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Southern California faces the risk of damaging debris flows as a potentially strong atmospheric river is expected to arrive in the area this week.
Rain may arrive by Wednesday morning, but a major threat is expected early Thursday and continues until Thursday night or Friday morning. Storms threaten to cause significant amounts of rain, predictors say – perhaps more than 2-4 inches along the coast and valleys, and more than 4-8 inches on mountain and hills.
Santa Barbara and San Luis Obispo counties have 60% of rainfall of that magnitude, with a 40% chance of being in Los Angeles and Ventura counties. The risk of significant rainfall in LA and Ventura County has worsened from forecasts on Saturday, explaining a 30% chance of high rainfall.
Rain of that size would bring wonders to an end what was particularly devastating winter fire season, but it also threatens to cause another disaster that is too familiar to many Californians. There is.
Here’s what you need to know about landslides:
Why is California at risk?
Landslides have always been a threat to those living near the mountains. What it needs is heavy rain in a short amount of time to create a deadly and damaging flow of mud and debris.
The California mountains present specific challenges. Because they are very tall and the altitude drops rapidly when water is discharged into the ocean. The situation has become more dangerous as residents develop and live on these paths of destruction.
How do wildfires exacerbate landslide risk?
If the hills are lush and healthy, the vegetation can lock the soil in place, even during a severe storm. But when that protective blanket burns out, the hillside becomes vulnerable to erosion, and the slopes collide with rapids of mud, rock, rock, dead branches, and a home or anyone can hit the road. It may be.
The heat from the fire also makes it difficult for water to penetrate the top soil layers, US Geological Survey Hydrologist Jason Keene said in a previous interview.
Ash tends to clog the soil, and oily materials can interfere with the ground’s ability to absorb water. Impacts from the rain also help to seal the top layer of sediment.
As a result, the soil becomes a water repellent. Instead of permeating beneath the surface, Keene said the water begins to flow downhill.
It’s like water on the surface of a car park or play area slipping. He states: “When rain hits you, you run right away. …The rapid runoff from these bare hillsides can quickly pick up sediment and it can turn into a really nasty debris stream. ”
What are the different types of landslides?
“Landslide” is an all-encompassing term that can describe the downhill movement of rocks, dirt and debris.
However, there are several distinct varieties, as defined by hydrologists.
Mudflow: Run down with just mud. This is considered a type of shallow landslide less than 15 feet deep.
Debris flow: As water quickly flows downhill, picking up rocks, branches, and sometimes huge rocks and cars. This is also considered a type of shallow landslide.
Hydrologists usually don’t use the term “landslide,” but they are widely used and understood by non-scientists. The term is portemanteau of mudflows and landslides, and is commonly used to describe debris flows and mudflows.
How the debris flows
How fast does the rain have to fall to cause a stream of debris?
A rainfall rate of 0.5 inches per hour is a typical starting point for cases where it can cause debris flow in recently burned areas.
Ryan Kittel, a meteorologist with the National Weather Service in Oxnard, said:
Can a landslide occur after the rain stops?
yes. Shallow landslides include saturated hillsides that fell but are not very moving. Perhaps they’ll fill the road next to them with dirt and rocks. They could happen an hour after heavy rain.
Even if there is no history of recent wildfires, debris flows can also occur.
To what extent can debris cause damage or lethal flow?
About a year ago, thrilling rain caused debris and mudflows in many hillside neighborhoods in Southern California. These include Viva Liguren, Studio City, Tarzana, Baldwin Hills and Hacienda Heights.
Some of the state’s most deadly debris streams were struck after a wildfire.
In January 2018, 23 people died, and numerous structures were destroyed when a river of mud and rock flowed through Montecito. One of California’s most destructive records, the fire burned 282,000 acres in Ventura and Santa Barbara counties and destroyed more than 1,000 structures.
In 1982, a stream of garbage from the Santa Cruz Mountains collided with Love Creek. The area was not burned in advance, but heavy rains made the slope unstable. Ten people, including two children, were killed and 30 homes were destroyed.
At the moment on New Year’s Day in 1934, massive floods and streams of debris sailed down the Lacresenca Valley, caused by heavy rains that continued fires in the nearby Angeles National Forest. According to Los Angeles County archives, 20 feet of mud and rock walls thundered from the canyon, killing 45 people and destroying more than 400 homes.
How long does it take for a mountainside to recover from a wildfire?
The risk of debris flow after a wildfire is the first year or two after that.
However, after burns, the flow of burns is more likely to occur for 3-5 years of burns. This is the time normally required for vegetation to grow. With each additional year of recovery, the risk decreases, experts say.
What is the most predictable type of landslide?
A type that can be hit on dry days.
In areas with very deep bedrock, rainwater can penetrate deep underground during multiple storms. During a series of repeated, intense storms, water can accumulate and pressure can accumulate, Keene said.
Pressure can destabilize the entire mass of land and break downhill. Landslides occur slowly, displaying warning signs such as cracks and subtle movements, allowing people to escape. However, even on days without rain from the end of winter, they can attack rapidly without warning.
This is called a deep-sitting landslide, which contains landslides that exceed 15 feet deep. In many cases, deep-seated landslides attack in areas with a history of such events. The USGS warns that such landslides could become active in a few months in a very wet winter.
Animated infographics show how debris flows and deep sea landslides occur
What example of a deep-sitting landslide?
One was at Bluebird Canyon in Laguna Beach in June 2005, on a foggy morning after heavy rain from December to February before. No rain occurred during or just before the landslide. Seventeen homes were destroyed and eleven homes were seriously damaged.
The Bluebird Canyon had a devastating landslide history. In October 1978, the neighborhood suffered from sliding and destroyed more than 20 homes. California geological surveys believe that heavy rain between December 1977 and April 1978 played a role along with the history of landslides and erosion on the site and the history of rock weakness.
The deep-seated landslide also collided with the Ventura County settlement in Ranchita in 1995 and 2005. The second landslide occurred at the end of a severe 15-day rainy season when we saw a severe sediment in Southern California. It came without warning, buried ten people and killed them.
Times staff writers Haley Branson Potts, Suzanne Last, Joseph Serna and Rosannasia contributed to this report.
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