O.C. Warned of Santa Ana River Flood Threat

Although the Santa Ana River has stayed within its banks this winter, flood control officials warn that a catastrophic storm could happen any time, inundating most of Orange County with several feet of water.

Usually bone-dry, the Santa Ana River channel poses the worst flood threat west of the Mississippi River, according to the Army Corps of Engineers. That’s because it’s not built to withstand rare, freak winter storms that can dump massive amounts of runoff into the river and cause it to spill over its banks.

Fortunately, however, a major federal and local flood control project, which will take more than a decade to complete, began along the river several years ago. Although the improvements completed at this point won’t prevent flooding from severe storms, they will help.

The channel has been strengthened, widened and deepened from the mouth of the river in Huntington Beach to Adams Street in Costa Mesa. New levees are in place, and the bottom has been dredged. Eventually, the river will be widened along its entire 30-mile length in Orange County.

“We’re better off already, and we’re not done with it yet,” said William Reiter, public works manager at the Orange County Environmental Management Agency. “We’re comfortable that (the newly constructed areas) are rain-ready.”

The Army Corps of Engineers also plans within a few years to begin raising and bolstering Prado Dam, which feeds the river. The 50-year-old dam, just over Orange County’s border in Riverside County, holds the runoff from 2,000 square miles of land in the Inland Empire.

The Corps of Engineers has estimated that 3,000 Orange County residents would die if storms similar to those that caused the county’s great flood in 1862 struck the Santa Ana River today.

The corps says the hypothetical 100-year flood would happen if about 15 to 20 inches of rain fell on the river basin within four days.

Such a storm could cause $11 billion in damage and cover with three to four feet of water the river’s massive flood plain, which encompasses most of Anaheim, Garden Grove, Orange, Villa Park, Fountain Valley, Huntington Beach, Westminster, Santa Ana, Tustin, Costa Mesa and Newport Beach.

“The probability of something like that happening increases as the season goes on and the ground becomes more and more saturated, increasing the runoff,” Reiter said.

In 1938, storms along the Santa Ana River washed out bridges, flooded highways and buckled parts of Pacific Coast Highway. Nineteen people were killed. Almost immediately afterward, Congress authorized funds to build Prado Dam.

The last major flood along the river was in 1969, when storms dropped eight inches of rain over six days. The dam held, but some bridges and the asphalt lining of the river’s channels were demolished. Major construction work followed to pave the river’s banks and better control its flow.

The Santa Ana River poses such a severe threat because it is in the middle of a highly populated area and it makes a steep descent, dropping 380 feet in elevation as it crosses the 30 miles between Yorba Linda and the ocean.

Reiter described conditions at Prado Dam on Monday as much better off than at Los Angeles County’s Sepulveda Dam, which could not hold enough of the sudden flow from the strong winter rainstorm.

“They got a cloudburst, literally, and the facility there couldn’t handle that amount of rain,” he said. “But we’re not anticipating that here, at least not tonight. The good news is (that Prado Dam) is relatively empty right now.”