Spillways: Difference between revisions

Latest revision as of 18:00, 28 July 2023

Labyrinth weir spillways are commonly used to increase discharge capacity at existing dams.

(Image Source: Gannett Fleming)

“The basic purpose of the spillway is to provide a means of controlling the flow and providing conveyance from reservoir to tailwater for all flood discharges up to the spillway design flood (SDF). The spillway can be used to provide flood control regulation for floods either in combination with flood control sluices or outlet works, or in some cases, as the only flood control facility.^[1]

“One of the most common causes of dam failures is the inability to safely pass flood flows. Failures caused by hydrologic conditions can range from sudden failure, with complete breaching or collapse of the dam, to gradual failure, with progressive erosion and partial breaching”.^[2]

“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”.^[1]

Anatomy of a Spillway

Types of Spillways (Classified by Operation)

Types of Spillways (Classified by Function)

Examples

Learn about the importance of adequate spillway capacity from the failure of Laurel Run Dam (DamFailures.org)

Best Practices Resources

Selecting and Accommodating Inflow Design Floods for Dams (FEMA P-94), FEMA

Technical Release 210-60: Earth Dams and Reservoirs, NRCS

Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 2- Selecting and Accommodating Inflow Design Floods for Dams, FERC

Design Standards No. 14: Appurtenant Structures for Dams (Ch. 1: Introduction), USBR

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

Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420), USACE

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

Trainings

ASDSO Dam Owner Academy: Spillways & Outlet Works

On-Demand Webinar: Inspection and Assessment of Spillways

On-Demand Webinar: Introduction to Addressing Inadequate Conveyance Capacity at Dams

On-Demand Webinar: Hydraulics 101: Intro to Hydraulics for Dam Safety

On-Demand Webinar: Hydraulics 201 for Dam Safety

On-Demand Webinar: Inlet and Outlet Hydraulics for Spillways and Outlet Structures

On-Demand Webinar: Designing Spillways to Mitigate Failure Modes

Citations:

Revision ID: 7471
Revision Date: 07/28/2023

[EM_1110-2-1603-1] 1.0 ^1.1 EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992

[[[Fema_94|FEMA_94]]-2] Federal Guidelines for Dam Safety: Selecting and Accommodating Inflow Design Floods for Dams (FEMA-P94), FEMA, 2013

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[2]

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+[[Category:Water Conveyance]]
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-“The basic purpose of the spillway is to provide a means of controlling the flow and providing conveyance from reservoir to tailwater for all flood discharges up to the spillway design flood (SDF). The spillway can be used to provide flood=control regulation for floods either in combination with flood-control sluices or outlet works, or in some cases, as the only flood-control facility.<ref name="EM 1110-2-1603">[[Hydraulic Design of Spillways (EM 1110-2-1603) | EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992]]</ref>
-“One of the most common causes of dam failures is the inability to safely pass flood flows. Failures caused by hydrologic conditions can range from sudden failure, with complete breaching or collapse of the dam, to gradual failure, with progressive erosion and partial breaching”.<ref name="FEMA 94">[[Selecting and Accommodating Inflow Design Floods for Dams (FEMA P-94) | Federal Guidelines for Dam Safety: Selecting and Accommodating Inflow Design Floods for Dams (FEMA-P94), FEMA, 2013]]</ref>
+{{Picture
+<!-- Add image file name (ex.image.jpg) -->
+|image=BDCD.jpg
+<!--Add link if applicable -->
+|link=
+<!-- Add picture caption -->
+|caption=Labyrinth weir spillways are commonly used to increase discharge capacity at existing dams.
+(Image Source: Gannett Fleming)
+}}
-“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”.<ref name="EM 1110-2-1603" />
-==Anatomy of a Spillway==
+“The basic purpose of the spillway is to provide a means of controlling the flow and providing conveyance from reservoir to tailwater for all flood discharges up to the spillway design flood (SDF). The spillway can be used to provide flood control regulation for floods either in combination with flood control sluices or [[Outlet Works|outlet works]], or in some cases, as the only flood control facility.<ref name="EM 1110-2-1603">[[Hydraulic Design of Spillways (EM 1110-2-1603) | EM 1110-2-1603 Hydraulic Design of Spillways, USACE, 1992]]</ref>
-#[[Spillway Approach | Approach]]
-#[[Control Section]]
-#[[Spillway Conveyance | Conveyance]]
-#[[Energy Dissipation]]
-==Types of Energy Dissipation==
+“One of the most common causes of dam failures is the inability to safely pass flood flows. Failures caused by hydrologic conditions can range from sudden failure, with complete breaching or collapse of the dam, to gradual failure, with progressive erosion and partial breaching”.<ref name="[[Fema 94|FEMA 94]]">[[Selecting and Accommodating Inflow Design Floods for Dams (FEMA P-94) | Federal Guidelines for Dam Safety: Selecting and Accommodating Inflow Design Floods for Dams (FEMA-P94), FEMA, 2013]]</ref>
-“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 safely at high discharge 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”.<ref name="EM 1110-2-1603" />
-Outlet erosion control structures such as headwalls/endwalls, impact basins, USBR Type II or Type III basins, baffled chutes, or plunge pools are all considered energy dissipating devices.<ref name="ASDSO">[https://damsafety.org/dam-owners/outlet-erosion-control-structures ASDSO, 2022]</ref>
+“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”.<ref name="EM 1110-2-1603" />
-Headwalls/endwalls located at or close to the end of the discharge conduit provide support and reduce the potential for undermining. Headwalls/endwalls are typically made of concrete and should be founded on bedrock or have an adequate foundation footing to provide support for stability. A headwall/endwall can experience undermining and become displaced if not adequately designed. Displacement of the headwall/endwall can lead to separation from the conduit at the joints which could affect the structural integrity of the conduit itself.<ref name="ASDSO" />
+==Anatomy of a Spillway==
+#[[Spillway Approach Hydraulics|Approach]]
-A concrete impact basin is an energy dissipating device located at the outlet of the spillway in which flow from the discharge conduit strikes a vertical hanging baffle. Energy dissipation occurs as the discharge strikes the baffle; thus, performance is not dependent on tailwater elevation. Most impact basins were designed by the NRCS and the USBR.<ref name="ASDSO" />
+#[[Spillway Control Structures|Control Section]]
+#[[Spillway Chute Hydraulics|Chute]]
-Type II and Type III basins reduce the energy of the flow discharging from a spillway and allow the water to enter the outlet channel at a reduced velocity. Type II basins contain chute blocks at the upstream end and a dentated (tooth-like) endsill. Baffle piers are not used in a Type II basin because of the high velocity water entering the basin. Type III energy dissipators can be used if the entering flow velocity is not high (less than 50 feet per second (Hydraulic Design of Stilling Basins and Energy Dissipators, USBR, 1984)). Type III basins contain baffle piers which are located on the stilling basin apron downstream of the chute blocks. Located at the end of both types of USBR stilling basins is an endsill, which may be leveled or sloped, that creates a tailwater to reduce the velocity of the flow leaving the basin. Baffled chutes require no initial tailwater to operate effectively and are located downstream of the control section. Multiple rows of baffle piers on the chute prevent excessive acceleration of the flow and prevent the damage that occurs from a high discharge velocity. A portion of the baffled chute usually extends below the streambed elevation to prevent undermining of the chute. A plunge pool is an energy dissipating device located below the outlet of a spillway. Energy is dissipated as the discharge falls into the plunge pool as a free-flowing jet. Plunge pools are commonly lined with rock riprap or other material to prevent excessive erosion of the pool area. Discharge from the plunge pool should be at the natural streambed elevation. Typical problems may include movement of the riprap, loss of fines from the bedding material, and scour beyond the riprap and lining. If scour beneath the outlet conduit develops, the conduit will be left unsupported and separation of the conduit joints and undermining may occur.<ref name="ASDSO" />
+#[[Energy Dissipation]]
 ==Types of Spillways (Classified by Operation)==
@@ Line 33: / Line 35: @@
 #[[Auxiliary Spillways]]
 #[[Emergency Spillways]]
+See also: [[Hydraulics]] > [[Hydraulic Performance of Spillways]]
 ==Examples==
-{{Website Icon}}
+{{Website Icon}} [https://damfailures.org/case-study/laurel-run-dam-pennsylvania-1977/ Learn about the importance of adequate spillway capacity from the failure of Laurel Run Dam (DamFailures.org)]
-==Best Practices Resources==
+<noautolinks>==Best Practices Resources==</noautolinks>
-{{Document Icon}}
+{{Document Icon}} [[Selecting and Accommodating Inflow Design Floods for Dams (FEMA P-94) | Selecting and Accommodating Inflow Design Floods for Dams (FEMA P-94), FEMA]]
+{{Document Icon}} [[Technical Release 210-60: Earth Dams and Reservoirs | Technical Release 210-60: Earth Dams and Reservoirs, NRCS]]
+{{Document Icon}} [[Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 2- Selecting and Accommodating Inflow Design Floods for Dams | Engineering Guidelines for the Evaluation of Hydropower Projects: Chapter 2- Selecting and Accommodating Inflow Design Floods for Dams, FERC]]
+{{Document Icon}} [[Design Standards No. 14: Appurtenant Structures for Dams (Ch. 1: Introduction) | Design Standards No. 14: Appurtenant Structures for Dams (Ch. 1: Introduction), USBR]]
+{{Document Icon}} [[Design Standards No. 14: Appurtenant Structures for Dams (Ch. 3: General Spillway Design Considerations) | Design Standards No. 14: Appurtenant Structures for Dams (Ch. 3: General Spillway Design Considerations), USBR]]
+{{Document Icon}} [[Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420) | Hydrologic Engineering Requirements for Reservoirs (EM 1110-2-1420), USACE]]
+{{Document Icon}} [[Hydraulic Design of Spillways (EM 1110-2-1603) | Hydraulic Design of Spillways (EM 1110-2-1603), USACE]]
 ==Trainings==
+{{Video Icon}} [https://youtu.be/TuQUf-nieVY ASDSO Dam Owner Academy: Spillways & Outlet Works]
 {{Video Icon}} [[On-Demand Webinar: Inspection and Assessment of Spillways]]
+{{Video Icon}} [[On-Demand Webinar: Introduction to Addressing Inadequate Conveyance Capacity at Dams]]
+{{Video Icon}} [[On-Demand Webinar: Hydraulics 101: Intro to Hydraulics for Dam Safety]]
+{{Video Icon}} [[On-Demand Webinar: Hydraulics 201 for Dam Safety]]
+{{Video Icon}} [[On-Demand Webinar: Inlet and Outlet Hydraulics for Spillways and Outlet Structures]]
+{{Video Icon}} [[On-Demand Webinar: Designing Spillways to Mitigate Failure Modes]]
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