Zone 3


RAINFALL ENDING 12/18/2018 15:04 PST

Fagan Canyon West
15-min Total: 0.00"
30-min Total: 0.00"
1-hour Total: 0.00"
2-hour Total: 0.00"
3-hour Total: 0.00"

Rainfall Intensity Required to Generate a Debris Flow

Light rainfall following a wildfire is essential for the regrowth of vegetation. However, high-intensity rain is detrimental to the watershed and may be devastating to properties and lives in its path.

.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

.75” 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

Known Hazards

  • Potential for debris flows into canyons blocking channels, storm drain inlets
  • Localized flooding in and below canyons.

What is a Debris Flow?

 A debris flow is a mass of loose mud, sand, soil, rock, debris, and water that travels down a slope under the influence of gravity. To be considered a debris flow, the moving material must be loose and capable of flowing. The following environmental conditions must be present to generate a debris flow:

Steep Slopes

Ventura County has an abundance of hills and mountains that make up our watershed. In a post-fire condition, these slopes provide a perfect environment for Mother Nature to develop debris flows.

Loose Debris

Following the Thomas Fire, various types of loose debris remained including burned plant material, ash, and physical property.

Source of Moisture

Rainfall more than .50”/hour can collect within a watershed and gain speed as it travels down steep slopes. As the water gains speed, loose debris is picked up and carried with the flow of water. Within channels, this water and debris mix to produce a thick flow of material.

Sparse Vegetation

Due to recent wildfires, vegetation in our local hills has been destroyed or is no longer present in many areas. Vegetation can reduce raindrop impacts to the soil because the plant roots help to hold the soil material in place.

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.


If you live in a flood-prone area, consider making permanent changes to your home such as constructing retaining walls and raising your furnace and electrical panel above potential flood levels.

  • Assemble an emergency supply kit as recommended by the American Red Cross.
  • Register your phone number(s) at so you can receive emergency messages.
  • Consider purchasing flood insurance. Many homeowner’s policies do not cover flood damage.
  • Create a list of items to take with you if you are asked to evacuate. In addition to your emergency supplies, consider items such as: keys, cash and credit cards; photos; insurance papers; computers; prescriptions; and pet supplies.
  • Clear debris from roof gutters, downspouts and drains so water can flow and drain properly.
  • Have a supply of sandbags and other flood prevention materials such as plastic sheeting, plywood and tarps.
  • Check the roof for leaks or damage. Pay special attention to areas where separation could occur, such as around the chimney.

 How to GET SET

  • Gather emergency supplies, evacuation items and review evacuation routes in case you are asked to evacuate.
  • Be aware that flash flooding can occur during periods of heavy or extended rain. If conditions warrant, take action immediately and evacuate the area.
  • Fill the gas tanks in your vehicles.
  • Bring in outside furniture that could be carried away by floodwaters.
  • Move furniture and other valuables to high points – upper floors if possible – in your home.
  • Fill and place sandbags if necessary.
  • Monitor gutters, drains and other areas that could cause flooding.


Properly placed sandbags will redirect water, mud and debris but they will not completely seal out water. Sandbags should be used for low-flow protection (up to about two feet). Ventura County fire stations maintain only a limited supply of sandbags to be used during an emergency. Homeowners should not depend on that supply. Sand and sandbags can be purchased at many home improvement and hardware stores. Purchase sandbags early and make them a part of your emergency supplies so they will be available if you need them.