August 19, 2023 - With the state bracing for wet weather arriving this weekend from Hurricane Hilary, the California Governor’s Office of Emergency Services (Cal OES) is reminding people about the dangers of mudslides and debris flows.

Hurricane Hilary is forecast to deliver potentially substantial rain across large areas of the state. Because of this, communities in flash-flood-prone areas and those still recovering from previous wildfires must prepare for a higher likelihood of mudslides, debris flows and flash floods.

Burn Scars

In the years after a wildfire, areas that are often left charred by flames and devoid of vegetation can render the soil non-permeable to rainwater. Where intense fires occur, soils can develop a layer that repels water, like rain on pavement, due to the charred remnants of organic material. Rainfall that would normally be absorbed by the soil will instead quickly run off.

Consequently, much less precipitation is required to produce a flash flood, and the potential for mudslides and debris flows increases with the loss of organic material that holds the soil in place. Because of this, locations that are downhill and downstream from burned areas are highly vulnerable to rain that can cause mudslides and debris flows, especially in and around steep terrain.

Debris Flow

Californians who live on or below hillsides, especially in areas impacted by recent wildfires, should be aware that precipitation increases the probability of potentially dangerous debris flows.

A debris flow is a fast-moving mass of material — slurries of water, rock, soil, vegetation, and even boulders and trees – that moves downhill by sliding, flowing and/or falling.

Debris flows range from a few square yards to hundreds of acres in size, and from a few inches to many dozen feet deep. Even smaller ones can be dangerous. Imagine trying to walk through a 3-inch deep mass of wet concrete moving at 30 mph.

Mudslide vs. Debris Flow

• Mudslides: a type of landslide that result from the failure of a slope, and often occurs due to the accumulation of water from prolonged rainfall and/or saturated subsurface conditions.
• Debris Flows: described as a “sediment-dominated slurry,” debris flows are mostly made up of soil, resulting from short-duration, high-intensity rainfall events.
• A mudslide is a localized slope failure, while a debris flow is a runoff event, such as a flash flood, that entrains sediment from a broad area.
• Debris flows and mudslides can occur many years after wildfires. Both happen fast, so heed evacuation warnings immediately

Prepare for Mudslides and Debris Flows 

• Pay attention to local emergency response messaging and heed evacuation notifications immediately
• Sign-up for local emergency alerts
• Monitor incoming storms, especially if you live in burned areas or downstream/downslope of a burned area
• Have an evacuation plan in place for you and your pets

During Mudslides and Debris Flows  

• Remember – local authorities may indicate it is safer for you and your family to shelter in place if flash flooding is not impacting your neighborhood
• Do not walk through moving water – just six inches of water can sweep an adult off his/her feet
• Do not attempt to drive through a flood, debris flow or into flooded areas. It takes only a foot of water to float or sweep away most vehicles.
• If you live on a hill, do not sleep in bedrooms that are on the ground floor which face the hazardous slopes.

Additional information on how Cal OES helps communities plan and prepare for disasters including flooding, mudslides and debris flows can be found here.

More in-depth information on flooding, flash flooding and disaster preparedness can be found here. [will insert Diana’s blog]
Source: Cal OES