Study Widens Risk Zone of Hypothetical Major Flood

A hypothetical 100-year flood would overwhelm Los Angeles County’s flood control system and blanket up to 100 square miles with water 2 to 10 feet deep, according to a recently completed study by the Army Corps of Engineers.

The area subject to flooding--home to 750,000 people and the site of 145,000 buildings--includes portions of downtown Los Angeles and about 60% of the city of Long Beach. The area is 50% larger than what was considered at risk in estimates made as recently as 1985, according to Ira M. Arzt, manager of the flood study project.

The corps, which has been working on the $5.9-million study for a decade, estimates that the floods would cause up to $2.5 billion in damage and that the risk justifies spending up to $500 million to alleviate the threat.

Worst-Case Flood

During the next six months, the corps will be working with local governments to come up with tentative construction projects to contain flooding. A major portion of the funding--possibly as much as 75%--would come from the federal government, with the remainder from state and local funds.

A 100-year flood is the worst-case flood with at least a 1% chance of occurring in any single year. Such a flood would send a torrent of 150,000 cubic feet of water a second along the Los Angeles River.

“We are sure the levees would break from a 100-year flood,” said Arzt.

During any of several scenarios leading to a 100-year flood, people in the areas subject to flooding could have as little as one hour’s warning before rapidly rising waters went over levees, undermined their footings and went through in full flood. Flood waters can rise several feet an hour along the southern sections of the Los Angeles River.

Heavy Impact on Long Beach

In Long Beach, the Los Angeles River, Coyote Creek and the San Gabriel River would put about three-fifths of the city at risk during a 100-year flood, according to the study.

Long Beach City Manager James C. Hankla said city officials intend to work with Los Angeles County and Army engineers to come up with a flood prevention plan that could be presented to Congress.

Although most of the flooding would occur in southeast Los Angeles County, the report says that flooding also could affect the Tujunga Wash and Burbank in the San Fernando Valley. Cities outside the Valley that also would be subject to flooding are Bellflower, Carson, Cerritos, Compton, Downey, Gardena, Glendale, Lakewood, Long Beach, Los Angeles, Lynwood, Montebello, Paramount, Pico Rivera, Signal Hill, South Gate and Torrance.

In addition to damaging thousands of homes, businesses and public facilities, the report says, roads and railroad tracks could be rendered impassible and floods might cut off electricity and telephones.

Previous planning for 100-year floods in the 2,000-square-mile watershed of the Los Angeles River has relied on 500 miles of channels, 20 flood control dams and 125 debris basins that were built between 1938 and the 1960s.

Increased urbanization and more efficient storm drains have diminished the capacity of the existing system to the point where it no longer can protect residents from a 100-year flood, he said.

In 1980, the system received its most severe test when six winter storms swept over Los Angeles, one after another. Churning flood waters tossed debris onto the top of a 20-foot-high levee in Long Beach. A seventh storm was expected but never materialized. If it had, the floods might have reached the 100-year level, Arzt said.

Possible measures to contain flooding include building levees and dams higher and stronger, constructing new reservoirs, digging tunnels and modifying bridges.

One project high on the list of corrective measures is “armoring” with concrete the unprotected back sides of levees along the Los Angeles River south of the Rio Hondo confluence. Arzt said the armoring project would cost about $100 million.

Before drawing up plans, Army engineers will meet with residents and public officials for local input.