The force of 12.5 billion gallons of water sent giant chunks of the St. Francis Dam hurtling down-canyon. By the time the reservoir finished draining, the canyon was stripped of all vegetation up to the water line.
The view is to the west as seen from Bee Highline Road. The prominent blocks are No. 13 at left and No. 11 at right. According to dam engineering expert J. David Rogers,
"The bottom of Block 11 revealed a very small shear key had been excavated into the underlying Pelona Schist, ostensibly to increase the basal shear resistance. One can also catch a glimpse of the flood damage to San Francisquito Canyon Road, ascending the dam's left abutment, at lower middle left."
Jason Brice provides the diagram above to explain that they dug a hole or trench into the schist and poured the concrete into it and went up from there, in an effort to lock it into the remaining schist.
Rogers' sketch, at right, shows the relative size of the fairly small shear key, as well as the position of the 8-foot-high cofferdam incorporated into the upstream heel of the dam
(click to enlarge).
At left, Rogers' image shows "the bottom of Block 7 — the upmost part of the dam's left abutment — which shows the same shear key feature,
excavated into the Pelona Schist. Note the huge shrinkage (tension) crack as well,
which was noted before the dam's failure." Click to enlarge.
Photograph shot on or shorly after March 13, 1928. The dam collapsed at 23:57:30 on March 12 and it took about 72 minutes for the 12.5-billion-gallon reservoir to empty (Outland 1977).
About the image: One of seventeen 5x7-inch glossy photographs found in a box in the Photography section of the San Francisco Public Utilities Commission's Engineering Archives in 2013. There was no record of the origin of the photographs and no writing on the back to identify subjects or dates. Current SFPUC staff members did not know the reason for their inclusion in the archive.
Ownership transferred via email communications (on file) from SFPUC to SCVTV-SCVHistory.com. Prints received July 11, 2014; subsequently transferred to the Santa Clarita Valley Historical Society.
1. A hand-written notation by current San Francisco PUC staff indicates the images were emailed Aug. 8, 2013, to someone who identified them as the St. Francis Dam. SFPUC staff alerted us to their existence in the Spring of 2014.
Essentially the same view in March 2014. Photo by Jason Brice. Click to enlarge and expand.
Construction on the 600-foot-long, 185-foot-high St. Francis Dam started in August 1924. With a 12.5-billion-gallon capacity, the reservoir began to fill with water on March 1, 1926. It was completed two months later.
At 11:57:30 p.m. on March 12, 1928, the dam failed, sending a 180-foot-high wall of water crashing down San Francisquito Canyon. An estimated 411 people lay dead by the time the floodwaters reached the Pacific Ocean south of Ventura 5½ hours later.
It was the second-worst disaster in California history, after the great San Francisco earthquake and fire of 1906, in terms of lives lost — and America's worst civil engineering failure of the 20th Century.