ABAG Dam Failure Inundation Hazards Guide

The Problem

Dam failures can result in the worst flood events. A dam failure is usually the result of neglect, poor design, or structural damage caused by a major event such as an earthquake. When a dam fails, a gigantic quantity of water is suddenly let loose downstream, destroying anything in its path. For example, in 1889, more than 2,200 lives were lost as a result of the Johnstown, Pennsylvania flood caused by an upstream dam failure. Billions of dollars of property damage can also occur as a result of a dam failure.

More recently, in 1971, during the San Fernando earthquake, shaking caused a major slide of the top thirty feet of the Lower San Fernando Dam. The dam was very close to completely failing. Eighty thousand people living downstream of the dam were immediately ordered to evacuate. At the time, there were no dam failure inundation maps available showing the areas which would be affected by a dam failure, and there were no planned evacuation procedures to follow.

Solutions

As a result of the near failure of the Lower San Fernando Valley Dam, the Dam Safety Act was passed into law. This new law required dam owners to create maps showing areas that would be flooded if the dam failed. The California Office of Emergency Services (OES) approves the maps and distributes them to local governments, who in turn adopt emergency procedures for the evacuation and control of areas in the event of a dam failure.

Besides the passage of the Dam Safety Act, other improvements concerning dams have been made throughout California as a result of the near-failure of the Lower San Fernando Valley Dam. Hydraulic fill dams, the type of dam that the Lower San Fernando Valley Dam was, were deemed to be unsafe and have been replaced with other types of dams (usually rolled earth dams in the Bay Area). Various other standards for dam structures have been improved and applied.

What can you do to help prepare for the possibility of a dam failure? You can begin by identifying the areas in your community which would be affected by a dam failure, click here to select a Bay Area dam failure hazard inundation map for viewing. Note however, the precautions about using the maps before choosing one to view.


ABAG, the Association of Bay Area Governments, is the regional planning and services agency for the nine-county San Francisco Bay Area.

This page was last updated 8/27/03 by jbp.