Resources by Topic: Hazard and Size Classification
The hazard potential classification for a dam is intended to rank dams in terms of potential losses to downstream interests if the dam shold fail for any reason. The classification is based on the incremental adverse consequences (after vs. before) of failure or mis-operation of the dam, and has no relationship to the current structural integrity, operational status, flood routing capability, or safety condition of the dam or its appurtenances. The hazard potential classification is based on potential adverse impacts/losses in four categories: environmental, life line, economic, and/or human life.
FEMA Publication No. 333 has adopted three hazard potential classification categories for dams as follows: LOW, SIGNIFICANT, and HIGH HAZARD POTENTIAL, listed in order of increasing incremental adverse consequences. When loss of one or more human lives is probable, High Potential Hazard classification if required. Some regulators use numbers or letters in lieu of these titles and may have more than three hazard potential classifications based on legislative requirements or agency history.
The selection of a hazard potential classification for a dam should be made using a phased approach utilizing three levels of effort: presumptive, incremental hazard assessment (dam break studies), and risk based assessment. It is intended that the engineer making the classification determination will proceed from the simplest method (presumptive) using existing data and field reconnaissance, to the most complex (risk based assessment) in a step sequence. In most cases, all three methods will not be required.
The hazard potential classification for a dam may change over time. New downstream development, raising of a dam to increase storage, the finding of an endangered or threatened species (plant or animal), revisions to National Weather Service Hydrometerological Reports, or downstream land use changes could warrant changing the hazard potential classification of the dam. Thus, it will be necessary to periodically review and update the classification of each dam based on the prior documented classification. It is recommended that the hazard potential classification review cycle for each dam correspond to the inspection frequency adopted by the regulatory agency.
For projects with several independent elements (dams, spillways, powerhouse, low level outlet, etc), the overall Project hazard potential classification will be that assigned to the highest rated project element.
Size classification is based on either structural height or reservoir storage capacity, whichever gives the higher classification. Size classifications are SMALL, INTERMEDIATE, or LARGE. Height and/or storage capacity are used by many states to legislatively define State Dam Safety office jurisdictional or non-jurisdictional dams. In some states, very high or large storage dams are automatically assigned High Hazard Potential. The size classification is also used to define dams listed in the US Army Corps of Engineers National Inventory of Dams.
The Hazard Potential and Size classifications of a project depend on the legislative authority, rules, and regulations of the project regulatory authority, and can vary significantly between state and federal agencies.
Sound engineering judgment and experience are required when assigning a hazard potential or size classification to a dam. Guidelines provide recommendations for hazard potential or size classifications based on various concerns, but not all possible scenarios and criteria are covered. A qualified engineer must consider any unique situations that arise at a specific dam based on a physical reconnaissance of the dam and downstream channel. The engineer must be knowledgeable concerning the size and hazard potential classification criteria in the rules and regulations of the project jurisdictional authority.
Classroom and Web-Based Training
- U.S. Bureau of Reclamation, Safety Evaluation of Existing Dams (SEED) Short Course, given annually
- Self-Directed Training: Interagency Committee on Dam Safety. Training Aids for Dam Safety
- Other Training:
Guidelines and Suggested References
State Publications and Guidelines
Federal Publications and Guidelines
- Federal Emergency Management Agency. Federal Guidelines for Dam Safety (1998): Hazard Potential Classification Systems for Dams [FEMA 333]
- Federal Emergency Management Agency. Federal Guidelines for Dam Safety (1998): Selecting and Accommodating Inflow Design Floods for Dams [FEMA 94]
- Federal Energy Regulatory Commission, Engineering Guidelines for the Evaluation of Hydropower Projects, Chapter 1.2 (1991) Project Classification
- U.S. Department of the Interior, Bureau of Reclamation, Assistant Commissioner, Engineering and Research (ACER) Technical Memorandum No. 11, Downstream Hazard Classification Guidelines (December 1988)
- U.S. Department of the Interior, Bureau of Reclamation, Safety Evaluation of Existing Dams (SEED) Short Course. Materials from the annual course.
- U.S. Department of the Interior, Bureau of Reclamation, Design of Small Dams, (1987)
- U.S. Department of the Interior, U.S. Fish and Wildlife Service, Low Hazard Dams Standing Operating Procedures (2008)
- Public Law 92-367, 92nd Congress, H.R. 15951, “National Dam Inspection Act,” August 8, 1972.
- U.S. Army Corps of Engineers, “Recommended Guidelines for Safety Inspection of Dams, Appendix D,” Title 33CFR Part 222 (1979)
- U.S. Army Corps of Engineers, Safety of Dams-Policies and Procedures,(ER 1110-2-13) (2011)
For additional resources, search the ASDSO Bibliography. Suggested search terms: hazard classification, risk, mapping, development, hazard creep, hazard potential, decision-making