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Recent Storms Bring San Francisco Bay Region to Beginning of the 2002-2003 Landslide Season
Released: 12/19/2002

Contact Information:
U.S. Department of the Interior, U.S. Geological Survey
Office of Communication
119 National Center
Reston, VA 20192
Raymond Wilson 1-click interview
Phone: 650-329-4892

Following a series of intense rainstorms over the past weekend (Dec.13-17), the annual winter landslide season has begun once again in the San Francisco Bay region. The U.S. Geological Survey (USGS) advises that moisture levels in local soils have now reached the point (soil saturation) that is considered a precondition for future landslide activity in most of the region. This threshold of soil saturation is based on rainfall and stream flow data and numerical models of moisture in steep hillslope soils. This means that heavy rainstorms in the coming winter months could cause landsliding on susceptible slopes.

Fast moving flows of mud and rock, called debris flows or mudslides, are among the most numerous and dangerous types of landslides in the San Francisco Bay Area. Intense prolonged rainfalls on steep hillslopes saturate soils causing them to become unstable and move rapidly downhill as debris flows. These catastrophic flows are capable of destroying homes, washing out roads and bridges, sweeping away cars, knocking down trees, and obstructing streams and roads with thick deposits of mud and rocks. Past damaging debris flows in the Bay region occurred in 1982, 1986, 1995, and 1998. Because debris flows can begin suddenly with little or no warning, it is essential to be prepared.

Persons living in hilly or mountainous areas of the Bay Area should watch for fresh cracks or fissures in the soil, new or increased tilting of trees or poles, or any other signs of recent earth movement. Also watch for changes in the patterns of storm-water drainage on nearby hillslopes and streams—obstructed drainage may increase the potential for landslides. If these conditions are found, property owners should immediately request an inspection by a licensed engineering geologist or soils engineer. Local residents should also monitor National Weather Service alerts during periods of heavy rain for information on the likelihood and possible areas of landslides.

USGS issues this as an advisory message only; a reminder that landslides are very common during the rainy season in the Bay Area. Residents are advised to seek information on the stability of your property, and be prepared to avoid problems before they occur.

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