Cofferdam Failure
In early 1986 a huge flood began pouring down from the Sierra Nevada mountains. In what would later be considered one of the largest regional floods in recorded history, Placer County was quickly designated a Federal Disaster Area. Rampaging streams and rivers incurred some $7.5 million in damages within the county. Ten inches (254 mm) of rain fell on the Sacramento region in 11 days. Flooding was so catastrophic, that the rating for the city's systems, supposedly designed to prevent a 125-year flood, was dropped to a 78-year flood. The floods tore out long stretches of levees along the American, Sacramento and Feather Rivers through the Sacramento Valley, and the city of Sacramento was spared by a close margin. Folsom Lake again filled to capacity, and began spilling 134,000 cubic feet per second (3,800 m3/s) by late February.
The flood rapidly filled the pool behind the Auburn cofferdam to capacity, as the diversion tunnel could not handle the extra water pouring into the reservoir. At about 9:00 A.M. on February 18, the rising water overtopped the cofferdam near the right abutment, creating a waterfall that quickly eroded into the structure. Although the cofferdam was designed with a soft earthen plug to fail in a controlled manner if any such event were to occur, the outflow reached 100,000 cubic feet per second (2,800 m3/s) by noon; several hours later the maximum discharge was reached at 250,000 cubic feet per second (7,100 m3/s), completely inundating the construction site and destroying almost half of the cofferdam. When the 265-foot (81 m) high cofferdam collapsed, its backed-up water surged downstream into already-spilling Folsom Reservoir less than a mile downstream, deposited the dam debris and raised the lake level suddenly. Folsom Dam outflow exceeded the design capacity of levees downstream from re-regulating Nimbus Dam downstream from Folsom, but the levees were not overtopped and severe flooding was avoided through Sacramento. The flood events spurred renewed interest in the Auburn Dam, since a permanent dam would have helped Folsom Dam store water without the flood from behind the cofferdam, but it became clear that the system of levees, dams and diversions on the American River needed to be re-engineered.
Read more about this topic: Auburn Dam, Construction
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