In my previous post, I discussed just how damaging an unnatural crown fire is not only to the ecology of an area, but also to the soils. It is this soil damage that poses the greatest risk to people living in areas burned by crown fires.
If heavy rains occur after the fire, the bare soils will erode. With no plant matter to help shield soil from the rain and no roots to absorb the water, erosion will be significant and widespread as the soil is knocked loose and carried away by the rain. This can happen on bare soils of any kind, but it is especially true of the hydrophobic soils that develop in the intense heat of crown fires. This water-resistant soil layer forms beneath the surface, causing the vulnerable topsoil to become saturated with water. Once it is waterlogged enough, it will slide downhill with the rain in a phenomenon known as sheet erosion.
This widespread erosion will pollute rivers, lakes, and streams downhill from the burned area. In a healthy forest or meadow, water runs from a point of high elevation to lower elevation, finally reaching a channel, stream, creek, or river. If soil is included in that runoff at significantly elevated levels, the waterways will become polluted with excess sediment. This can reduce visibility in these waterways as well as alter the water chemistry of the affected areas, which can harm wildlife (and humans) that rely on them.
That’s if we’re lucky. If we’re not, a full-on landslide can develop, which can cause loss of life and damage property as quickly as fires can. We witnessed this in the Southern California community of Montecito, where 100+ structures were destroyed and over a dozen people lost their lives to the mudflow.
Now let’s assume the rains are still heavy, but landslides don’t occur. Water will still slide off the impenetrable soils with virtually no absorption, resulting in flash floods.
Make no mistake: you are not out of the woods just because your home is still standing. According to the National Weather Service, you may be at risk if you can look uphill from your house and see a burned area. Until the landscape heals sufficiently enough to absorb rainwater, resist erosion, and sustain vegetation, you must keep your guard up. Catastrophe is still possible.
Stay tuned for my next post, where I’ll discuss what you can do to mitigate your risk. In the meantime, if you are in an affected area, have your vehicles packed and ready. If heavy rain is in the forecast, act with discretion and find a place to stay for the night if necessary. If heavy rains come unexpectedly, act with discernment. Your life is too important to leave it to chance.
Mejia, Brittny; Hamilton, Matt; Tchekmedyian, Alene; and Chang, Cindy. 2018. Up to 43 people still missing in Montecito; dead include four children. Los Angeles Times. http://www.latimes.com/local/lanow/la-me-ln-mudslide-recovery-mainbar-20180111-story.html
National Weather Service/National Oceanic and Atmospheric Association. Burn Scars Have an Increased Risk of Flash Flooding and Debris Flows. https://www.weather.gov/riw/burn_scar_flooding
National Weather Service/National Oceanic and Atmospheric Association. Post-Fire Ground Conditions. https://www.weather.gov/images/riw/postfire.jpg
National Weather Service/National Oceanic and Atmospheric Association. What Happens with Heavy Rainfall. https://www.weather.gov/images/riw/burnscar_ff.jpg
US Department of Agriculture, National Resources Conservation Service. Soil Quality Resource Concerns: Hydrophobicity. 2000. https://web.archive.org/web/20040726160247/http://lamar.colostate.edu/~rmoench/hydrophobic.pdf
Wikipedia. 2018 Southern California mudflows. 2018. https://en.wikipedia.org/wiki/2018_Southern_California_mudflows#/media/File:Santa_barbara_county_neighborhood_affected_mudslides_-_09_Jan_2018.jpg