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World Environmental and Water Resources Congress 2017 380

The City of Los Angeles’ Bureau of Waterworks and Supply (BWWS) engineers who
had worked on the design, construction, and maintenance of the St. Francis Dam and two
adjacent Powerhouses in San Francisquito Canyon were questioned at length about the St.
Francis Dam in late March and early April 1928 in downtown Los Angeles. A number of glaring
inconsistencies emerged from the testimonies of the various individuals queried by the jurors, the
County’s Deputy District Attorneys, and their technical experts, who produced their own report
(Mayberry et al., 1928). Some of these inconsistencies with what the Inquest jurors felt was
good practice are briefly summarized below.

BASE-TO-HEIGHT RATIO

BWWS prepared a preliminary design of the St. Francis Dam in July 1923, which was very
similar to the gravity arch structure that they had recently designed, then called Weid Canyon
Dam. Situated in the Hollywood Hills near the east side of Cahuenga Pass, it was referred to as
the Hollywood Dam during construction, which began in August 1923. It was renamed
Mulholland Dam when the structure was formally dedicated on March 17, 1925 (the body of
water it retains is known as Hollywood Reservoir).
The original design envisioned concrete monolith rising 175 feet above the bed of San
Francisquito Creek, with a “chopped toe,” shown in Fig. 1. The maximum reservoir capacity
was to be 30,000 acre-feet (ac-ft). In July 1924 the reservoir capacity was increased to 32,000 ac-
ft by adding 10 feet to the dam’s height, which required a wing dike extended westward about
600 ft from the dam’s right abutment. City crews began placing concrete in August 1924. In
July 1925 BWWS decided to raise the dam another 10 feet (to elevation 1835 feet), this time
increasing the reservoir capacity to 38,168 ac-ft.

Figure 1. Left pane shows the original maximum section through the St Francis Dam, made
in July 1923. Middle pane shows the as-built drawing released by BWWS after the failure,
showing a flared toe, up to 175 ft wide at elevation 1625. The right pane shows the actual
limits of the dam in red, with a base width of 148 ft.

The dam was subsequently raised 20 vertical feet, about 11% of its original height (175
ft) without any corresponding adjustment of the dam’s base width. The elevation of the stream
bed was about 1655 ft, and the maximum depth of excavation was about 16 ft, to an elevation of

World Environmental and Water Resources Congress 2017