Mountain Fire Town Hall
Mountain Fire Burn Area
The Mountain Fire started on Wednesday, November 6, 2024, at 8:51am in the Somis area near Balcom Canyon Road and Bradley Road. Over the course of several days, the fire burned 19,904 acres and destroyed 182 residential structures. Numerous other residential structures and outbuildings were also damaged as a result of the fire.
Post-fire soil conditions significantly increase the risk of flooding and debris flows by altering soil structure, hydrology, and surface conditions due to intense heat. Key factors that contribute to debris flow generation in burned areas include:
Hydrophobicity
High-intensity fires can induce a hydrophobic (water-repellent) layer in the soil, preventing infiltration and promoting rapid runoff. This condition is common in soils with organic matter that vaporizes and condenses in cooler subsoils.
Loss of Vegetation
Vegetation removal reduces root reinforcement and slope stability, leaving the soil vulnerable to erosion and mobilization. It also eliminates canopy cover, increasing rainfall impact on the soil surface.
Ash and Fine Sediments
Burned areas often accumulate ash and fine particles, which are easily transported by water. This material can clog soil pores, further reducing infiltration and enhancing surface runoff.
Soil Compaction and Weakening
Extreme heating can destroy soil aggregates and reduce soil cohesion, making it prone to detachment and movement during rainfall.
Increased Runoff Potential
Reduced infiltration combined with bare soil surfaces leads to higher runoff rates during precipitation events. Rapid runoff can trigger soil mobilization, especially on steep slopes.
Topography and Slope Steepness
Post-fire debris flows are more likely in areas with steep gradients, where gravity aids the downward movement of saturated or loose soil material.
Precipitation Intensity and Duration
Short, intense rainfall events shortly after a fire are particularly effective at initiating debris flows. Even moderate rainfall can trigger flows if it occurs on a highly burned and vulnerable landscape.
What is a Debris Flow?
A debris flow is a fast-moving, gravity-driven mass of water-saturated soil, rock, organic material, and other debris that flows downhill, typically following steep slopes and natural drainage paths like channels or gullies. These flows can range in size from small, localized events to large, destructive movements capable of damaging infrastructure, property, and landscapes.
Mountain Fire Burn Area Soil Conditions
Post-fire soil conditions in mountainous areas are well known for their ability to produce rock and debris flows. Rainfall intensity and soil type are two key factors that determine the flood and debris flow threat. Unlike the Thomas Fire in 2017 and the Springs Fire in 2013, the burn severity observed in areas impacted by the Mountain Fire did not result in the creation of a hydrophobic layer.
Within the hills of the Camarillo Heights area, the combination of a low burn severity coupled with soils consisting largely of sand and silt are expected to reduce the likelihood of debris flows. Flooding however will remain a possibility, especially during high-intensity rainfall where nearby drains and culverts become clogged with debris and sand.
How Much Rain is Too Much?
Vegetation may take several years to re-establish in burned areas. Residents are encouraged to monitor weather forecasts for potentially hazardous rain events impacting the burn area. When rainfall intensity exceeds 0.60” per hour, the likelihood for flooding and debris flows increases.
Flooding and debris flows can occur even during lower-intensity storms, particularly after successive rain events. Given the potential for rapidly changing conditions, residents should remain vigilant and not wait for official evacuation orders before deciding to leave.
0.60 inches of Rainfall Per Hour
- Mud and burned debris may clog drains and culverts, potentially impacting nearby properties and roadways.
- Steep slopes near homes and roadways may erode and result in damage.
- Evacuation warnings and/or orders may be issued by local authorities when conditions warrant.
>0.80 inches of Rainfall Per Hour
- Extreme risk of mud flows and flooding on properties and roadways. Residents should be prepared for rapidly changing conditions.
- Potential for debris flows to develop in steep terrain.
- Roadways will likely become impassable due clogged drains and overtopping of culverts.
- Evacuation orders may be issued by local authorities.
Rainfall thresholds are higher than post-Thomas Fire due to low burn severity of the area, the sandy absorptive nature of the soils, and recent advancements in debris flow and storm modelling. However, prolonged or cumulative rainfall will increase the potential for debris flow, landslide and flooding.
Pre-burn areas not typically prone to flooding may now be at a higher risk because vegetation debris could clog or restrict drainage channels, inlets and culverts. Restricted access due to localized flooding or debris in roadways should be anticipated, but impacts may vary by elevation.
Higher elevations may experience landslides and localized flooding due to increased slope erosion and blocked drainage channels, while blocked culverts and inlets may result in overtopping and localized flooding in lower elevations.
Responsibility to Protect Property
The protection of private roadways and hillsides is the responsibility of individual property owners, as government agencies are legally prohibited from using public resources to fortify private properties. Such actions could be considered a misuse of public funds and pose liability concerns. Property owners should take proactive steps to protect their land, including installing sandbags, clearing drains, and implementing other flood and debris flow mitigation measures. For complex protective needs, residents are encouraged to consult professional firms specializing in erosion and flood control.
Public Works Departments are limited to protecting public property and infrastructure by maintaining and improving drainage systems, clearing debris from culverts and roadways, reinforcing embankments, and installing flood mitigation structures like sandbags or barriers.