A Retrospective Evaluation of the Performance of the Lower San Fernando Dam

As a result of the M 6.7 1971 San Fernando earthquake the Lower San Fernando Dam (LSFD) suffered significant damage that could have resulted in a breach and uncontrolled release of the reservoir (URR). The damage to the LSFD was a result of liquefaction that led to a massive slide on the upstream slope into the reservoir. The slide lowered the crest of the dam well below the reservoir level. However, a section of the downstream slope remained, with about 5 ft of freeboard. Fortunately, aftershocks did not cause further significant damage and the reservoir was lowered soon after the earthquake, avoiding URR into the heavily populated San Fernando Valley below. Experience suggests that system failures are a result of a combination of events or circumstances (a sequence of events) that collectively conspire to produce the outcome that occurred. This study takes a retrospective look at the events at the LSFD as a result of the 1971 earthquake to assess just how rare (in terms of its annual frequency of occurrence) it was. This retrospective look at the 1971 earthquake provides insights and lessons for conducting prospective seismic risk studies in the future.