Rainfall Intensity Required to Generate a Debris Flow
On November 8, 2018 the Hill Fire started south of Santa Rosa Valley and quickly scorched 4,531 acres. Approximately 20 minutes later, the Woolsey Fire ignited near the Santa Susana Field Laboratory and within a matter of days burned 96,949 acres. Both fires have resulted in the destruction of plants and trees that play a key role in stabilizing soil and absorbing rainfall in nearby hills. Many of the areas impacted are well known for their ability to produce rock and debris flows especially under post-fire conditions. These rock and debris flows form rapidly during high-intensity rainfall and often threaten life and property. In addition to areas that are historically prone to flood, new areas that have not been prone to flooding may develop due to higher than usual flows in nearby streams and creeks.
Debris flow thresholds for the Woolsey Fire Burn Area have been set at first year, post-fire values by the United States Geoglogical Survey. Those thresholds are as follow:
.50″ of Rainfall Per Hour
- Intensity sufficient to produce flooding/debris flows
- Voluntary evacuation orders may be issued
- Roadways may be restricted or closed
- Residents with access & functional needs should leave early
1″ of Rainfall Per Hour or More
- Sustained intensity will cause flooding and debris flows that may result in injury or death
- Mandatory evacuation orders will be issued
- Roadways will be affected and may be hazardous
- Abide by recommended actions issued by authorities
Watershed Recovery Takes Time
Following a significant wildfire, damage to the local watershed includes the destruction of vital vegetation and root structures responsible for water absorption during a rain event. These plant materials burn and seep into the top layer of soil forming an impermeable layer of dirt also known as the hydrophobic layer. This soil layer prevents rainfall from easily penetrating the ground, particularly during high-intensity events. The remaining ash and burnt material is swept away by the rainwater and washed into nearby creeks, streams and drainages.
On average, it takes 3-5 years for vegetation to re-establish itself to a point where water absorption and soil stabilization returns to the pre-burn condition. Nearby properties remain at increased risk for flooding and debris flows for 1-3 years following a wildfire.