Handling the Deluge : System Easily Absorbs Storm’s Runoff : * The Ballona Channel is the final conduit of a multibillion-dollar drainage network. Although it is virtually invisible, the system has tamed an area that was once subject to massive flooding.

The Westside and everyone in it is getting soaked this week. Traffic stinks. Roofs leak. There are mudslides in the hills.

But look on the bright side: The region’s flood control system is keeping most areas from being inundated. Runoff from Wednesday’s storm took up only about a fifth of the system’s capacity.

“It’s definitely water but it’s not unusual,” said Allen Gribnau, head of the planning section at the operations center of the Los Angeles County Public Works Department.

He said that instruments near the mouth of the Ballona Creek Channel showed a flow of about 9,000 cubic feet per second Wednesday morning. The concrete-covered creek, which collects most of the runoff from the Westside and sends it off to sea, can carry as much as 46,000 cubic feet of rushing rainwater a second.

Completed in 1939, the year after massive floods killed between 87 and 113 people (reports differ) and ruined hundreds of homes, the Ballona channel is the final conduit at the end of a multibillion-dollar system that drains most of the Westside.

Other key elements were completed by 1962, but work continued until the late 1980s, funded by a 1970 revenue bond issue. Among the later projects was the excavation of Pan Pacific Park in the Fairfax District, which doubles as a rainfall storage basin.

Starting with small, capillary-like pipes that take water from street-corner catch basins, excess rainwater from much of the Westside surges through underground concrete “boxes” as big as 11 feet high and 9 feet wide.

Then it spills into open-air channels like the one visible under the Fairfax Avenue off-ramp from the eastbound Santa Monica Freeway, and eventually pours into Ballona Creek.

Two separate systems are also in place. One drains the excess rainfall from the canyons above Pacific Palisades, Brentwood and northern portions of the city of Santa Monica.

Another, the infamous Pico-Kenter storm drain, takes rainwater and an unknown quantity of pollutants from the rest of Santa Monica and parts of West Los Angeles, and dumps it out at a popular bathing beach.

“This is an urbanized area, there’s a lot of oil and dirt on the street, and people have the unfortunate habit of dumping oil and paint into our catch basins,” Gribnau said. “People use them as trash disposals, and the rains wash it all into the ocean.”

Although it is virtually invisible, the system has allowed the domestication of an area that once was subject to massive flooding.

Indeed, the big flood of 1868 covered the entire plain of what is now central and western Los Angeles and changed the course of the Los Angeles River. Before the flood, it followed the course of Ballona Creek into Santa Monica Bay. When the water receded, it emptied into the Pacific in Long Beach.

Fortunately, not many people lived here at the time.

“Without this system, we’d have had a tremendous amount of flooding,” Gribnau said. “When it works, we never miss a beat.”

Despite the region’s six-year drought, it is not feasible to capture any of the Westside runoff and save it for, well, a non-rainy day.

In the eastern part of the county, the Public Works Department is able to retain much of the runoff. A complex of spreading grounds at the Whittier Narrows Basin allows the water in the San Gabriel River to sink back into the ground, where it is tapped by local users.

But the heavy clay soils of the Westside make a similar basin impossible.

“Generally for ground-water recharge to work you’ve got to have granular, porous material from the ground to the water table,” Gribnau said. “In the western part (of Los Angeles County), you just don’t have that.”

Instead, residents of most of the Westside get their drinking water from reservoirs high in the hills, filled by water brought in by aqueduct from the Eastern Sierra.

The rain has little effect on the open-air reservoirs and there is no danger of overflowing, according to Marty Adams, water control engineer for the city’s Department of Water and Power.

“If we get an inch of rain, the surface goes up an inch, but at this particular time, because the aqueduct is down (for a few days of repairs), all reservoirs are lower than normal,” he said.

Two of the biggest city reservoirs, Stone Canyon above Westwood and Encino on the San Fernando Valley side of the mountains, are subject to mudslides and other debris during a heavy rain, he said. Barriers have been built to keep out the biggest rocks and tree limbs.

“Stone and Encino typically get the most water, so we typically keep the top five feet free so there’s always a lot of room for rainwater,” Adams said. “It would have to be an incredible rain for it to overflow.”

The water is treated again with chlorine before it leaves the reservoir, he said.

Flood Controls

A huge network of dams, basins and canals that channels most runoff water to the ocean protects the 4,000-square-mile Los Angeles County basin from major flooding during storms. The Sepulveda Dam Basin, which was flooded on Monday, is one of five such facilities operated by the U.S. Army Corps of Engineers in the county. In addition, the county Department of Public Works operates 15 dams.

Water running off from mountains and canyons is trapped by the dams, then channeled into the Los Angeles, Rio Hondo and San Gabriel rivers. 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.

Despite heavy rain this week, the system is not near capacity. 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.

Officials say there also has been no danger of the Los Angeles River flooding downstream. At their peak, more than 51,000 cubic feet of water per second rushed through the lower portion of the river to the ocean; the river could have taken as much as 146,000 cubic feet.

The county dams are monitored electronically and some have workers who live on the premises. The areas around some of the dams are used for recreational purposes during dry periods.

A Separate Drainage System

The Westside is in 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 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 Wednesday storm.