How about we discuss a site that dramatically changed dam safety? This is the Van Norman Bypass Reservoir. It’s a place tied to important lessons learned. The Van Norman Dams, also called the San Fernando Dams, once marked the end of the Los Angeles Aqueduct. They supplied about 80 percent of Los Angeles’ water.
Let’s step back in time. The Lower Van Norman Dam’s construction began in 1912. It used hydraulic and rolled fill techniques. These techniques were considered sound engineering at the time. Hydraulic fill was placed up to approximately 1,105 feet elevation. The shells consisted of silty sand, sandy silt, and clay layers. The core was made from puddled clay. Rolled earth fill was added later, raising the dam’s crest to 1,144.6 feet.
Fast forward to February 9, 1971. The San Fernando earthquake struck. This earthquake caused about $550 million in damage and killed 50 people. The earthquake’s shaking caused a major slide in the Lower Van Norman Dam’s upstream slope. It moved towards the left abutment. This slide significantly reduced the dam’s freeboard. The embankment spread about 250 feet beyond its toe.
At the time of the earthquake, the reservoir was at 1,109 feet. This was 25 feet below the spillway. It was also about 4 feet above the hydraulic fill. After the slide, the remaining freeboard was only about 5 feet. A caretaker noticed the reservoir was quiet, without waves or sloshing.
Imagine the immediate response. Sandbags were rushed to the site. About 80,000 residents were evacuated from the area below the dam. This evacuation lasted four days. The reservoir’s water was lowered using pumps. A major catastrophe was narrowly avoided. About 10,000 acre-feet of water was safely released.
The near failure of the Lower Van Norman Dam led to major changes. Public agencies and engineers reassessed seismic safety. They focused on embankments made of fine sands and silts. They also reviewed numeric dynamic analysis of dams. Hydraulic fill was effectively no longer considered suitable for embankment dams in seismic zones.
The Los Angeles Dam and Reservoir were built to replace the failed Lower San Fernando Dam. They were constructed in 1976-78. The old Lower San Fernando Dam was reconstructed to serve as a stormwater holding basin. In 1994, the Northridge earthquake struck. The remnant Lower San Fernando Dam suffered heavy damage again. However, the Los Angeles Dam performed well.
The Van Norman Bypass Reservoir stands as a testament to engineering evolution. It highlights the critical importance of seismic safety in dam construction. It is a reminder that even in engineering, lessons are often learned through adversity.