As 'monsoon moisture' continues to hit Colorado, the public has been repeatedly warned to avoid burn scar areas, leaving many to wonder why these spots are so dangerous.
One big reason that burn scar areas can present issues related to flash flooding during heavy rain is due to the way fire changes the layering of soil. While rainfall is typically absorbed into the ground, fire can create a hardened, water-repellant layer, typically covered by debris that isn't very stable. Not only does this mean water is unable to penetrate the ground, giving it no where to go, it also means that there's often ample material for rain to carry downhill as this standing water moves with gravity. This is why flash floods can be more common in burn scar areas, as well as why chances of mudslides, rockslides, and debris flow can increase on angled slopes that a fire has recently scorched.
This same issue with soil being non-permeable can also become problematic when severe dryness is present.
According to the National Weather Service, all it takes is a half-inch of rain in less than an hour to cause flash flooding in a burn scar area.
The lack of vegetation that results from a wildfire can also be problematic, as this vegetation aids in water absorption and distribution.
It's also crucial to note that the impact of flash flooding can be seen many miles away from rainfall. For example, heavy rainfall in the area of a burn scar may pull debris into a river that eventually results in blockage or flooding in a more narrow section far downstream.