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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

  1. Approach
  2. Control Section
  3. Chute
  4. Energy Dissipation

Types of Spillways (Classified by Operation)

  1. Controlled Spillways
  2. Uncontrolled Spillways

Types of Spillways (Classified by Function)

  1. Principal Spillways
  2. Auxiliary Spillways
  3. Emergency Spillways

See also: Hydraulics > Hydraulic Performance of Spillways


Learn about the importance of adequate spillway capacity from the failure of Laurel Run Dam (

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


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


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