But it had all been planned \u2014 or at least anticipated \u2014 years before.<\/p>\n
Here\u2019s what to know about the Auburn Coffer Dam collapse on Feb. 18, 1986.<\/p>\n
Raging water spews through a coffer dam that collapsed on the North Fork of the American River and dwarfs a pickup truck on Feb. 18, 1986. Morgan Ong Sacramento Bee file Why was the coffer dam built?<\/p>\n
The coffer dam was built in 1972 to divert water around construction of what was intended to be a full-scale Auburn Dam on the North Fork of the American River.<\/p>\n
Congress authorized the project<\/a> in 1965 and construction began in 1967, the American River Authority said in a 2008 report.<\/p>\n The arch concrete dam was intended to join Folsom Dam, Nimbus Dam and other flood protections for the Sacramento area. It also was designed to provide water and power.<\/p>\n But an earthquake near Oroville in 1975 raised questions about the safety of the dam design, and engineers said in a 1976 report that such a quake could cause the proposed dam to fail.<\/p>\n New designs were later proposed, but Congress refused to reauthorize the project following intense opposition from environmental groups, the report said.<\/p>\n Why did the coffer dam collapse?<\/p>\n A combination of heavy rainstorms and snowmelt<\/a> caused the water held back by the coffer dam to overtop an emergency spillway, the Placer County Water Agency said.<\/p>\n The storms were part of a Pineapple Express<\/a>, carrying warmer rain from Hawaii, or what is today called an atmospheric river, KCRA reported.<\/p>\n In normal conditions, water behind the coffer dam flowed through a diversion tunnel, but the rain and snowmelt overcame the tunnel\u2019s capacity.<\/p>\n The volume of water began to erode the earthen spillway before eating into the side of the dam itself, undermining its foundation.<\/p>\n Huge chunks of earth collapsed into the raging torrent<\/a> as the dam failed, a Placer County Water Agency video of the collapse shows.<\/p>\n The water unleashed by the dam\u2019s gradual collapse filled Folsom Lake nearly to its 1 million acre-foot capacity, The Sacramento Bee reported.<\/p>\n In order to keep water from spilling over Folsom Dam, operators released a record 125,000 cubic feet per second into the American River.<\/p>\n The water strained downstream levees, including those protecting Sacramento, The Bee said. Volunteers joined patrols along the levees to watch for breaches.<\/p>\n Emergencies were declared in parts of 19 counties and five cities in Northern California.<\/p>\n In the end, the February 1986 series of storms killed 13 people and caused $400 million in damage to the state, KCRA said.<\/p>\n While the Feather River flooded the town of Linda in Yuba County a few days later, destroying 895 homes, the levees protecting Sacramento held, preventing an even worse disaster.<\/p>\n Was the collapse anticipated?<\/p>\n Oddly enough, the seemingly catastrophic collapse of the coffer dam had always been part of the plan, the Placer County Water Agency video said.<\/p>\n The emergency spillway had been installed as a kind of \u201csafety plug\u201d years earlier. It was designed to slowly erode over hours in an emergency to prevent a full-scale breach.<\/p>\n By spreading out the release of water from behind the coffer dam over 12 hours, it gave Folsom Dam operators time to prepare for the influx.<\/p>\n In addition, the water behind the coffer dam also had always been factored into the capacity of Folsom Lake, anticipating that the earthen dam would someday fail, the video said.<\/p>\n Were Sacramento flood protections strengthened?<\/p>\n Following the floods and the decision not to proceed with the Auburn Dam, work began on other flood safety improvements.<\/p>\n