Dam Safety Education - Dams 101

What is a dam?

An impounding structure or dam is defined as a manmade structure, whether a dam across a watercourse or structure outside a watercourse, used or to be used to retain or store waters or other materials.

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Benefits of dams

• Dams enable storage of water for drinking, hydroelectric generation, flood control and recreation.
• Dams enable agriculture in drier regions and provide a more reliable water source for farmers.
• Dams can improve wildlife habitat and provide food for migratory birds.

Parts of a dam

Most dams consist of most or all the components shown below. This image shows components of a typical earthen dam.
Refer to the U.S. Department of the Interior, Bureau of Reclamation Design of Small Dams () for more information about dam components. Source: FEMA P-911

Types of dams

Embankment dams (hydraulic fill, homogeneous or zoned)

• These dams are earthfill or rockfill dams. The strength of this type of dam is partly a function of the materials used in its construction.

Concrete gravity dams

• A concrete gravity dam has a triangular cross section with the base much wider than the crest. The dam is configured to provide enough mass and a sufficiently wide base to resist sliding and overturning in response to the force of water pushing against it. If the upstream face of the dam is sloped, a component of the water force pushes downward on the dam, contributing favorably to the stability of the structure. Concrete gravity dams are often used for hydroelectric power projects.

Concrete buttress dams

• Concrete buttress dams are a specific type of gravity dam. They have a solid upstream side supported on the downstream side by a series of buttresses. Water pressure forces are diverted to the dam foundation through vertical or sloping buttresses.

Concrete arch dams

• Concrete arch dams are usually constructed of a series of vertical blocks that are keyed together; barriers that stop water from flowing are provided between the blocks. These dams have a convex curve into the reservoir to transfer water forces to the abutments.

Roller-compacted concrete (RCC) dams

• RCC is generally defined as a no-slump concrete that is placed by earth-moving equipment and compacted by vibrating rollers in horizontal lifts up to 12 inches thick (USBR, ). RCC can be used to build stability buttresses for masonry, gravity and concrete arch dams; overtopping protection and upstream slope protection for embankment dams; new gravity dams; new spillways and spillway stilling basins; tailrace dikes and overflow weirs.

Masonry dams

• Masonry dams are dams mainly made out of cut quarry stone, brick or concrete blocks that may be joined with a binder such as mortar. They are typically configured as gravity or arch-gravity dams. Masonry dams are most often found in parks and municipal areas.

Lined fill dams

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• Lined fill dams have embankments usually constructed primarily of earthen or rockfill materials. The upstream slope face and the area extending along the impoundment upstream is lined with concrete, geomembranes, asphalt or other low-permeability materials.

Tailings dams (starter dams or dykes; upstream, centerline or downstream construction)

• Tailings dams are used to impound waste materials. They are constructed of industrial or mining waste or waste mineral processing materials. Tailing dams are often the most significant environmental liability for a mining project.

Coal combustion residuals impoundments

• Coal waste dams are constructed to store waste materials. Ash impoundments (also called ash ponds) store ash mixed with water primarily from the combustion of coal. They are considered a waste management facility. In these impoundments, coal ash settles out and is eventually removed or disposed of as slurry or sludge. Water at the surface level is discharged through an outlet structure to a nearby stream or water-processing plant.

Other types of dams

Timber dams

• Timber dams are commonly constructed for agricultural uses, such as livestock ponds. Over time, the wood can weaken and, depending on the amount of rock and mud that has collected among the timbers over the lifetime of the dam, can collapse.

Sheet-pile dams

• A sheet-pile dam is typically a temporary structure used during construction projects. Some dams use sheet piles to reduce seepage.

Virginia law requires dam owners to operate and maintain dams in a safe manner. To accomplish this, the owner has the responsibility to make periodic inspections and perform routine maintenance and repairs.

Owners of high and significant hazard dams are required to have an Emergency Action Plan (EAP), while owners of low hazard dams are required to have an Emergency Preparedness Plan (EPP). During storm events and emergencies, these plans provide critical action steps for dam owners to use to help protect life and property.

Regulations and requirements of dam ownership are in the Virginia Administrative Code.

Read more about EAPs at the Dam Failures page.

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