Description & Background

The ability to quickly and safely perform reservoir drawdown can be crucial to the protection and preservation of a dam. A reservoir low level outlet works and/or drain system with adequate capacity should be provided in all dams to provide a method of lowering the reservoir level in an emergency within a reasonable period. In several instances, dam failures have been averted by lowering the reservoir in response to emergency conditions detected at dams.

Common types of reservoir drains include gated openings in a riser or control structure, a low level conduit through the dam with a valve or gate at either end of the conduit, or stoplogs located in a control structure.  For conduits through embankment dams, it is industry practice to provide a means for upstream closure. In special cases where a reservoir draining system was not incorporated into the original design and construction of a dam, or where the reservoir drainage system is inoperable, siphons or pumps have been installed and used to drawdown the reservoir.

Cross-sectional view of a typical earthen embankment dam with a riser structure and conduit through the embankment. Typically, a riser structure would be used to regulate normal pool level and serve as a low level outlet works should the reservoir need to be drained.

Reservoir drawdown by an outlet works draining system can be necessary in a variety of circumstances. Should serious problems that threaten the immediate safety of the dam occur, the reservoir drain system may be used to lower the reservoir level to reduce the likelihood of dam failure. Examples of such emergencies include clogging of the spillway that could lead to failure of the dam by overtopping, development of internal erosion and uncontrolled seepage in an embankment that could lead to failure of the dam, or a structural problem with a spillway or other critical feature of the dam that could lead to its failure.

Repairs to spillways, embankments, or other dam appurtenances or performance of standard maintenance procedures may also require the drawdown of the reservoir.

In addition to providing a means of drawdown, the outlet works is needed to control the reservoir level during first filling.


References:

(1) Ohio Department of Natural Resources. (1999).  Dam Safety:  Lake Drains, Fact Sheet 93-26.  Columbus:  Ohio Department of Natural Resources, Division of Water, Dam Safety Engineering Program.

(2) Morrison Maierle, Inc. (2012).  Guidelines for Use of Pumps and Siphons for Emergency Reservoir Drawdown.

 

Summary

Photos

Videos

Case Studies

Austin (Bayless) Dam (Pennsylvania, 1911)

Austin Dam (also known as Bayless Dam) was constructed between May and November of 1909 just outside Austin, Pennsylvania, a town of approximately 2,500 people in Potter County.

Learn More

Penn Forest Dam (Pennsylvania, 1994)

Penn Forest Dam was constructed between 1956 and 1958 to be one of the main water supply reservoirs for the city of Bethlehem, Pennsylvania. The dam was originally an earthfill embankment structure, stood 145 feet high, 1,930 feet long, and stored 6.4 billion gallons of water...

Learn More

South Fork Dam (Pennsylvania, 1889)

South Fork Dam was an earth- and rock-fill dam located about 8 miles east of Johnstown, PA. Originally constructed in 1852, the dam’s primary purpose was to provide a source of water for a division of the Pennsylvania Canal. The dam was approximately 72 feet high...

Learn More

Teton Dam (Idaho, 1976)

Teton Dam was located in southeastern Idaho about 15 miles from Rexburg in the valley of the Teton River. The dam and its reservoir were the principal elements of the Teton Basin Project designed by the Bureau of Reclamation to control flooding as well as provide a source of...

Learn More

Other Resources

Low Level Drains

Author: New York State Department of Environmental Conservation

NYDEC online resource

Dam Safety: Lake Drains, Fact Sheet 93-26

Author: Ohio Department of Natural Resources

Fact Sheet published by ODNR

No Drain to Pull? Well that Sucks! Pennsylvania's Emergency Response Teams May be Able to Help!

Author: R. Reisinger

Technical paper published by Association of State Dam Safety Officials