California rainstorm brings risk of dangerous mudslides


The rainy season produces a higher risk of danger debris flows – often called mudslides or mudslides – for those who live on or below hills, especially in areas scorched by recent forest fires.

The bomb cyclone that hit northern California Sunday produced a record amount of rain in the Sacramento Valley and the Sierra Nevada foothills.

Now residents, authorities and emergency officials are left with potential damage from debris flows after forest fires in the area.

What is a debris flow?

A debris flow is a mixture of water and sediment. And rain as small as three-tenths of an inch falling for at least 15 minutes can cause a reaction.


This map shows where debris flows are most likely to occur in the area of ​​the Caldor fire, with the probability expressed as a percent probability in a storm with a maximum intensity of 15 minutes of about one inch per hour. .

Menu: NATHANIEL LEVINE | Source: US Geological Survey

According to Jason Kean, research hydrologist for the United States Geological Survey, after a forest fire, steep slopes no longer have the protective layer of vegetation and the ground also becomes water repellent.

As a result, this area receives a lot of runoff.

“Sometimes that’s all you have. You have a lot of runoff and a little bit of sediment transport with it, and then you have a flood, ”Kean said.

“Other times it picks up a bunch of sediment and it swells and it swells so it’s half sediment and half water and that’s what we call a debris flow.”

The streams can include “water, rocks, soil, vegetation, and even rocks and trees,” according to the California Department of Conservation.

Put simply: “Debris flows are rapid landslides that are particularly dangerous to life and property because they move quickly, destroy objects in their path and often strike without warning,” according to the website of the ‘USGS.

Before fire and rain, the soil is trapped on rocky hills steeped in vegetation. But during the summer fire season, this vegetation is scorched, causing sediment to roll over the steep hills.

In a few hours or a day, the bottom of the canals is laden with loose sediment which moves through the steep canals during heavy rains.

“It’s one of the most dangerous things that can happen after a wildfire,” Kean said.

Forest fires promote flooding and if enough sediment is picked up, a debris flow begins. And while flooding and debris flows can occur at the same time – debris flows are deeper and can be faster than an average person can run – which makes them more dangerous than a flood.

“A debris flow can carry a car-sized boulder and pass through your house,” Kean said.

debris flow areas.jpeg
The maps illustrate the probability of debris generation and estimates of the magnitude of the flow in locations where debris begins. The models do not predict downstream impacts, potential debris flow paths, and the extent of the debris or flood zone. USGS

Kean said there is often not much warning before a debris flow begins, as it can start instantly when heavy rain hits a steep slope that has been scorched by a forest fire at moderate to high burn severity.

“It is often too late to get by. It’s really best to be out of the way before heavy rain hits the scorched area, ”Kean said. “This is why the National Weather Service is working to put these watches and warnings out in advance so that people are aware.”

The Sacramento National Weather Service extended a flood advisory to southern San Joaquin County, Stanislaus County, southeast Calaveras County and southwest Tuolumne County until 5 a.m. Tuesday.

But Kean warned as the skies began to clear that residents should remain cautious as winter approaches.

According to the National Weather Service Post Wildfire Flash Flood and Debris Flow Guide, the risk of flooding remains significantly higher until vegetation is restored – up to five years after a wildfire.

“We’ll have to be on our toes here for the rest of the winter for the next storms to come,” Kean said. “This will be with us for a while until these scorched areas recover and it often takes two years or more.”

Here’s what anyone preparing for the next storm should know about reducing the risk of property damage and injury, according to the National Weather Service and Habitat for Humanity.

Plan ahead

  • Gather supplies in case of a storm and plan evacuation routes.
  • Make sure you keep important papers in a secure, waterproof place and take photos of your belongings.
  • Review your insurance coverage and make sure you have a flood insurance policy.

Visit the National Flood Insurance Program to learn more about individual flood risks and coverage options.

After a debris flow

  • Get away from the toboggan area as there might be more for me.
  • Avoid using the phone except in a serious emergency
  • Remember that flooding can occur after a debris flow
  • Inspect your home
  • Leave your home if you smell gas or chemicals
  • Check damaged walls, electrical system, or water pipes
  • Clean flammable liquids
  • Report any broken utility lines or damaged roads to authorities
  • Replant damaged soil as soon as possible to reduce the risk of flooding

For more advice on the consequences of debris flows, visit Habit for Humanity.

What do you want to know about life in Sacramento? Ask our California Utility Team your most important questions in the module below or send an email to [email protected]

This story was originally published 25 October 2021 4:12 pm.

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Brianna Taylor is a reporter with the Sacramento Bee Public Services Office. A former Bee intern, Brianna has also reported in Missouri and Maryland. She graduated from Morgan State University.

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