4.2.3 St. Francis Dam

St. Francis Dam was completed May 1926 and reservoir was filled. This dam was 205-ft high, 700-ft long. Dam was failed 11:57 p.m., March 12, 1928. Official death toll was 495.

St. Francis Dam was the key facility of the Los Angeles Bureau of Water Works and Supply system. The arched concrete gravity structure was located approximately 45 miles north of LA. The reservoir, which had a capacity of 38,000 acre-feet was nearly full at the time of the failure.

No witness of the actual failure survived the dam’s final minutes. At 11:47 pm, only 10min. preceding the disaster, the operator at the powerplant upstream from the reservoir called the lower plant, and nothing unusual was reported. At 11:58 pm, a break occurred in the power line of the Southern California Edison Company in the canyon downstream. The dam failure undoubtedly happened almost instantly. In about 70 min., practically the entire reservoir storage had been discharged. The flood wave attained an estimated maximum depth of 125ft in the first mile downstream from the dam. Peak outflow probable was greater than 500,000 cfs.

St. Francis dam after failure, Courtesy by DSOD

Blocks of the dam were washed thousands of feet from the site. A wide slide was active on the left abutment for at least two weeks following the disaster. At the time of its failure, St. Francis Dam was less than two years old. The 205-ft-high dam was arched on a radius of 500 ft to the upstream face at the top. The dam had no inspection gallery, and the foundation was not pressure-grouted. Uplift relief under the dam was provided only at the river channel.

Lessons Learned

- Site exploration must include a search for old or potential slides and an evaluation of their possible effects on the dam.
- Consideration must be given to foundation capability under a full range of foreseeable conditions, including shearing and weakening due to wetting.
- The need for foundation grouting has to be carefully assessed.
- Inspection galleries may be of high value in concrete dams.
- The engineering and the surveillance of an important dam should be assigned to an interdisciplinary team so that all potentially adverse conditions can be fully evaluated.


Sources:

Robert Jansen, The St. Francis Dam Failure, Advanced dam engineering for design, construction, and rehabilitation, 1988, Van Nostrand Reinhold, NY.
CA DSOD files (Courtesy of S. Verigin).