As storms roll in across the state, the California Governor’s Office of Emergency Services (Cal OES) is reminding people about the dangers of mudslides and debris flows, especially in burn scar areas.
The National Weather Service is forecasting significant rain across Southern California through the end of the week. Communities in flash-flood prone areas and those still recovering from the recent Los Angeles wildfires must prepare for a higher likelihood of mudslides, debris flows and flash floods.
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. Turn around, don’t drown.
- If you live on a hill, do not sleep in bedrooms that are on the ground floor which face the hazardous slopes.
Burn Scars
In the days, weeks, months and even 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.
Significant less rain is required to produce a flash flood, and the potential for mudslides and debris flows increases with the loss of plants and vegetation 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, 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.