Dam Failure

Definition

Dam failure is the uncontrolled release of impounded water resulting in downstream flooding, which can affect life and property. Flooding, earthquakes, blockages, landslides, lack of maintenance, improper operation, poor construction, vandalism, or terrorism cause dam failures.

History

In recent years, dam failures in the United States have prompted renewed public and government concern and action. Public Law 92-367, the National Dam Inspection Act, resulted in the inventorying of dams in the United States and the inspection of non-federal dams nationally. See Table 9 for a table of Dam Failures and Incidents in Washington State.

Hazard Identification and Vulnerability Assessment

The Department of Ecology, Dam Safety Office, in its 1998 Report to the Legislature stated that the responsibility for the 1025 dams in Washington State rests with several agencies. Dam safety units within the respective federal agencies inspect the 69 federally owned and operated dams. Private engineering consultants inspect the 76 non-federal hydropower dams licensed by the Federal Energy Regulatory Commission. There are currently 880 dams in Washington State under the sole jurisdiction of the Dam Safety Office.

Of the dams inspected by the Dam Safety Office, 296 are situated above populated areas. Nearly all of the 115 dams located upstream of three or more residences (high downstream hazard potential) have previously been inspected and are supposed to be on a six-year inspection cycle. However, a decrease in dam safety engineering staff in 1997 resulted in fewer inspections than necessary to meet the six-year inspection cycle. There are 181 dams that have a significant downstream hazard potential where one or two homes are at risk in the event of dam failure. These projects are currently on an inspection cycle of 10 to 12 years. As of February 1999, 14 high downstream hazard potential dams and 14 significant hazard dams existing in Washington State have safety deficiencies.

In general, periodic inspections and follow-up engineering analysis are conducted to:

  • Identify defects, especially due to aging
  • Evaluate dam operations and maintenance
  • Assess dam structural integrity and stability
  • Determine the adequacy of the spillways to accommodate major floods
  • Assess the stability of dam structures under earthquake conditions

As with any hazard, a community should consider upstream dams when considering building permits or development. On average, Washington State experiences a dam failure approximately once every two years. The majority of failures result from inadequate maintenance and monitoring of the facilities. Failure of a dam can have many effects such as loss of life and damage to structures, roads, utilities, crops, and the environment. Economic losses also can result from a lowered tax base and lack of power profits.

Conclusion

Three state statutes deal with safety of dams and other hydraulic structures: Chapters 43.21A, 86.16, and 90.03 RCW. These laws provide authority to approve plans for dams, inspect construction, inspect hydraulic works, and require appropriate changes in maintenance and operation. In addition, regulations, policies and procedures, and guidelines have been adopted. They serve to clarify the mission of the Dam Safety Office and to assist the agencies in their efforts to build, operate, and maintain safe dams.

The failure to implement a suitable operation and maintenance program at dams is a common thread in dam incidents occurring in Washington State. Many municipalities operate old reservoir systems and find it difficult to fund effective operation and maintenance programs. While the failure of projects with a high potential for loss of life are increasingly remote, the number of failures of low hazard projects that provide important infrastructure roles are on the rise. With increasing population in the state, homes are frequently being constructed below dams. These dams were not built to the more stringent requirements of high hazard dams, and these present the greatest potential threat to public safety. Dam Safety Office is attempting to examine these smaller dams and get them on a schedule for comprehensive inspections and repair.

Periodic inspections are the primary tool for detecting deficiencies at dams that could lead to failure. Experience shows that corrections of these safety deficiencies in a timely manner can prevent dam failure and other serious incidents from occurring. Periodic inspections help identify dams where significant development has occurred downstream, resulting in the need for more stringent building and planning codes due to greater population at risk.

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