Virginia impounding structure regulations specify that each dam be classified based on potential loss of human life or property damage if it were to fail. Classification is based on a determination of the effects that a dam failure would likely have on people and property in the downstream inundation zone. Hazard potential classifications decend in order from high to low, high having the greatest potential for adverse downstream impacts in event of failure. This classification is unrelated to the physical condition of the dam or the probability of its failure. The hazard potential classifications are:
Safety standards become increasingly more stringent as the potential for adverse impact increases. For example, a high hazard dam -- that is, one whose failure would cause probable loss of human life -- is required to meet higher standards than a dam whose failure would not be as likely to result in such severe adverse consequences. Classification, however, is not static. Downstream conditions, including land use, can and often do change. Although a dam itself may remain relatively stable, it is subject to reclassification if a change occurs in the downstream inundation zone. For example, if new homes are built in the downstream inundation zone of a Class II, III or IV dam, the dam could be reclassified to Class I.
A change in hazard classification can create a dilemma because if a dam is reclassified, it usually does not meet the higher standards of the new hazard classification. To meet the required higher standards, the owner of the dam is often required to make expensive modifications. Any dam that does not meet the most extreme standards of a high hazard dam could become deficient in the future if land use in the downstream inundation zone changes.
To avoid the need for some of these expensive modifications, all affected parties -- dam owner, engineer, downstream land owners, and local governments -- need to work together. People should be aware of the impacts development downstream can have on the required standards of a dam. It is better and cheaper to address this potential problem beforehand rather than wait and deal with modifications later.