As the death toll and number of missing continue to soar as a result of California’s deadliest fire on record, relief is finally on the way. But as we count our blessings this Thanksgiving, could the positive rainfall forecast bring too much of a good thing? Burn scars are especially susceptible to mudslides and debri flows. Hundreds of thousands of acres of recently charred ground will pose a major risk.
Potential for significant travel delays on Wed afternoon into Wed night due to projected rain impacts combined with already very busy holiday traffic. Threat of rockslides/mudslides, especially Hwy 1 and canyon roadways in #WoolseyFire and #HillFire burn areas. #LArain #cawx pic.twitter.com/ndZKru8Y9p
— NWS Los Angeles (@NWSLosAngeles) November 18, 2018
Let’s break down the threat and how it could play out. First off, we have to understand the land prior to a debris flow.
When dealing with an area of terrain that has vegetation, you’re working with a certain depth of soil that is relatively strong and healthy. A healthy ground promotes more growth and sustains the growth that is already there. A
After a period of extreme drought or after a wildfire, the ground becomes weak and vulnerable to any kind of water. When water flows over that ground – it becomes hydrophobic (A.K.A. water resistant). When this happens, the ground becomes solid like concrete. Here’s a visual from the NWS in Boise, ID:
In the above video, water is poured onto a recently burned area. After the fire, the ground cannot absorb any of the water. It runs off, collects any loose debris, and heads downhill. Eventually coming to a halt downstream. When this happens in real life on a larger scale, boulders, trees, and other large debris get picked up and they become extremely dangerous.
For more information about the dangers of debris flows and what causes them. Here’s WeatherNation’s John VanPelt:
Interesting facts about debris flows:
~most common within 2 years of a wildfire
~normally range between 600 and 300,000 cubic meters. (about the size of a football field filled with 65 meters worth of mud and debris.)
~the size of the area burned, the gradient of the slope in which the burn happened, and the properties of the soil all contribute to how bad a debris flow can be
~rainfall conditions also play a crucial role (rain rate mainly – how much rain falls in a certain amount of time)
What should people who live near a burn scar do to protect themselves from potential flash flooding and debris flows?
-Have an evacuation/escape route planned that is least likely to be impacted by Flash Flooding or
-Have an Emergency Supply Kit available
-Stay informed before and during any potential event; knowing where to obtain National Weather Service (NWS) Outlooks, Watches and Warnings via the NWS Riverton website, Facebook, Twitter, or All Hazards NOAA Weather Radio
-Be alert if any rain develops. Do not wait for a warning to evacuate should heavy rain develop.
-Call 911 if you are caught in a Flash Flood or Debris Flow
For more information visit: Debris Flow Info
For WeatherNation, Karissa Klos