Dam Hazard Potential Classification
Impoundments and dams are evaluated relative to their hazard potential classification, which affects the design criteria incorporated in the planning, development, and construction of dams. Consistent with the hazard potential classification system and criteria for dams in use by federal agencies, the three hazard potential classifications for dams are as follows:
- Low hazard potential – Facilities where failure would result in no probable loss of human life and low economic and/or environmental losses. Such facilities are usually located in rural or agricultural areas where losses are limited principally to the owner’s property or where failure would cause only slight damage to farm buildings, forest and agricultural land, and minor roads.
- Significant hazard potential – Facilities where failure would likely not result in loss of human life, but can cause economic loss, environmental damage, or disruption of lifeline facilities. Such facilities are generally located in predominantly rural areas, but could be in populated areas with significant infrastructure and where failure could damage isolated homes, main highways, and minor railroads or disrupt the use of service of public utilities.
- High hazard potential – Facilities where failure will probably cause loss of human life. Such facilities are generally located in populated areas or where dwellings are found in the flood plain and failure can reasonably be expected to cause loss of life; serious damage to homes, industrial and commercial buildings; and damage to important utilities, highways, or railroads.
The purpose of hazard potential classification is not to determine the likelihood of a failure occurring, but rather to assess the potential impacts should a failure occur and to establish appropriate criteria for use in the design and operation of the facility. Thus, more conservative design and operations criteria apply as the potential for loss of life or property damage from failure increases. For example, more subsurface exploration and material property testing is normally performed for a facility with high hazard potential than for one with low hazard potential. An impoundment or dam with high hazard potential would be designed to accommodate the probable maximum flood (PMF), while a dam with low hazard potential would be designed for a smaller storm event.
Determination of possible damage due to failure of an impoundment or dam must be based upon an evaluation of conditions for an appropriate downstream distance. This distance is normally determined by performing a breach analysis that defines an inundation area resulting from a breach of the impoundment or dam. This analysis is particularly important in mountainous mining areas where complementary industrial, commercial, and residential developments are usually located in the valley bottom.
An impoundment or dam’s initial hazard potential classification is determined by the designer, mine operator, or MSHA. In many instances, the expected results from a dam break cannot be known with certainty, thus the use of the word “probable” in the hazard classification definitions. Unless site specific dam break analyses with inundation mapping are conducted, it is prudent and conservative to assign a higher hazard classification.
One of the major issues facing dam owners and regulators is known as "hazard creep." This term refers to a dam originally designed, constructed, and operated as low or significant hazard potential that is now reclassified as high hazard potential due to new downstream development. Such dams often do not meet design, inspection, and maintenance requirements for high hazard potential dams and must be improved or removed. It is customary to periodically review the hazard potential classification for a dam to ensure downstream conditions have not affected the classification.