A massive storm hits Southern California this week, poses the risk of debris flowing through the firebeat. Rain begins on Wednesday, but the most harmful rain is expected on Thursday.
Here are four things you need to know about the potential debris flow:
1. 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 penetrating underneath the surface, the water begins to flow downhill.
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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. ”
The risk of debris flow after a wildfire is the first year or two after that.
However, after a fire, the burn flow remains prone to occur for 3-5 years after the fire, and this is the time normally required for the vegetation to grow. With each additional year of recovery, the risk decreases, experts say.
2. Preparation
Below are some basic tips for preparing for landslides from the California Department of Conservation.
After a wildfire, prepare for a stream of debris for 2-5 years. Heavy rain on recently burning slopes (usually about half an hour per hour – like a thunderstorm) can cause debris flow. You can pay attention to the official weather forecast. If only a few minutes of heavy rain falls, the debris can begin to flow. The National Weather Service will issue flash flood monitoring or warnings in your area if the rain is expected to be severe. Whether it’s not raining or sprinkling where you live, it’s the rain in the mountains that starts the debris flowing. Don’t rely on what you see in the flow of past wreckage. Debris flows can hit new areas or return to previous areas. Next time it might be smaller or bigger. Whatever happened before, the next time could be different. Leave before the storm arrives. The flow of debris moves fast! If you wait to see if the stream of debris is on your path, it will be too late to leave safely. You cannot overtake the stream of debris. If you need to evacuate in place, please select your location in advance and leave a warning. Find the best nearby points (such as a two-storey room or roof) and are ready to get there in no time. Hurry and listen to water, mud and unusual sounds and be careful. Survivors describe cracks, destruction, roaring, or freight train sounds prior to the flow of debris. Debris flow can start anywhere you have never seen before. They can leave the stream channel and plow the neighborhood. If the flow of debris is small, people can control it with walls, K-rails, sandbags. If the debris flow is large enough, it cannot be stopped. Exclude other flood risks. Storms that can cause debris flow can also pose a more common flood risk. Debris flow can also occur if there is no fire during wet winters. Note that the soil can get flooded and more rain can cause debris flow. Excessive surface erosion of the watch for new springs and penetration, and slopes within and around the site. If there’s a nearby stream, do they look more muddy than usual? Do not sleep in the bedroom under the side of the house facing the slope. Debris flow can fill people sleeping in low-bedroom bedrooms adjacent to dangerous slopes.
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3. Different kinds 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
4. How does that happen
A rainfall rate of 0.5 inches per hour is a typical starting point for when debris flow is caused in recently burned areas.
Ryan Kittel, a meteorologist with the National Weather Service in Oxnard, said:
Shallow landslides include saturated hillsides that are lying but not very moving. They could happen an hour after heavy rain.
Even if there is no history of recent wildfires, debris flows can also occur.
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.
5. Big concern: “Sitting deep” landslide
The type of landslide that can become one of the most unpredictable is one that can attack 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
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. Without warning, 10 people were buried and killed.
Times staff writers Haley Branson Potts, Suzanne Last, Joseph Serna and Rosannasia contributed to this report.
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