The Van Norman Dams, also known as the San Fernando Dams, were the terminus of the Los Angeles Aqueduct, supplying about 80 percent of Los Angeles' water, until they were damaged in the 1971 San Fernando earthquake and were subsequently decommissioned due to the inherent instability of the site and their location directly above heavily populated areas.

Construction

The Upper Van Norman Dam initially was constructed with 42 ft of hydraulic fill. In 1922, the dam was raised 18 ft with rolled fill. The Lower Van Norman Dam was constructed with hydraulic and rolled fill. Hydraulic fill height was about 102.4 ft, while rolled fill was added at least five times in the dam's history, each time increasing the dam's height, totaling 39.6 ft rolled fill. The last addition was made in 1929–30.

1971 San Fernando earthquake

The 1971 San Fernando earthquake significantly damaged the dams, resulting in evacuation of thousands of people from the San Fernando Valley immediately below. 80,000 were evacuated for three days. Later, it was estimated that a dam failure could have killed 123,400.

Upper Van Norman dam

The Upper Van Norman reservoir was operating at about one-third capacity at the time of the earthquake. The quake lowered dam height 3 ft and displaced the dam laterally 5 ft.

Lower Van Norman dam

Originally, the Lower Van Norman reservoir was operated near full capacity of 1134.6 ft. However, the maximum operating height was reduced to 1125 ft in 1966 following seismic hazard review. Fortuitously, at the time of the 1971 San Fernando earthquake the water height was 1109 ft (about half capacity: 3.6 e9USgal of water) as a large landslide fell into the reservoir along with 30 ft of the crest and upstream face reducing the freeboard to about 5 ft. This failure was predominantly due to liquefaction of the hydraulic fill. To reduce the risk of catastrophic failure, the water level was lowered as rapidly as possible, 13.5 ft in 3 1/2 days, at the rate of 700 cuft/s. This rate was limited by earthquake damage to the outlet lines and drainage towers.

Aftermath

Reconstruction was proposed, but abandoned after geologic evaluation showed the inherent instability of the dams' foundations. As a replacement, the Los Angeles Dam was constructed between the original Lower and Upper Van Norman Dam structures in a more stable location. During the 1994 Northridge earthquake, the Lower Van Norman reservoir area was again severely damaged, but as then it was in use only as a holding basin, the consequences were minor.

Lessons learned

The near failure of the Lower Van Norman Dam brought about major changes in the way public agencies and engineers viewed seismic safety, particularly regarding embankments of fine sands and silts and numeric dynamic analysis of dams. Also, it resulted in many mandated dam safety reassessments.