Several storms rained damage down on three Northern California counties earlier this week flooding rivers, saturating the earth and spawning mudslides that tore at least several homes from their foundations.
With property damage estimated at more than $300 million in just the Sonoma, Napa and Marin counties, Gov. Schwarzenegger declared states of emergency in 20 more Golden State counties as damage tallies were incomplete.
From the San Francisco Bay Area counties north to Sonoma and Napa counties several days of rain spawned nearly a dozen deaths, breached levees, uprooted trees, downed power lines and closed roads with mud, uprooted vegetation and debris.
By midweek, as the weather cleared, officials warned residents to be on the look out for saturated earth, high water levels, and general moisture-laden conditions that remained ripe for fueling more mudslides.
Mud and landslides are most common on hilly and steeply sloped areas, but they can occur wherever there is a slope as shallow as 15 degrees or more, experts say. The slides can be backyard size or miles long, shallow or deep and they can move a few inches or several miles in a flash.
They range from soil and mud flows to rock and boulder falls, all of which can take vegetation, structures, vehicles and parts of roadways and other debris -- as well as people -- along for the ride.
Properly engineered site development and building can decrease the possibility of sliding.
To help prevent slides:
- Do not place loose soils on steep slopes.
- Do not channel or concentrate surface drainage onto slopes.
- Do not cut the toe or bottom of a slope without replacing it with a retaining wall.
- Do not strip the vegetation from a slope. Deeply rooted plants that do not require extensive watering make the best type of landscaping for hillsides.
Too often, however, there is no real engineered protection from slides in slide-prone areas. Sooner or later the hillside will bow to nature and give way. Not building where slides are likely to occur is the sanest protection, and in many cases, the only protection.
Unfortunately, cheap land and the cheap housing that can be built there often leads home buyers and builders to overlook a location's potential for danger.
Landslides are so potentially disastrous, insurance protection is rare and prohibitively expensive. Mudslides or mud flows on the other hand, a highly liquefied form of landslide, is covered by flood insurance through the federal National Flood Insurance Program. Flood insurance policies are mandated for certain high flood risk areas, but available to anyone who wants to purchase it. Coverage, however, is usually limited to an amount that, in California, won't cover the loss of a mudslide totaled home.
That means residents in hilly or mountainous regions must remain vigilant about the potential for slides especially after a barrage of rain storms.
Initially, most slides aren't fast-moving and can be measured in inches or feet a day, giving you plenty of time to react. Of course, once the already creeping ground is saturated, the process accelerates.
Geologists say earthquakes can trigger landslides, but slides are most commonly triggered when a combination of factors exist -- heavy rainfall, steep slopes, and loose or soft soil. The water can come from rainfall as it has in recent days, but broken pipes, intensive landscape watering, private septic system-laden land or misdirected run-off can also contribute to setting off a slide.
Where natural slopes have been disturbed by cutting away at the bottom of the slope, there is a higher chance of sliding in that area relative to the undisturbed hillsides. Landslides also can be man-made, caused by cutting roadways and building pads, or placing improperly engineered fill on steep slopes.
Home buyers and current owners who are concerned they are in a slide-prone area, may want to hire a professional to assess the potential for disaster. That could include a structural engineer to examine the home for signs of landslide activity and a soils engineer to inspect the earth.
Tell-tale signs the earth may be about to give way to gravity, include:
- Foundation cracking, doors slightly rotated so they jam -- both on the downhill side -- are indications the land may be creeping.
- An ominous cracking of the ground in an arcuate fashion, like an arch shaped crack rather than a linear crack.
- Hillside scalloped shapes with a berm forming at the bottom of the slope is also an indication the hillside is succumbing to gravity.
- Hillsides with large areas without trees are likely bare because the trees slid down the hill. Hilly areas left barren of grasses, plants, shrubs and trees are particularly vulnerable to slides. Conversely areas dense with old trees indicates stability.
- New above-ground springs, excessive ponding, cracked soil or rocks, bulging slopes, new holes or bare spots on hillsides, tilted trees and muddy waters.
Only very early landslide prevention efforts are successful at holding back the earth. Once the earth starts to move you can protect yourself, but it's almost always too cost prohibitive to stop the slide.