Prado Dam May Break in Flood, Engineers Warn

The U.S. Army Corps of Engineers has found that Prado Dam falls short of national dam safety standards and probably would collapse in the event of the heaviest flood likely ever to occur along the Santa Ana River.

The engineers’ report, delivered to Orange County officials this month, says failure of the dam would send water rushing past it down the river channel, and most of central Orange County would be flooded with three feet of water. Moreover, the rushing water would cross the San Gabriel River and flood portions of Los Angeles County, the study found.

Corps officials called the likelihood of such a severe flood “extremely remote,” something that occurs only once in 10,000 years. It would take 26 inches of rain within three days to produce such a condition, they said.

The corps and most other agencies use that standard, known as the “maximum probable flood” standard, in evaluating dam safety. Prado, on the Santa Ana River just east of the Orange County boundary with Riverside County, is one of a number of dams throughout the nation that have been found to fall short of the safety standard, according to corps spokesman Dennis Majors.

Preparing for the Worst

“It’s conceivable that (such a flood) could happen, and we’re covering a contingency that if it happens, we want to be prepared,” Majors said. “It potentially could threaten lives, simply because it would occur relatively fast.”

The corps has proposed raising the dam and spillway walls by 15.5 feet, a $7.8-million undertaking, and the Orange County Board of Supervisors this week called for “expeditious modification” of the dam as “an important near-term safety improvement.”

Corps officials said Prado Dam, built in the early 1940s, met federal standards at the time, but advancements in scientists’ ability to predict major floods gradually made it clear that the dam was inadequate.

Corps officials concluded several years ago that a major flood could happen along the Santa Ana River, even with Prado Dam in place, and they recommended a $1.2-billion flood-control project to raise Prado by 30 feet, build a new dam upstream in San Bernardino County, and improve the river channel through Riverside and Orange counties.

‘Standard Project’ Flood

That flood protection plan, for which Congress has not voted funds, is designed for full control of a flood that would occur about once every 200 years, known as a “standard project flood.”

But corps officials decided to do even more refined studies of Prado, and they measured the dam’s capacity in the event of the maximum probable flood. They found that the dam’s spillway would not be able to hold back that much water.

Even with the spillway releasing a maximum 400,000 cubic feet of water each second, water would eventually flow over the top of the dam, erode it and, eventually, cause the dam to collapse, corps officials said. At that time, the flow of water from the dam would increase “almost instantaneously” to 1.3 million cubic feet per second, Majors said.

Much of Orange County would have already sustained flood damages with a 400,000-cubic-foot-per-second flow, he said. But the flow after the dam failed would flood much of Anaheim, Fullerton, Orange, Garden Grove, Buena Park, Westminster, Stanton, Santa Ana, Costa Mesa, Huntington Beach and Los Alamitos with about three feet of water. Then it would cross the San Gabriel River and flood La Mirada, Artesia, Cerritos and east Long Beach, the study found.

Typical quantities of releases

from Prado Dam during the rainy season are about 5,000 cubic feet per second.

Majors emphasized that the chances of a flood of the magnitude they are planning for are “remote . . . . It’s a very, very high standard we’re designing to, but obviously, we want to design a fail-safe system with our spillways.”

Orange County officials say they are concerned, nonetheless. County Public Works Director Carl Nelson said he received notice of the corps’ report “only about 10 days ago, and I was alarmed enough that I sent this immediately on to the Board of Supervisors.”

Murray Storm, director of the Orange County Environmental Management Agency, explained, “We think it is important, even though, theoretically, it’s something that is a remote possibility. With storm conditions, you never know when these things can happen.”

Corps officials said the interim plan to raise Prado Dam by 15.5 feet would eventually be tied in with the overall flood-control plan to raise the dam by 30 feet and offset the eventual cost of that project.