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New threat in burn zones: flash flooding

unnamed-7While the post-fire cleanup makes progress throughout the Sonoma Valley, a new threat will emerge with any wet winter weather – flash floods in the burn zones.

Wildfires dramatically alter the terrain and soil conditions and leave the ground unable to absorb water, creating ideal conditions for flash flooding and mudflows. Periods of intense rainfall magnify that threat, according to the National Weather Service.

To help residents assess their risk and prepare for flooding, Sonoma County has created a post-fire hazard assessment map. See it here.

Sonoma County is also posting road signs and sending out postcards alerting residents and anyone entering the burn areas of the increased threat of flooding and mudflows.

Owners of properties in the burn areas identified as medium and high risk for flash flooding, mudflows and debris flows will receive a postcard alerting them to the increased probability of these hazards in the event of heavy rainfall.

Residents in locations in and around the burn areas are advised to keep their cell phones turned on at all times, and to have their notifications switched on in order to receive Emergency Warnings from the National Weather Service (NWS) and SoCoAlert. Information on NWS Advisories and Watches is available at weather.gov/alerts.

Residents are encouraged to have an evacuation plan in place and make sure all family members are familiar with it. The risks for residents in and around the burn areas include:

  • Flash floods: A rapid flooding of a low-lying area in less than six hours, which can be caused by intense rainfall. Flash floods are known to roll boulders, tear out trees and destroy buildings and bridges.
  • Mudflows: Rivers of liquid and flowing mud on the surface of normally dry land, often caused by a combination of vegetation loss and subsequent heavy rains. They can cause significant damage.
  • Debris flows: A moving mass of mud, sand, soil, rock and water. They can travel very quickly, and can be very powerful and destructive.

 

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