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GUARDING AGAINST LEVEE WOES HERE EARTHEN DAMS INSPECTED, TESTED.

Byline: Judy O'Rourke Special to the Daily News

SANTA CLARITA - With the levee collapses in the Gulf Coast focusing attention all over the country on planning for potential disasters, local attention has long been focused on the seven earthen dams sitting above Santa Clarita when locals envision the worst-case scenario.

The Castaic Lake and Bouquet dams are expected to withstand an 8.0 magnitude earthquake, but neither has been tested beyond the computer modeling method. The largest quake ever to hit the region was about that magnitude, occurring in 1857, about 40 miles to the north.

Those two dams are inspected twice a year, while seven smaller earthen dams that ring the area are inspected annually. The experts are certain they will hold because the stakes are huge: More than 200,000 people - and potentially many more - live downstream.

``We can't allow failure,'' said Fred Sage, chief of the field branch in the Division of Safety of Dams, under the state Department of Water Resources. ``We are conservative when we allow dam design to be built. It's our engineering judgment that in the event of an earthquake these dams would not have any catastrophic failure or loss of water.''

The dam safety division was created in 1929 in the aftermath of the 1928 St. Francis Dam disaster in Saugus. About 450 people downstream were killed when a 200-foot wall of water rushed through the canyon and swept to the coast.

Modern dams are insulated by an impervious clay core barrier that looks like a curtain and is sandwiched between two layers of gravel. Inspectors regularly scan for cracks, but rely on instruments built into the structures that detect movement. All dams have some seepage, but that is measured to make sure it does not exceed an acceptable amount.

Since the 1960s, the standards in dam design have been significantly heightened, Sage said. However, the criteria is not one size fits all. Each dam is built or retrofitted to withstand the worst shaking it could undergo, with magnitude 8.0 as the gold standard.

``Think of it like hurricanes,'' Sage said. ``(Along the Gulf) Coast in Mississippi and Alabama, they should be building dams to withstand hurricane winds, but away from the coast they don't need to withstand as much.''

U.S. Geological Survey seismologist Lucy Jones said earthquakes have caused dam failures around the world.

The San Andreas Fault runs down the state, through Gorman and near Palmdale. While the Bouquet Canyon dam sits within seven to 10 miles of the great fissure, all seven dams are pretty close to it.

A concentration of faults smaller than the San Andreas is located between Sylmar and Santa Clarita, and half a dozen faults are within 20 miles of Santa Clarita.

The area boasts a dubious geological distinction. ``Santa Clarita is among the most active places in the nation outside of Alaska,'' Jones said.

Dams settle slowly after they are built, and cracks that emerge usually result from the construction process, not from quakes, Sage said.

Dam owners are required to survey the structures after the shaking subsides, measuring the height to detect any variance. Sage said two-tenths of an inch is the greatest post-quake movement that has been detected, and this would be considered normal.

Inspectors cannot simulate the shaking on site, so they test structural integrity on a computer program.

The water contained in dams is measured by the acre-foot. If a football field were filled with water to a depth of one foot, that would measure about one acre-foot. The area's three largest dams are Castaic, Pyramid Lake and Bouquet Canyon. Castaic is 340 feet tall and holds 323,000 acre-feet of water. Pyramid, at 386 feet tall, is half the width and holds 180,000 acre-feet. Bouquet Canyon is 190 feet tall and holds 36,000 acre-feet of water.

While earthquakes pose an obvious threat, flooding from the aftermath of storms poses an even greater danger in this area and all over the world, Sage said. Many reservoirs are required to have flood storage. During the recent storms, a number of flood basins approached the 100-year flood mark.

Officials who manage the Castaic Dam lowered the water in the reservoir during last winter's heavy rain - more than 50 inches fell in Santa Clarita - so that water did not overflow and cascade down the concrete spillway. The water was released to other water agencies.

The historic Castaic Elementary School that stood on Ridge Route Road was ordered demolished in the late 1990s because Federal Emergency Management officials said it would be in the path of a disastrous flood if the 25-year-old Castaic Dam failed.

The site, which stood 1.7 miles south of the dam, could have been wiped out if billions of gallons of water overflowed from a structural failure. FEMA provided a $6.6 million grant to relocate the Castaic elementary and middle schools.

A Castaic district risk study said the area could be flooded with 50 feet of water if the dam broke.

Many people who live downstream from dams rarely give them a thought and are not aware of the potential consequences of a failure.

Probably the most graphic failure that nearly occurred was at the lower San Fernando Dam just south of Santa Clarita. The dam settled about 20 feet after the 1971 Sylmar Earthquake, and water came within 4 feet of breaching the dam. That event prompted significant changes in dam design to guard against quake-caused ruptures.

``We want to make sure all dams in California could survive something like that,'' Sage said. ``That wasn't supposed to happen.''

The county's Office of Emergency Services requires dam owners to create flood maps that illustrate which areas would be flooded should a failure occurs. A copy of the maps is filed with the OES. Since the Sept. 11 terrorist attacks, dam owners have been asked to step up security for their facilities and to be vigilant about breaches and vulnerabilities.

Judy O'Rourke, (661) 257-5255

judy.orourke(at)dailynews.com

REGIONALS DAMS

Castaic Dam is 340 feet tall and holds 323,000 acre-feet of water. It is owned by the California Department of Water Resources and was completed in 1973. Lake Oroville feeds its water supply.

Pyramid Dam is 386 feet tall and holds 180,000 acre-feet in its reservoir. Like Castaic, it was built as part of the bond-funded State Water Project. It is owned by the state and was completed in 1973. Its water supply originates in Lake Oroville where it flows down the Sacramento River into the delta. From there, the water is pumped south into the California Aqueduct.

Bouquet Canyon Dam is 190 feet tall and holds 36,000 acre-feet of water. It was built in 1934 and is owned by the city of Los Angeles.

Santa Felicia Dam sits above Lake Piru, off state Route 126. It stands 213 feet tall, and its reservoir holds 100,000 acre-feet of water. It was built in 1955 and is owned by the United Water Conservation District, a public entity.

Elderberry Forbay Dam is located in the upper reach of the northwest arm of Castaic Lake. It is 179 feet tall and holds 28,000 acre-feet of water, which are fed into it from Pyramid Lake. The city of Los Angeles owns the dam and a power plant on the premises. It was built in 1974.

Dry Canyon Dam sits northeast of Santa Clarita. It is 66 feet tall, and its reservoir holds 1,100 acre-feet of water. It was built in 1912 and is owned by the city of Los Angeles.

Drinkwater Dam is 10 miles east of Castaic. It is 105 feet tall, and its reservoir capacity is 92 acre-feet. It was built in 1923, and it is owned by the city of Los Angeles.

Source: Fred Sage, chief of the field branch in the Division of Safety of Dams, state Department of Water Resources.

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REGIONAL DAMS (see text)
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Publication:Daily News (Los Angeles, CA)
Date:Oct 2, 2005
Words:1331
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