Burn Scars and Flash Flooding: How much rain does it take?

Posted at 4:14 PM, Jul 05, 2018
and last updated 2018-07-05 18:14:36-04

With all of the recent fires around Colorado we have a lot of burn scars left behind. These burn scars can have an impact on flooding potential for years to come. With rain expected throughout the evening on Thursday, we’re taking a look at some of the flooding concerns with our local burn scars.

When an area is burned, especially a forest, the trees and vegetation are wiped out and can no longer soak up rainwater as it falls and moves through the area. There is very little to keep rainfall from moving downhill very rapidly, sometimes bringing rocks, mud, and debris with it. This water then moves towards the lower elevations as well as creeks, streams, and rivers that are not used to handling so much water.

A burned area will not hold as much water as an area with abundant trees and vegetation, so the amount of rain needed to cause flooding, especially flash flooding, is severely decreased. With fresh burn scars and active fires like the ones we are currently seeing across our state, it generally takes a storm total of .3" or a rainfall rate of .5"/hour to cause flooding. Rain like that is pretty common, especially during monsoon season. These low thresholds for rain apply to current fires, like the Spring fire and Chateau fire, as well as very recent burn scars like those from the Junkins and Hayden Pass fires.

As time passes, we get more regrowth of vegetation in areas previously destroyed by fire. With the growth comes a higher tolerance for water. For example, the Waldo Canyon and Black Forest burn scars can now typically handle an inch or more of water with a storm before flooding becomes a major concern. 

Flash floods, by definition, happen quickly and often without much of a warning with water and debris spilling downhill. If you live in, near, or downstream of a burn scar you want to be prepared for any flood threat. Have an evacuation plan ready to go and head to higher ground away from scars as quickly as possible. Avoid low-lying areas and ditches, creeks, rivers, streams, etc. Never play, swim, or walk through flood water. Just 6 inches of rushing water can knock a person off of their feet. Never drive through flood water. Turn around and find another way to go.