Have you ever heard of a burn scar? Did you know that these remnants of wildfires can cause dangerous flash floods and debris flows? Well, today we’re going to learn all about it.
While this is a topic most relevant to the west, it’s good information for anyone to know that might visit a mountainous region. Wildfires have lasting effects on the landscape, both in the immediate area and for locations several miles away. Areas downhill and downstream from burned land are susceptible to flash flooding and debris flow, especially near steep terrain. Rainfall that would normally be absorbed will run off extremely quickly after a wildfire. This is because burned soil can be as water repellent as pavement. As a result, much less rainfall is required to produce a flash flood.
This layer of water-repellent soil is formed when organic matter such as trees, shrubs, and plants burn at high intensity. When this happens, water repellent compounds are vaporized and condense on cooler soil layers below, which prevents soil from absorbing water. This will not only cause flash floods, but debris flows. As water runs downhill through burned areas, it can create major erosion and pick up large amounts of ash, sand, silt, rocks, and burned vegetation.
The force of the rushing water and debris can damage or destroy culverts, bridges, roadways, and buildings even miles away from the burned area. The time required for a flash flood to begin depends on how severe the fire was, how steep the terrain is, and the precipitation rate. A good rule of thumb is that half an inch of rainfall in less than an hour is sufficient to cause flash flooding in a burn area, but it depends on all the previously mentioned factors. Light precipitation on steep terrain that was severely burned can cause flash flooding within minutes!
If you are in an area vulnerable to flooding and debris flows, plan in advance and move away from the area. There may be very little time to react once the storms and rain start. A good rule of thumb is: “If you can look uphill from where you are and see a burnt-out area, you are at risk.”