‘100-Year Flood’ Plan Perils Sepulveda Basin

Federal officials may force Los Angeles to gouge a huge amount of earth from farm and recreation areas in the Sepulveda Basin because a dike to be built around a sewage treatment plant there will reduce space for storing water in the event of a major flood.

City officials, who want to avoid the $5-million to $7-million excavation project and the damage it would cause, contend that the dike around the Donald C. Tillman sewage plant will have a minuscule effect on flood heights during a major storm.

They said that during a “100-year flood”--a storm so severe it has only a 1% probability of occurring in any year--the dike would raise the water level in the basin just over 1 1/2 inches.

No Disruption in Basin

“We don’t want to disrupt what’s in the basin,” whether it’s agriculture or recreation, said Clark Robins, assistant division engineer with the city Department of Public Works.

But Wanda Kiebala, Sepulveda Basin project manager for the Army Corps of Engineers--which owns the basin and the flood-control dam there--said the city probably will have to go ahead with the excavation to offset the effect of the dike.

“We have a flood-control project which is there to control floods, and not to inundate people who are adjacent to it,” Kiebala said. She added that a 1 1/2-inch rise in water levels in the 2,000-acre basin during a flood would be significant.

Recent estimates by the engineering corps show that its flood-control channels along the Los Angeles River and tributaries no longer can meet their design goal of confining a 100-year flood. Rainfall today creates bigger storm flows than similar amounts in the past, partly due to replacement of open fields by streets and buildings and the use of storm drains to speed drainage to the channels.

The dike is to accompany a major addition to the Tillman plant in the northeast corner of Sepulveda Basin. Ground breaking on the project, which will double Tillman’s capacity to 80 million gallons a day, is scheduled next year. The expansion will cost $63 million to $75 million. Another 40 million-gallon expansion is likely in 20 years.

But the city must include the protective dike because estimates show that Tillman--considered out of reach of floodwaters when it was designed in the 1970s--is now vulnerable to the 100-year flood. The dike, which will ring the plant east of Woodley Avenue and south of Victory Boulevard, will be 20 to 80 feet wide at the base and vary in height from 2 to 15 feet, Robins said.

Enough to Bury a Field

The approximately 480,000 cubic yards of earth that must be removed to compensate for the dike is enough to bury a football field to a depth of about 260 feet, city officials say.

As an alternative to the massive excavation, city officials have asked the engineering corps to allow them to build small berms and levees at key points around the basin--in effect, creating a taller tub to contain the rising water.

Kiebala said the engineering corps has not finally ruled out this alternative, but that compensatory excavation “has been the corps’ standing policy.”

If the excavation is ordered, earth would be stripped from farmland near the Tillman plant and possibly from a wildlife area next to it, Robins said. Topsoil would be stockpiled and replaced on the lowered land surface once the excavation is done.

Officials said they would try to avoid recreational areas, although there is a chance the model airplane field near Woodley Avenue would be torn up. Despite reports that Encino or Balboa golf courses could be dug up, city and corps officials said the chance is extremely remote.

While city officials are unhappy at the thought of the excavation, they are not as glum as Bud vom Cleff, owner of Valley Sod Farm.

Take a Year Off?

Vom Cleff farms corn and sod on 200 acres leased from the corps, including some of the land near Tillman where earth would be removed. Although officials have said farming would be halted for only a year, Vom Cleff is not appeased.

“What do I do with all my employees who have been with me for 15 to 20 years?” asked Vom Cleff, who said he has 25 full-time workers. “Do I tell them to take the year off?”

The calculations that show Tillman needs protection also show that many areas below Sepulveda Dam would be inundated in a 100-year flood.

According to corps estimates, a flood of that magnitude would cause about $2.3 billion in damage along the Los Angeles River and its tributaries--although the lion’s share of damage would be on the lower river around Long Beach.

Ira Mark Arzt, project manager for the corps’ area flood-control study, said hundreds of structures along Tujunga Wash in North Hollywood, near the confluence of the wash and Los Angeles River in Studio City, and along the river near Forest Lawn and Griffith Park could be inundated by a foot or two of water during a 100-year flood.

The damage, Arzt said, would reach $97.2 million along Tujunga Wash and $19.2 million along the river from Studio City to Arroyo Seco, a creek south of Griffith Park.

Not a Minor Problem

“We happen to feel it’s a very important problem,” said Polly Ward, president of the Studio City Residents Assn.

Arzt said while the Valley would experience “minor flooding compared to what happens down on the lower end of the river,” it would not be minor to the people being flooded.

And, he said, because the potential damage from the flood may not be that great in the Valley, major flood-control improvements are considered unlikely.

To build flood-control works, the corps must be able to show flood losses would exceed construction and maintenance costs. Arzt said preliminary indications are that major construction in the Valley would not pass this benefit-cost test.

A draft version of the corps’ area study of the flood-control system is expected to be finished early next year, Arzt said. In August, 1989, a final report recommending specific changes will go to corps headquarters in Washington, D.C.

Arzt said costly structural improvements, such as raised channel walls, probably will be recommended for portions of the lower river.

Arzt said a few detention basins--open water-collection areas along stream channels--may be considered for the Valley along with changes in the way the Sepulveda and Hansen dams are operated during major storms.