Sorry, Noah, Flood Control Worked

The onslaught of record rains marked the first serious test of $220-million worth of improvements made over the last few years to some of the most vulnerable stretches of Southern California’s flood-control system, and officials said the network of dams, rivers and storm drains has so far withstood the downpours.

The region historically has been susceptible to flooding during intense storms, and officials have been working to shore up flood-control infrastructure across the Southland. They have paid particular attention to areas most likely to flood, such as a swath of southeast Los Angeles County covering parts of Long Beach, Lakewood, Downey, Compton and surrounding communities.

Los Angeles County and the Army Corps of Engineers raised 21 miles of levee and modified other stretches of the Los Angeles and Rio Hondo rivers to protect against a failure that, in a massive storm, could inundate 82 square miles.

That work was completed in 2002, but the region was then in a dry period that only ended last fall with a series of storms that could make the 2004-05 rainy season the wettest on record.

“We didn’t even see minor flooding in our community,” said Don Waldie, spokesman for the city of Lakewood, which lies in flats long considered at high risk for flooding.

But despite the success, Waldie and other water experts said the storms so far have not produced the kind of intense downpours that could cause parts of the flood-control system to fail.

Los Angeles County’s network of 15 dams, debris basins and concrete-encased rivers was designed to withstand a “100-year storm.” Improvements made in recent years to the Santa Ana River in Orange County were also intended to provide protection from such an epic storm.

A “100-year storm” might bring 10 inches of rain over 24 hours to downtown Los Angeles, said Kenneth Pellman, spokesman for the county’s public works department. The record daily rainfall in downtown is 5.88 inches. And the most intense rain to fall this season occurred on Dec. 28, when 5.55 inches were recorded.

So while this season’s storms -- which have dumped 34.51 inches so far -- have been wet, they have not brought destructive concentrations of rain, experts said.

Pellman said such a storm would be rare but far from impossible.

“You can never say never, because there could be some really freaky weather,” Pellman said.

Heavy rains in the early part of the 20th century flooded large portions of Los Angeles and Orange counties, prompting officials to turn natural rivers such as the Los Angeles and Santa Ana into concrete channels.

In his essay “Los Angeles Against the Mountains,” author John McPhee described the extreme microclimates that inspired the channelization of rivers and the raising of towering dams and huge debris basins.

“In one year out of every four over the past century, rainfall in Los Angeles has been under 10 inches, and once or twice it was around 5,” he wrote. But certain storms “pick up huge quantities of water from the ocean and just pump it into the mountains.”

“These are by no means annual events, but when they occur,” McPhee continued, “they will stir even hydrologists to bandy the name of Noah.”

Waldie, the author of several books about Southland history who has written extensively about the Los Angeles River, said a series of intense storms could create problems despite the recent improvements to the flood-control system.

He describes a scenario in which wildfires scorch vast expanses of the San Gabriel Mountains and foothills followed by inordinately high volumes of pounding rain -- an inch an hour -- over a day. The combination of intense rain that brings debris flowing down rivers could be disastrous.

“The whole system wouldn’t fail, but large chunks would,” Waldie said. “It’s poker, it’s gambling, it’s Las Vegas odds. Every resident who lives near the San Gabriel and L.A. rivers in effect goes to Las Vegas and pulls the big slot machine during the rains. The odds are very low, but in fact, someone does ‘win.’ ”

William Patzert, a meteorologist for the Jet Propulsion Laboratory, said it would probably take an event such as a hurricane to cause widespread flooding.

It’s not as improbable as it sounds, he said, although the last hurricane to strike Southern California hit San Diego in 1858.

“It’s not a preposterous scenario. The oceans are warming, so the possibility of eastern Pacific tropical hurricanes is there,” Patzert said. “It’s going to happen, we’re going to have a hurricane.”

Despite its reputation for endless sunny days, Southern California has seen its share of weird weather. In 1983, a tornado destroyed homes and businesses south of downtown before ripping off a portion of the L.A. Convention Center.

In November 2003, a freak storm hit South Los Angeles. Residents awoke to frontyards piled high with ice and homes severely damaged by flood waters.

“You want to talk about 100-year storms,” Patzert said. “Now, that was a 500-year event that happened in our lifetime.”

The flood-control system is not perfect. Even with the improvements, heavy rains continue to sometimes flood homes and intersections.

The storm-drain system, less stalwart than the flood-control channels, has frequently been overwhelmed in parts. Officials have struggled for years with how to improve areas that are most susceptible to flooding, including Sun Valley and the Antelope Valley.

Engineers continue to improve the system as storms highlight the weak points.

In 1980, rains caused the lower end of the L.A. River -- roughly from Vernon to Long Beach -- to reach its flood-control capacity. The Army Corps of Engineers discovered that some of the most vulnerable portions of the river were protected only against much smaller storm and flood events -- the kind that could strike the region every 25 years.

That 21-mile stretch of the river had been leveed but remained at high risk of massive flooding in a large storm.

The levees were raised by as much as 8 feet and the level of protection was increased so that the river could withstand the kind of flooding that could occur every 133 years, said Joseph Evelyn, chief of the hydrology and hydraulics branch of the Corps of Engineers.

Construction of culverts was among the improvements that followed flooding in 1992.

Officials are completing a $1.3-billion project designed to better protect Orange County and Inland Empire residents against flooding by widening Santa Ana River channels and raising the Prado Dam.

The flood that is largely credited with creating the modern flood-control system occurred in 1938.

Fueled by El Nino -- a weather system involving warm Pacific waters -- five relentless days of rainstorms caused thousands of square miles of Southern California to be flooded. More than 100 people died and thousands were left homeless.

Large portions of Orange County were under water for days. During the worst stretch, 5.88 inches of rain fell in downtown L.A. over 24 hours -- the wettest day recorded in the civic center.

The event accelerated efforts to improve the flow of runoff from the mountains to the ocean, such as turning natural creeks and rivers into concrete channels.

“The system is not very aesthetically pleasing to most of us, and it would be nice if it had more natural-looking channels,” Evelyn said. “But it does the job it was intended to do extremely well.”

Still, experts said all these contingencies don’t take into account the kind of rare epic storm that could overwhelm large portions of the system.

“Every century or two or three we could have the 25 inches of rain in a 24-hour event which could overwhelm everything,” Patzert said. “Think of the Old Testament raining down on us.”