A dam may be designed to contain a flood at a location on a stream that has a certain probability of occurring in any one year. If prolonged periods of rainfall and flooding occur that exceed the design requirements, that structure may be overtopped and fail. Overtopping is the primary cause of earthen dam failure in the United States.
Dam failures can also result from any one or a combination of the following causes:
• Inadequate spillway capacity resulting in excess overtopping flows;
• Internal erosion caused by embankment or foundation leakage, or piping or rodent activity;
• Improper design;
• Improper maintenance;
• Negligent operation; and/or
• Failure of upstream dams on the same waterway.
Water released by a failed dam generates tremendous energy and can cause a flood that is catastrophic to life and property. A catastrophic dam failure could challenge local response capabilities and require evacuations to save lives. Impacts to life safety will depend on the warning time and the resources available to notify and evacuate the public. Major loss of life could result as well as potentially catastrophic effects to roads, bridges, and homes. Electric generating facilities and transmission lines could also be damaged and affect life support systems in communities outside the immediate hazard area. Associated water supply, water quality and health concerns could also be an issue. Factors that influence the potential severity of a full or partial dam failure are the amount of water impounded; the density, type, and value of development and infrastructure located downstream; and the speed of failure.
The California Department of Water Resources Division of Safety of Dams assigns hazard ratings to large dams within the State. The following two factors are considered when assigning hazard ratings: existing land use and land use controls (zoning) downstream of the dam. Dams are classified in three categories that identify the potential hazard to life and property:
• High hazard indicates that a failure would most probably result in the loss of life.
• Significant hazard indicates that a failure could result in appreciable property damage.
• Low hazard indicates that failure would result in only minimal property damage and loss of life is unlikely.
According to data provided by National Performance of Dams, there are 26 dams in Sacramento County constructed for flood control, storage, electrical generation, and recreational purposes. Of the 26 dams located inside the County, 11 are rated as high hazard, 6 as significant hazard, and 9 as low hazard.
Inundation Map and Evacuation Route Map