By Kathe Malouf
Special to the Sun
The history of dam safety and the lessons learned from previous failures was the topic of a presentation to the Kern River Valley Historical Society during their monthly meeting last week.
Anthony Burdock, Project Manager for the Isabella Dam Safety Modification Project, presented a program outlining catastrophic dam failures and how those failures were used to mold the dam safety regulations that now govern the nation’s dams, including Isabella Dam.
Burdock presented an overview of four failures: The South Fork Dam in 1889, the St. Francis Dam in 1928, Teton Dam in 1976 and the Van Norman Dam in 1971.
A dam failure can be attributed to several factors, Burdock said including: overtopping, foundation defects, cracking, inadequate maintenance and piping.
The primary cause of dam failures in the U.S. is overtopping, Burdock said, when water comes over the dam, due to inadequate design of the dam or spillway. Debris blockage and settlement of the dam crest can also lead to overtopping.
Burdock noted that cracking in an earthen dam is caused by the natural settling of the dam. Inadequate maintenance is another cause of failure and was the primary reason that the Oroville Dam overtopped and failed. The building up of sediment along piping can also create problems.
“All dams will leak,” Burdock said. “But when it gets to a large degree, we can’t stop it. It is very difficult to stop a failure once it starts.”
The failure of the South Fork Dam in Pennsylvania occurred in May,1889, following a period of heavy spring rains which caused overtopping and the erosion of the dam. Burdock noted that the dam had previously failed in 1862. It was repaired with changes not originally planned, such as the lowering of the dam’s crest, removal of low water outlets and reduction of the spillway capacity. Those changes led to the dam failure. The entire structure was washed away and 2,200 people were killed.
“This was the worst man-made disaster in U.S. history, until September 11, 2001,” Burdock said.
The St. Francis Dam was a concrete dam located near Los Angeles designed and constructed by the Los Angeles Bureau of Waterworks and Supply from 1924 to 1926 under the supervision of William Mulholland, a self-taught engineer with experience in earthen dams, but limited experience with concrete dams.
The dam failed on March 13, 1928, approximately 12 hours after the last inspection by Mulholland. Burdock said that cracks and leaks were observed in the main dam and abutments, but were dismissed.
The failure was attributed to poor foundation rock. It was considered the worst engineering disaster of the 20th Century with the death of 432 people.
“Because of Mulholland’s status in the community, his design was not questioned,” Burdock said. “What we learned is the importance of alternate technical reviews. We hire outside experts specifically for safety reasons.”
Following the failure of St. Francis, California enacted the professional registration for civil engineers and Congress required that all non-federal dams be reviewed.
The Teton Dam in Idaho was completed in Nov. 1975 and failed in June of 1976, during its first filling. Although a spillway existed, the powerhouse, auxiliary spillway and outlet works were unfinished at the time of the filling. Seepage was noted, which expanded to a large hole and ultimate failure. The water release inundated 300 square miles and traveled 155 miles downstream. Eleven people were killed.
“There were multiple problems. Designers did not take site-specific geologic calculations to ensure that the dam was sufficient,” Burdock said. “There was no way to release the water and the dam was designed by one team and handed over to another team for construction without much communication.”
He pointed out that design and construction teams at Isabella are in constant communication, noting that they have engineers on site that evaluate and review the design of work at Isabella.
There were five major dam failures during 1972 to 1977, which led to the study of all dams by the National Dam Safety Program, Burdock said.
Several audience members remembered the fourth case presented: The Van Norman Dam located 20 miles northwest of Los Angeles.
The dam failed in February, 1971, following a 6.5 earthquake along the San Fernando fault. The quake caused severe liquefaction and failure of the dam’s embankment slope and 30 feet of embankment slid away from the dam. Evacuation orders were issued for 80,000 people downstream. Fortunately, there were no fatalities.
“Because they had low-water outlets, they were able to lower the pool levels,” Burdock said.
Burdock then discussed the lessons learned from previous failures.
Lessons such as the requirement that all dams have an operational means of drawing down the reservoir; dams located in seismic areas must be evaluated for cracking, liquefaction and potential fault offset; dams must undergo periodic risk assessments to include site inspections, construction and performance; earth and rockfill embankment dams must be stable under full range of anticipated loading conditions; emergency action plans must be updated and practiced regularly; and the first filling must be planned, controlled and monitored.
Focusing on Isabella, Burdock stated that the dam is being raised to ensure that it can withstand a catastrophic flood. He stated that the new labyrinth weir that is part of the modification will likely never be used, as it would be used only if the dam is about to overtop. While the raised height of the dam will not increase the storage capacity, the modification is all about the Corps’ primary focus: public safety.
Following the presentation, questions were asked about Isabella’s project ranging from the status of the visitor’s center to the night time lighting at the construction campus.
“The lighting is necessary as we have workers on site at night, so the lighting is for the safety of our crews and security,” he explained. Regarding the visitor’s center, Burdock said that the Corps is still looking at multiple sites for the center and will be going through a study process to evaluate the pros and cons and cost of each site.