News & Columns > News Team > Debris Flows Possible In Burn Areas

Debris Flows Possible In Burn Areas

Thunderstorms Predicted for Mountain Areas

By News Team
Thursday, July 10, 2008

The National Weather Service and other forecasting agencies are predicting possible thunderstorms for our local mountain areas over this weekend. These are common for summer storms but are somewhat early for this year.

Therefore, the county is reminding residents about the dangers of possible debris flows in and below the areas recently burned by the Slide and Grass Valley wildfires in October 2007.

Flooding possible in burn areas. (Photo by Michael P. Neufeld.)

Debris flows are among the most hazardous geologic phenomena; debris flows that followed wildfires in southern California in 2003 caused tens of millions of dollars of property damage.

A short period of even moderate rainfall on a burned watershed can lead to debris flows. Rainfall that is normally absorbed into the hill slope soils can run off almost instantly after vegetation has been removed by wildfire.

This causes much greater and more rapid runoff than is normal from creeks and drainage areas. Highly erodible soils in a burn scar allow flood waters to entrain large amounts of ash, mud, boulders, and unburned vegetation. Within the burned area and downstream, the force of rushing water, soil, and rock can destroy culverts, bridges, roadways, and buildings, potentially causing injury or death.

The United States Geological Survey (USGS) has a report from December 2007 that addresses the potential hazards of the debris flows as a result of the Slide and Grass Valley wildfires in October 2007. Their website contains more in depth information on this report; it can be viewed at http://pubs.usgs.gov/of/2007/1416/.

The County distributed information packets, conducted community meetings and notified residents in the affected areas of potential hazards after the fires and during the winter storm events. The County Department of Public Works is again asking residents to stay clear of flood control debris basins and channels.

Residents are urged to be aware of their surroundings and approaching weather systems, and review and update their emergency and evacuation plans. If roads leading away from their property are in danger of being washed out, residents should identify the nearest high ground.

In addition, residents are reminded to not drive where water is over the road as flood waters can rise rapidly and sweep a car and its occupants away. Taking these precautions will help protect lives and property.