Debris Flow Modeling

Wildfires can increase the risk of debris flows.

The USGS Landslide Hazards Program performs emergency assessment of post-fire debris flow hazards for select fires. The program has assessed 22 fires in WA since 2013.  Use the link above to view debris flow hazard maps from past fires, or to inquire about having a new assessment done.IMG_0778

A note on the science of debris flows….

Debris flows, like this one in 2016 below the Las Lomas fire in California (courtesy of the USGS), have a specific, geologic definition that is often misused. Most observed “debris flows” are actually debris-laden flash floods or “hyperconcentrated flows.”

Flash floods, especially those that originate from recently burned areas, are often described as “debris flows” due to the appearance of sediment-laden water transporting woody and vegetative debris, trash, and carrying gravel, cobbles, and occasionally boulders. Though “debris flow” may be an observer’s description of the event, a true debris flow has specific properties, behaviors, and characteristics that significantly differentiate them from flash floods. What are often describe as a “debris flow” are actually a sediment-rich flash flood called a hyperconcentrated flow (HCF). A HCF is the transition between a flash flood and debris flow. One way geologists differentiate the three is by the percent of sediment (by volume) carried by the flowing water, so a flood contains less than 5 percent sediment by volume, a HCF is around 5 to 60 percent sediment by volume, and a debris flow exceeds 50 percent sediment by volume.

Debris flows are often described as appearing similar to flowing, wet concrete and travel quickly in steep, convergent channels. Debris flow speed may exceed that of the water flowing in the same channel. A moving debris flow can be very loud because they can buoy cobbles, boulders, and debris to the front and sides of the moving debris flow. The sound is often described as similar to that of a freight train and may cause the ground to vibrate. In the post-fire situation, a debris flow may start as a flash flood surge that entrains (picks up) sufficient sediment to transform into a HCF and, if conditions are suitable, (typically very steep and convergent slopes with significant, unconsolidated sediments) can transform into a debris flow.

Evidence of debris flow deposits tend to be distinct and include channel-adjacent levees of gravel, cobbles, and boulders; channel-adjacent trees display upslope damage such as scarring to bark from rock or debris impact; mud and gravel may be splashed onto trees and other channel adjacent objects; and (or) debris flow deposits that display coarse gravel, cobbles, and boulders “suspended” in fine-grained sediments (sand and finer).

Debris flow and flooding hazards

Because of the ability of a debris flow to buoy cobbles, boulders, and woody debris to the front of the moving mass, debris flows are extremely dangerous to public safety and infrastructure. The hazard is typically limited to first and second order channels, so exposure by the public may be limited in wilderness areas and forestlands. The hazard of HCF and flashfloods should not be discounted because they are not debris flows. Both flashfloods and HCF can mobilize large volumes of woody debris and HCF can transport large volumes of coarse sediment. Both flashfloods and HCF can inundate areas not typically wetted by regular flows, so channel-adjacent roads, trails, building, and other infrastructure can still be damaged by impact from debris, sedimentation, water erosion, and (or) water inundation.