Storm Proves Value of Flood Controls : Engineering: The sprawling network of dams and canals has functioned well during the rains, experts say.

Floating cars, raging creek beds and people being plucked from muddy streams by helicopter are uncommon sights in the Southland because of an intricate flood control system built around the area’s diverse terrain.

A sprawling network of dams, basins and canals that channel most runoff water to the ocean is designed to protect the 4,000-square-mile Los Angeles County basin from major flooding during storms. Flood control experts say the system has functioned well so far during this week’s deluge, but the ultimate control over its success lies with nature and not with man.

“The best flood control practice is that you release the water (from the dams) up to the point where it doesn’t overflow the system below,” said Donald F. Nichols, chief of the Hydrology and Water Conservation division for the Los Angeles County Department of Public Works. “But when a storm goes on long enough, you exhaust the ability to drain the reservoirs . . . and you can’t control it anymore. Then it’s in God’s hands.”

The area’s massive flood control system was built in pieces over the past 65 years to prevent flat areas below the county’s mountain ranges from turning into lakes during torrential downpours.

A look at the county’s flood control system reveals engineers’ adaptations to the surrounding topography, ranging from a network of catch basins in the South Bay flatlands to a series of hillside dams in the mountainous San Gabriel Valley.

One example is the Sepulveda Dam Recreation Area, which was built to blunt the impact of the floods that periodically ravaged the San Fernando Valley, most recently in 1938. That flood inundated the surrounding area with water, claiming 49 lives and resulting in more than $40 million in property damage.

The Hansen Dam, on Tujunga Wash, is the other major flood control structure in the San Fernando Valley. They determine the outflow from the Valley into the Los Angeles River.

“Those are the only two structures that control water up there,” said John Peterson, hydraulic engineer for the U.S. Army Corps of Engineers.

Although the Los Angeles River this week overflowed its banks in the Sepulveda Dam basin, trapping 48 people in their cars on adjoining roadways, experts say the system did what it was designed to do--hold runoff water during a storm.

Flood control authorities say that although the existing system is vast and complicated, the basic principle behind it is simple.

Water running off from mountains and canyons is trapped by a system of dams and then channeled through the basin’s three major rivers--Los Angeles, San Gabriel and Rio Hondo. The amount of water running through the channels is controlled at the dams, which means that only a massive “storm of the century” would force water over the banks of these rivers and cause severe flooding, officials said.

The Los Angeles River collects water from the northwestern portion of the county. Water from the east flows into the Rio Hondo and San Gabriel rivers. Supplementing the dams are 2,370 miles of underground storm drains that collect runoff from streets, curbs and gutters.

The Sepulveda Dam basin is one of five such facilities in the county operated by the U.S. Army Corps of Engineers. In addition, the county Department of Public Works operates 15 dams.

Despite heavy rain this week, the system was not near capacity, officials said Wednesday. The 15 county dams have a storage capacity of 116,000 acre-feet of water. As of Tuesday--the most recent day when figures were available--about 40,000 acre-feet of water were in the dams. One acre-foot equals 325,850 gallons.

But the storm has been fierce enough for flood control experts to set up a command watch to monitor the rising reservoirs. Except for drills, it was the first time since 1983 that the county Department of Public Works has staffed its storm “war room” on the second floor of its 13-story headquarters in Alhambra.

“We have moved from a water conservation mode to a flood control mode,” Nichols said Wednesday.

The county dams are monitored electronically and some have workers who live on the premises.

Nearly a dozen of the county’s major dams are tucked away in the San Gabriel Mountains, where rainfall can reach up to 40 inches per year, about three times the average rainfall in downtown Los Angeles.

“Could the Sepulveda situation happen in the San Gabriel Valley?” Nichols said. “Sure, given the right weather patterns.”

The Westside of Los Angeles has an independent drainage area, separate from the vast San Gabriel River and Los Angeles River watersheds.

An elaborate system of pipes, catch basins, open-air channels and underground concrete “boxes,” as big as 11 feet high and 9 feet wide, drain rainwater from the southern slopes of the Santa Monica Mountains, the Hollywood Hills and the rest of the Westside into Santa Monica Bay. During a big storm, some water is kept temporarily at West Hollywood’s Pan Pacific Park, which doubles as a flood-control basin.

Most of the runoff ends up in the concrete covered bed of Ballona Creek, which pours into the ocean at Marina del Rey. The capacity of Ballona Creek is 46,000 cubic feet per second, more than five times the volume registered during the storm Wednesday.

The South Bay’s largely flat terrain and a vast network of storm drains protect that area from the types of mudslides and flooding that have hammered parts of Malibu, Ventura County and the San Fernando Valley, officials say.

“There is very little concern about the South Bay area (because) the existing facilities should handle the rainstorm,” said John Mitchell of the Los Angeles County Flood Control District.

Along the eastern border of the South Bay, for example, the Dominguez Channel has greatly increased storm protection for many local communities because it serves as a drainage point for parts of Gardena, Torrance, Carson and other cities that once had vast areas of marshland.

In the Southeast and Long Beach areas, the flood control system is largely controlled by the Whittier Narrows Dam and several smaller tributaries. Officials there were confident that they would not suffer major flooding unless storm conditions worsened dramatically.

During heavy downpours, the U.S. Army Corps of Engineers uses the Whittier Narrows Dam to hold back water from the north and control the flow of water down the San Gabriel, Rio Hondo and Los Angeles Rivers to the ocean.

Nichols said the basin’s flood control system works primarily because hydrology experts have had nearly 100 years of trial and error in their battle to control water flow in Los Angeles County.

“The game that we play all over the basin is that when we get surplus water flows, we bleed them out,” Nichols said. “Water used to rip this joint apart all the time. This system was not built by accident.”

Times staff writers Mathis Chazanov, Richard Holguin, Berkley Hudson, Greg Krikorian and Jim Herron Zamora contributed to this story.

FLOOD INFORMATION: For more detailed local flood control information, please refer to stories today in The Times’ San Gabriel, Southeast/Long Beach and Westside sections, and the South Bay Edition.

Back in Time

Here are some major floods and rainstorms this century in the L.A. area.

1914: January flooding inundated 1,094 acres of farmland and damaged 100 to 200 highway bridges, railroad lines, public utility wires and pipelines. The flooding prompted the state Legislature to create the Los Angeles County Flood Control District.

1934: In a New Year’s Day flood, 40 people died and thousands of tons of rock and debris fell into Glendale, Montrose and La Crescenta. More than 242 homes were destroyed. Although the storm dropped 12.86 inches of rain in Pasadena in 48 hours, the Rose Parade continued. The theme that year: “Tales of the Seven Seas.”

1938: The city’s worst flooding on record; 100 people died and $35 million in property was damaged. Bridges collapsed into the Los Angeles River, Van Nuys was isolated by high water, communications and roads were disrupted.

1952: Nearly 10 inches of rain over a two-week period in January caused at least eight deaths and the evacuation of thousands. Schools closed and trains were rerouted. In the San Fernando Valley, floodwaters caused 1,000 cesspools to collapse and overflow, creating a health hazard.

1969: In the heaviest deluge since 1938, more than 22 inches fell in the Los Angeles Basin in a six-week period of January and February. At least 90 deaths were linked to storms across Southern California. In Ventura, a wall of water surged down the Santa Clara River and destroyed the city’s marina. At one point, 2,000 workers were deployed to clear roads in the city of Los Angeles.

1978: A series of February and March storms closed highways, caused mudslides and washed more than 30 corpses out of a Tujunga cemetery. At least nine deaths were blamed on weather in Southern California. More than 300 Los Angeles County homes were damaged.

SOURCE: Times files, Southern California Quarterly

Compiled by researcher Michael Meyers