One-Up the River

Kathleen Bullard eyed the sky nervously last weekend, wondering whether a few days of hard rain would wash away three years’ work.

Instead, the El Nino-linked storm vindicated her plan to turn a slice of Las Virgenes Creek into a sort of ecologically correct flood-control channel: natural enough to deserve its name, but strong enough to withstand the scouring rush of flood waters.

The project, believed to be the first of its kind in the Los Angeles area, proved a success during its baptism last week, when water in the creek rose several feet above the banks without causing damage from erosion.

And it has attracted the interest of the U.S. Army Corps of Engineers, which will look into whether the technique can be used elsewhere.

“It really held well,” said Bullard, executive officer for the Resource Conservation District of the Santa Monica Mountains. “We just let the natural processes take place.”

Since 1994, Bullard has led the attempt to restore a 500-foot run of the creek just off Lost Hills Road in Calabasas south of the Ventura Freeway. Originally, the idea was to reduce pollution. Developers dumping land by the creek decades ago had created a sheer, 25-foot cliff that was falling off in chunks into the stream below.

But district officials quickly decided to one-up Mother Nature. Not only would they return the creek to a more natural shape, they’d also strengthen it.

Beginning this October, workers trundled away 17,000 cubic yards of earth to shape a gentle slope. Then, they used a new specially woven mesh to create a stream bank that can withstand the high, fast-moving waters of a flash flood without the ugly concrete walls that jacket many Los Angeles watercourses.

The mesh works like re-bar in concrete. First, sheets of the mesh are laid down between layers of earth to create a bank for the river. Then, on top of those layers, trees and grasses are planted so that the roots grow down through 1-inch-square gaps in the mesh. The plants prevent surface erosion and stabilize the bank.

The result: a bank more stable than that of a natural stream, but more aesthetically pleasing than a concrete channel--which is exactly what the Las Virgenes Creek becomes a mile north of the district’s project. There, the creek flows through a narrow, box-like chute that backs up against condos and apartment complexes.

Bullard figures the total cost is about $440,000, and cheaper than concrete. And it may even be safer than the flood-control channels, where raging water and lack of anything to grab on to have resulted in numerous drownings.

“Imagine if the L.A. River looked like this instead of concrete channel,” said David Gottlieb, vice president of the district’s board, as he looked over the slope that one day will be covered with willow and cottonwood, lupine and wild poppy. Down in the creek, water tumbled across moss-covered pebbles.

“This is a different approach,” he said. ‘We hope it sets an example of a different way to deal with creeks.”

Flood control officials said they were, indeed, interested in the project. The Army Corps of Engineers had not heard of a similar technique in Los Angeles, though the mesh has been used frequently to shore up road embankments and slopes above roads.

It has also been used to strengthen stream banks in the East by groups like the Tennessee Valley Authority.

“This is really kind of a new technique for me,” said Chris Sands, a geotechnical engineer for the corps. “I’d like to take a look.”

Still, Sands and other flood-control experts said it was doubtful that the mesh would ever serve to replace existing flood-control channels.

For one thing, authorities questioned whether the technique is necessarily cheaper, since creating stream banks would require purchasing land on either side of the channels. The concrete channels make the best use of minimal land, they said.

But another problem is the type of floods that occur in Southern California. The manufacturer, North Carolina-based Huesker Inc., recommends using the mesh in areas where water flows at speeds less than 15 cubic feet per second. But as flooded creeks in this area rush from the mountains toward the Pacific Ocean, water can move at more than twice that rate.

To handle such high flows, the best option is concrete, civil engineers say. The smooth services are good at stopping surface erosion and allowing water to flow quickly and freely to the ocean.

The mesh is “not going to be a substitute because of the high water flows and erosion,” said Don Wolfe, deputy director of the Department of Public Works. “We get very fast-moving flows, typically where streams are restricted due to development. Those flows rip out anything that’s not heavily reinforced and tied down well.”

Still, Bullard hopes the project will prove the mesh can act as a flood-control alternative, at least for smaller streams.

In the future, the district plans to install a path so the area can be used by picnickers and joggers.

Pushing aside the branch of a willow tree on a recent tour, Bullard looked over the stream.

“It’s just magical here,” she said. “It really is.”