operates, the reservoir cannot be maintained above the control structure crest (base of the control structure containing the fusegates). This could result in the loss of reservoir storage until the fusegates have been reinstalled or replaced. Finally, this type of spillway is applicable to concrete, embankment, and composite dams. Examples of fusegate spillways include auxiliary spillways at the U.S. Army Corps of Engineers’ Terminus Dam (embankment) and Canton Dam (embankment).[2]
“Gated spillways – These types of control structures include drop inlet, free overfall, ogee crest, side-channel and bathtub, tunnel inlet, various shaped weirs, and orifice. These control structures are associated with regulated releases and suited for service, auxiliary, and emergency spillways. The most frequently used gates include radial gates, drum gates, wheel-mounted gates, and crest gates. Considerations include a firm foundation (typically rock), high degree of reliability of gate operation, limiting debris blockage potential, and favorable economics. In addition, gated spillways provide increased control of releases for a given reservoir water surface, allowing increased discharge where reservoir storage is limited, and/or to reduce the amount of reservoir water surface rise during a flood event. These considerations would apply to large volume inflows, where there is a relatively small reservoir storage capacity. Examples of these gated spillways include:
- "Gated drop inlet spillway - This type of control structure is mostly associated with a service spillway with radial gates. An example of a gated drop inlet spillway is Reclamation’s Gibson Dam (concrete). Other examples include the service spillways with ring gates at Reclamation’s Hungry Horse and Owyhee Dams (both concrete).[2]
- Gated free overfall spillway - This type of control structure is mostly associated with a service spillway with roller gates. An example of a gated free overall spillway is Reclamation’s Parker Dam (concrete). [2]
- Gated ogee crest spillways - This type of control structure is associated with a service and auxiliary spillway with gates. Examples of gated ogee crest spillways are Reclamation’s Shasta Dam (concrete) with drum gates and auxiliary spillway with radial gates at Reclamation’s Stewart Mountain Dam (concrete). Other examples include service and emergency spillways with radial gates at Reclamation’s Folsom Dam (composite) and the service spillway with fixed-wheel gates at Reclamation’s Keswick Dam (composite).[2]
Gated side-channel spillway - This type of control structure is mostly associated with a service spillway with gates. An example of a gated side-channel spillway is Reclamation’s Arrowrock Dam (concrete), which has drum gates.[2] Gated tunnel inlet spillway - This type of control structure is mostly associated with a service spillway with gates. An example of a gated tunnel inlet spillway is Reclamation’s Seminoe Dam (concrete) with fixed wheel gates. Other examples include the service spillways with radial gates at Reclamation’s Glen Canyon Dam and the service spillways with drum gates at Reclamation’s Hoover Dam.[2]
- Gated various shaped weir spillway - This type of control structure is mostly associated with a service spillway with gates. An example of a gated various shaped weir spillway is Reclamation’s Hyrum Dam (embankment) with radial gates.[2]
- Gated orifice spillway - With this type of control structure, flow is typically released from the spillway by one of two approaches:
- Siphon Spillway - These types of control structures are suited for service, auxiliary and emergency spillways, and/or outlet works. Siphon spillways have been used to help pass excess inflows (i.e., augment other hydraulic structure discharge capacity). These spillways can be designed to be self-priming or manually primed. The siphon has relatively small discharge capacity (pressurized operations), limited ability to drain the reservoir (limited hydrostatic head to less than atmospheric pressure, or about 30 feet), and is generally not suitable for cold weather climates (i.e., susceptible to ice blockage). Examples of siphon spillways include the auxiliary spillway at Reclamation’s McKay Dam (embankment) and the service spillway at Reclamation’s Salmon Lake Dam (embankment). Also, siphon spillways have been used to provide discharge capacity for small embankment dams to augment or replace existing outlet works. Considerations include fairly rapid installation involving shallow excavation through dam crest, and the reservoir does not need to be drained. Examples of siphon spillways include the service spillways at the BIA’s Horseshoe Cienega Dam (embankment) and Tsaile Dam (embankment).[2]
Examples
Best Practices Resources
Design of Spillway Tainter Gates (EM 1110-2-2702)
Design Standard No. 14: Appurtenant Structures for Dams (Spillways and Outlet Works)
Trainings
Citations:
Revision ID: 2420
Revision Date: 09/07/2022