Marin’s disaster triple threat

Marin’s disaster triple threat

27 September 2005

published by www.marinij.com


USA — Most of Marin is sandwiched between two major active earthquake faults, the San Andreas and the Hayward, both of which have generated great earthquakes in the past. A third fault, Rogers Creek, is less than 10 miles from northern Marin.

According to a study published by the U.S. Geological Survey in 2003, there is a 62 percent probability of major earthquake, magnitude 6.7 or greater, striking the Bay Area by 2032. In 1989, the magnitude 7.1 Loma Prieta earthquake triggered intense shaking in Marin communities 90 miles from the epicenter of the quake.

In the event an 8.3 magnitude quake occurs on the San Andreas fault, “there will be hundreds of deaths, thousands of injuries requiring hospitalization, and tens of thousands of minor injuries requiring first aid,” predicts the Marin Municipal Water District’s disaster plan.

In addition to casualties, the quake could cause secondary emergencies such as fires, flooding, tsunami, hazardous materials spills, landslides, car accidents and dam failures.

A 6.7 magnitude quake on the Hayward-Rodgers Creek fault would destroy 2,690 living units in Marin and leave 6,725 people homeless, according to projections by the Association of Bay Area Governments. A similar quake on the Hayward fault would leave 5,300 people in Main homeless by knocking out 2,125 housing units.

According to the county’s disaster plan, Marin faces electrical power failure throughout the county because of the potential loss of the Pacific Gas & Electric substation at Ignacio.

Mill Valley’s emergency plan notes that unreinforced masonry buildings in the downtown area will suffer moderate to severe damage. The plan states that “Pancaking of these older structures during business hours may entrap many persons. The city’s densely settled alluvial lowlands have a potential for heavy damage.”

Sausalito’s disaster plan states that “In the densely populated business district, damage potential is extreme.” Sausalito’s plan also notes that in the hill areas, “there are many older homes which will probably suffer great structural damage, along with cliff-hanger homes with one end of the house tied to the hillside and the other supported by wood or concrete posts, some being 50 to 70 feet high.”

Fairfax’s plan warns of landslides in steep hill neighborhoods, but says the greatest risk is in the downtown area where there are half a dozen reinforced masonry buildings and one non-reinforced masonry building.

Corte Madera’s plan states that “a large earthquake during or soon after a sustained period of moderate to heavy rainfall could produce a landslide problem of monumental proportion.”

Significant damage to the transportation system is expected, which will make getting the injured to local hospitals or bringing in help from outside the county difficult, if not impossible, for some time.

It is expected that the Golden Gate Bridge and Richmond-San Rafael Bridge may be unusable for 24 hours because of damage to the approaches.

Highways 1 and 37 may be impassable for at least 72 hours due to liquefaction, rock slides and debris in the roadway. Highway 101 may be impassable in spots for at least 12 to 24 hours due to collapsed bridges, settling of the roadbed in filled areas and rock slides. Lucas Valley Road and Sir Francis Drake Boulevard are expected to be impassable for the first 24 hours.

FIRE: THREAT OF BLAZES ALWAYS LOOMING OVER COUNTY

For many living in Marin, a fire on Mt. Tamalpais is the most likely and frightening disaster.

At particular risk are residents living on the eastern rim of the mountain in Mill Valley, Larkspur, Corte Madera, Kent Woodlands and the Ross Valley. Hundreds of homes are situated in box canyons surrounded by dense flammable vegetation.

The fourth largest but most destructive fire in Marin history was the Mill Valley Fire in July 1929. It burned 1,000 acres and destroyed 110 homes in Mill Valley. It caused more than $1 million in damage. More recently, the Mount Vision Fire in 1995 burned more than 12,000 acres of Point Reyes National Seashore, destroying 48 homes.

The Mount Vision Fire did nothing to diminish the fuel loads on Mt. Tamalpais that have been accumulating since 1929. Fires that would have occurred under natural conditions were suppressed as the population living in wildland areas burgeoned.

A vegetation management plan prepared for the Marin Municipal Water District in 1995 reported that vegetation in the area would support “a catastrophic wildfire under severe weather conditions.” It predicted that the fire would burn thousands of acres and large numbers of structures. “Fire departments would not be expected to control a wildfire under such conditions; the fire would ultimately be controlled when weather conditions changed,” the study stated.

In some areas, the fuel load has increased since 1995 due to the spread of sudden oak death. Dying oak trees also allowed forests to be increasingly dominated by more flammable trees such as bay and laurel.

The county’s disaster plan says that “Because of the massive build-up of fuel and the proximity of residences, any wildland fire has the potential to become a major inferno.”

Some Marin homes built prior to code changes in the early 1990s still have wood shake roofs that are highly combustible. Other homes built into hillsides using stilt construction can be destroyed if a fire ignites at the base of a hill and travels upslope faster than fire engines can respond.

Fires in undeveloped areas can be caused by lightning, arson, an accident or a structural fire that spreads to wild land. Should an earthquake hit during Marin’s six-month dry season, it could easily touch off a major fire.

Mill Valley’s emergency plan states, “In the event of an earthquake, water heaters may fall over, and landslides and lateral spreading of the ground may occur, causing gas lines in homes to rupture and fire to result.”

In addition to possibly starting a fire, a quake would make it harder to extinguish a blaze.

Sausalito’s disaster plan predicts that a major quake would cause water mains there to rupture “reducing or eliminating water pressure for fire fighting.” In ridgetop areas, water supplies can be rapidly depleted under normal conditions.

Evacuating people from threatened homes and getting fire engines in to fight the fire will be difficult even if there is no quake. Roads in the hillside areas are typically narrow and winding and often dead end. Where fire roads exist, they are often overgrown. Residential parking on narrow streets greatly limits access of emergency vehicles.

Directing evacuees to the correct route out and keeping roads clear will be a major challenge for public safety officers.

FLOOD: MUDSLIDES, WATER COULD OVERWHELM CONTROL MEASURES

The biggest natural calamity to strike Marin in recent memory was the 1982 flood.

On Jan. 4, 1982, a ferocious Pacific rainstorm unleashed 12 inches of rain on the county in 32 hours. The deluge caused landslides and turned streets into chest-deep rivers. Thousands of people were evacuated and several hundred housed at emergency shelters.

Before it was over, four people were dead, including residents in Tiburon, Sausalito and San Rafael whose homes collapsed in mudslides.

Slides blocked Highway 101 and other key thoroughfares. Thousands of San Francisco commuters were stranded. Power and telephone lines were snapped. Inverness and other remote neighborhoods were isolated for days. Communities bordering Corte Madera Creek, which has a history of flooding, were hit hard.

The storm destroyed 35 homes and damaged another 2,900. Twenty-five businesses were destroyed and 800 damaged. The damage total exceeded $80 million.

Flooding is a persistent menace. In recent years, the winter storms of 1970, 1973, 1983, 1986, 1998 and 2005 also caused significant damage.

Several flood control projects have been completed since the 1982 flood. Nevertheless, areas of Marin remain at risk, according to the county’s disaster plan.

“Although the current Corte Madera Creek Flood Control project is nearly complete, flooding will still occur for storms greater than a 40-year recurrence flood event and can cause significant infrastructure damage in all of the surrounding areas,” the plan states.

“Potentially all nine southerly and some centrally located communities of Marin County would be impacted,” the plan continues. “The northeast part of the county, densely populated around the flood plain zones, is threatened every winter and still experiences some damage during winter storms despite the completed Novato Creek Flood Control Project.”

The plan notes that low-lying areas such as Santa Venetia are extremely susceptible to flooding and rely on levees and pump stations to control flooding.

Storms aren’t the only possible cause of flooding in Marin. An earthquake could cause flooding by damaging one of Marin’s dams, or by setting off a tsunami.

Communities near five major dams in Marin would be flooded if they failed. The dams are on Stafford, Phoenix, Alpine and Kent lakes and Nicasio Reservoir. A quake could cause a dam to fail, or it could cause a wave that would top a dam.

Should Stafford Lake dam fail, 4,230 acre-feet of water would surge into Novato, reaching San Marin Drive and Sutro Avenue 11 minutes after dam failure. Thirty-two minutes after dam failure, the flood waters would begin to inundate the business district and city government buildings. The water would reach Highway 101 about 50 minutes after dam failure.

Failure of Phoenix Lake dam, which holds 411 acre-feet of water, would pose a serious threat to Ross. Flood waters would reach the first homes in Ross five minutes after dam failure. Fifteen minutes after dam failure, flood waters would inundate the business district and local government buildings of Ross.

As for tsunamis, the huge waves generated by disturbances of the sea floor, danger exists for all areas within one mile of the coast and less than 50 feet above sea level – if the disturbance occurs far away. If the disturbance is close by, all areas within one mile of the coast and less than 100 feet above sea level would be at risk.

The county is awaiting tsunami maps from the state to determine which areas of the county are most at risk.


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