Week of February 21, 2005- Update

In view of forecasts by the National Weather Service that has significant rainfall continuing in southern California into Wednesday, the U.S. Geological Survey is extending the landslide and debris flow advisory for southern California through Wednesday, February 23.

Significant debris-flow (mudflow) activity is likely in areas that are susceptible to debris flow (see map below) and receive intense rain (more than 2 inches in 6 hours in lowland areas, more than 4 inches in 6 hours for the mountains) on already wet slopes. Areas that have been burned in recent wildfires may be even more vulnerable, so that significantly lesser rainfall amounts over shorter time periods could produce hazardous debris flows. Counties that can potentially be impacted include: San Diego, Riverside, Orange, San Bernardino, Ventura, Los Angeles, Santa Barbara and San Luis Obispo.

Areas of potential danger from debris flow include areas downslope and downstream from the susceptible areas shown on USGS maps (http://geopubs.wr.usgs.gov/open-file/of03-17/) and from the recent burn areas. Fatalities, injuries, and property damage from debris flows commonly occur in low-lying areas such as canyon floors and near the mouths of canyons.

Because debris flows can begin suddenly with little or no warning, it is essential to be prepared. During the drier periods between storms, residents living in the foothills and mountains in southern California should watch for fresh cracks or fissures on hillsides, tilting of trees or utility poles, or other signs of recent earth movement. They should also watch for changes in the patterns of storm-water drainage on nearby hillslopes and streams. Obstructed drainage may increase the potential for landslides. During periods of heavy rain, residents should monitor local National Weather Service Flash-Flood watches and warnings, which will contain more detailed information on specific times and places of concern for debris-flow activity. Residents should stay awake and alert because many debris-flow fatalities occur when people are sleeping. Residents should listen for unusual sounds, such as rushing water, cracking trees, or rolling boulders; be prepared to move quickly; and be especially alert when driving because road cuts and embankments are often susceptible to debris flows and rock falls. Residents should also watch for fallen trees.

An additional consequence of the above-normal rainfall in January and February in southern California is the potential for activation of deep-seated, slow-moving landslides. Several areas have already experienced movement or reactivated movement of deep-seated landslides.   In the weeks and months ahead, we expect to see numerous additional deep-seated landslides activate in response to the near-record amounts of rainfall that have fallen in recent weeks. Although deep-seated landslides can result in property damage, they develop and move slowly, and are usually not as hazardous as the debris flows we saw in response to the early January storm. 

For additional information contact the U.S. Geological Survey Landslide Information Center at: http://landslides.usgs.gov or by phone at: 1-800-654-4966