The Van Norman Dams, also known as the San Fernando Dams, were the terminus of the

Los Angeles Aqueduct The Los Angeles Aqueduct system, comprising the Los Angeles Aqueduct (Owens Valley aqueduct) and the Second Los Angeles Aqueduct, is a water conveyance system, built and operated by the Los Angeles Department of Water and Power. The Owens Valley ...

, supplying about 80 percent of Los Angeles' water, until their damage in the

1971 San Fernando earthquake The 1971 San Fernando earthquake (also known as the 1971 Sylmar earthquake) occurred in the early morning of February 9 in the foothills of the San Gabriel Mountains in southern California. The unanticipated thrust earthquake had a magnitude o ...

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 of

hydraulic fill Hydraulic fill is a means of selectively emplacing soil or other materials using a stream of water. It is also a term used to describe the materials thus emplaced. Gravity, coupled with velocity control, is used to effect the selected depositi ...

. In 1922, the dam was raised with rolled fill. The Lower Van Norman Dam was constructed with hydraulic and rolled fill. Hydraulic fill height was about , while rolled fill was added at least five times in the dam's history, each time increasing the dam's height, totaling 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 The San Fernando Valley, known locally as the Valley, is an urbanized valley in Los Angeles County, California. Located to the north of the Los Angeles Basin, it contains a large portion of the City of Los Angeles, as well as unincorporated ar ...

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 and displaced the dam laterally .

Lower Van Norman dam

Originally, the Lower Van Norman reservoir was operated near full capacity of . However, the maximum operating height was reduced to in 1966 following seismic hazard review. Fortuitously, at the time of the 1971 San Fernando earthquake the water height was (about half capacity: of water) as a large landslide fell into the reservoir along with of the crest and upstream face reducing the freeboard to about . 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, in days, at the rate of . 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 1994 Northridge earthquake was a moment 6.7 (), blind thrust earthquake that occurred on January 17, 1994, at 4:30:55 a.m. PST in the San Fernando Valley region of the City of Los Angeles. The quake had a duration of approximately ...

, 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.

References

External links

* * {{Authority control Dam failures in the United States Dams in Los Angeles County, California History of Los Angeles History of Los Angeles County, California Los Angeles Aqueduct Reservoirs in California Earthquake and seismic risk mitigation Dams completed in 1921 San Fernando Valley 1971 in Los Angeles 1971 in California 1971 earthquakes History of the San Fernando Valley Geology of Los Angeles County, California