The Thomas fire started on December 4, 2017 and burned approximately 281,893 acres across Ventura and Santa Barbara Counties. Post-fire soil conditions in areas adjacent to mountains and hills are well known for their ability to produce rock and debris flows. These rock and debris flow form rapidly, and the outcome is often devastating during high-intensity rainfall. Additionally, areas that are not typically prone to flooding may now be at new risk due to higher than usual flows in nearby streams and creeks.
Rainfall Intensity Required to Generate a Debris Flow
Debris flow thresholds for the Thomas Fire Burn Area have been updated effective 12/28/2018 to year 2 values. The thresholds areas follow:
.75″ 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.