South California is confronted with a risk of damaging debris this week because a potentially strong atmospheric river is expected to arrive in the region.
The rain is expected to arrive on Wednesday morning, but the biggest threat is expected early Thursday and continuing until Thursday evening or Friday morning. The storm threatens to lower the large quantities of rain, depending on the forecasters – perhaps 2 to 4 inches or more along the coast and in the valleys, and 4 to 8 inches or more in the mountains and the foothills.
There is a 60% chance of precipitation of this magnitude in the counties of Santa Barbara and San Luis Obispo, and 40% chance in the counties of Los Angeles and Ventura, the National Weather Service said on Sunday. The risk of significant precipitation in the counties of Et Ventura has worsened compared to the forecasts on Saturday, which described 30% of high precipitation.
Although the rain of this magnitude makes wonders to put an end to what was a particularly devastating winter season, it also threatens to trigger another disaster which is too familiar for many Californians.
Here is what you need to know about the landslides:
Why is California at risk?
The landslides have always been a threat to those who live near the mountains. All that can take is an intense amount of rain in a short time to create deadly and damaging flows of mud and debris.
The mountains of California present a particular challenge, because they are quite large and the elevation falls quickly when the water drains towards the sea. The situation has become more dangerous as the residents have become more and more developed and inhabited these paths of destruction.
How do forest fires aggravate the risk of landslide?
When the hills are green and healthy, the vegetation can anchor the soil in place, even during heavy storms. But when this protective cover is burned, the hills become vulnerable to erosion and the slopes can crash into a torrent of mud, rocks and dead branches like rapids of living water, in danger of jeopardizing – or anyone – on their way.
The heat of a fire also makes it more difficult for water to infiltrate the ground layer of the soil, said US hydrologist Geological Survey, Jason Kean, in a previous interview.
Ashes tend to obstruct the soil and oily substances can hinder soil’s ability to absorb water. The impact of rain can also help seal the upper sediment layer.
Consequently, the soils become repulsive to the water. Instead of piercing under the surface, the water begins to run down – picking up rocks and debris in its wake, said Kean.
It is like water in a parking lot or the surface of a slide of playground, he said: “When precipitation strikes him, he extends immediately. … The rapid runoff of these naked slopes of the hill can quickly pick up the sediments, and this can be transformed into a really unpleasant flow of debris. »»
What are the different types of landslides?
The “landslide” is a global term which can describe any movement of rock, dirt or descent debris.
But there are some distinct varieties, as defined by hydrologists:
MUDE FLOW: The water rushing with only mud. This is considered to be a type of shadowlift – less than 15 feet deep.
Debris flow: When the water flows quickly and picks up rocks, branches, sometimes even rocks and massive cars. This is also considered to be a type of shallow shift.
Hydrologists generally do not use the term “mud flow”, but it is widely used and understood by non-sciences. The word is a portman of mud flowering and landslide, and is commonly used to describe debris flows or mud flows.
How a debris flow works
How fast should the rain fall to trigger a flow of debris?
The precipitation rates of half a pump per hour are the typical starting point of the moment when the flow rate can be triggered in the recently burned areas.
The higher the precipitation rate higher than this threshold, “this is where we could get more appreciable and important debris flows,” Ryan Kittell, meteorologist of the National Weather Service, told Oxnard.
Can landslides occur after stopping the rain?
Yes. A shallow landslide involves a saturated hill that collapses but does not move very far – perhaps buried a neighboring road with dirt and rocks. They can occur up to an hour after a burst of intense rain.
The flow of debris can also occur even if there is no history of recent forest fire.
How are the debris flows be damaging or deadly?
About a year ago, swinging precipitation sparked debris and harmful mud debris in a number of southern hillside districts in California, including Beverly Glen, City Studio, Tarzana, Baldwin Hills and Hacienda Heights .
Some of the deadliest debris flows in the state hit after forest fires.
In January 2018, 23 people died and many structures were destroyed when a mud and rock river crossed Montecito, which had been burned less than a month earlier in the Thomas fire. This fire, one of the most destructive in California, burned 282,000 acres in the counties of Ventura and Santa Barbara and destroyed more than 1,000 structures.
In 1982, a flow of debris struck Love Creek in the mountains of Santa Cruz. The area had not burned in advance, but heavy rains destabilized the slopes. Ten people died, including two children and 30 houses were destroyed.
A few moments in the day of 1934, a major flood and a flow of debris had the Crescenta valley sailed, triggered by heavy rain preceded by a fire in the neighboring Angeles forest. A 20 -foot mud and rock walls came out of canyons, killing 45 people and destroying more than 400 houses, according to the Los Angeles County archives.
How long does the hills take to recover from forest fires?
The highest risk of flow debit after a forest fire is the first year or two thereafter.
But the burning areas are even more prone to the flow of debris for three to five years after a fire – the time generally necessary for the vegetation to resume. Each additional year of recovery, the risk decreases, according to experts.
What is the least predictable type of landslide?
The genre that can strike a dry day.
In the areas where the foundation is very deep, rainwater can infiltrate deep underground for several rain storms. During a series of repeated heavy storms, water can accumulate and increase the pressure, Kean said.
The pressure can destabilize an entire piece of earth, which makes it collapse. The landslide can occur slowly and show warning panels such as cracking or subtle movements, allowing people to escape. But it can also strike quickly without warning, even one day without rain after the end of winter.
This is called a deep landslide, involving landslides greater than 15 feet deep. Often, deep landslides strike in areas with history of such events. The USGS has warned that these landslides can become active several months after a very humid winter.
The animated infographic shows how debris flow and deeply anchored landslides occur
What is an example of a deep landslide?
One occurred in a misty morning in June 2005 in the Laguna Beach Bluebird canyon after the heavy rains fell from December to February. No precipitation occurred during or just before the landslide. Seventeen houses were destroyed and 11 seriously damaged.
There was a story of devastating landslides in Bluebird Canyon. The district underwent a slide in October 1978 which destroyed more than 20 houses. The California Geological Survey said that the heavy rains between December 1977 and April 1978 would have played a role, as well as a history of landslides and erosion on the site, and weakness in the rock.
The deeply anchored landslides also struck the hamlet of the county of Ventura de la Conchita in 1995 and 2005. The second landslide occurred at the end of an intense rainy period of 15 days which experienced strong precipitation throughout southern California. He came without warning and buried 10 people, killing them.
The staff of Times Hailey Branson-Potts, Susanne Rust, Joseph Serna and Rosanna Xia contributed to this report.
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