As firefighters battled blazes in Southern California, onlookers kept hoping and praying for rain. But anything short of a drizzle has the potential to trigger new and horrific natural disasters. Heavy rains may trigger flash floods, mudslides, or mudslides after steep hillsides burn. The threat to life and property is severe. California state and local officials are busy studying the burned land, identifying where problems might arise next time it rains and issuing warnings to residents. But more time is needed to complete their work. There are limits to what their models can predict.
Jason Kean, a research hydrologist with the U.S. Geological Survey’s Landslide Hazards Program, told me about the coordinated effort to figure out who is at risk.
The first step was to compare infrared satellite images of the greater Los Angeles area taken before and after the fires to map the worst-burned areas. Of greatest concern are moderate or severe burns on steep slopes. Analysts fed data on burn severity, slope and soil composition into computer models showing how those areas might respond to heavy rains of varying intensities. Results will be sent back to the team to identify damaged homes and businesses. The National Weather Service will then track storms in dangerous areas that could cause disaster. The worst-case scenario is a sudden heavy rain in the hillside burn area.
Identifying steep slopes stripped of vegetation by wildfire is relatively simple. So does tracking storms heading into the area and issuing public warnings. But these warnings aren't perfect. If the hillside most likely to fail collapses, what path will the debris flow follow? Where will it end? A knowledgeable expert can make an educated guess, but ultimately it's not certain. "Maps showing the potential paths of debris are ideal for defining evacuation areas, but this is the missing piece in our current models," Keane explained. "We've been trying to fill that gap, and we're close, but we're not fully operational yet."
As a recent disaster demonstrated, the consequences of incomplete knowledge can be catastrophic. In a wildfire that started in December 2017, 282,000 acres burned near Santa Barbara. On January 8, 2018, before the fire was extinguished, a heavy rainstorm hit, triggering an emergency alert. In the early morning hours of January 9, half an inch of rain fell in the Santa Ynez Mountains in about 15 minutes. The rain caused more death and destruction than the fires themselves.
Imagine an Olympic-sized swimming pool perched high on a hillside. It was filled with dirt, rocks, sand, branches and boulders, some over 20 feet in diameter. If you spill the contents, it will flow down the hillside, gathering mass at speeds of up to 20 miles per hour. That morning, the actual volume of debris flow caused by rainfall was cubic meters. 272 Olympic-size swimming pools. Gravity pulled the debris nearly two miles down the mountain. Montecito is a small town with thousands of people living just down the road from it. 23 people were crushed to death, many while sleeping; 167 people were injured; 408 homes were damaged. Highway 101 is covered in dirt. The mud was 12 feet deep in some places, almost as deep as the overpass. Even though officials knew that heavy rainfall in the burn area would likely disperse large amounts of debris, they were shocked by the extent of death and destruction.
Today, local, state and federal officials are focusing on the Montecito disaster as they assess burn areas in Los Angeles. When Keene and I spoke Tuesday, teams were trying to map initial burn severity as quickly as possible (January and February are typically the rainiest months in Los Angeles), but their work was slowed and complicated by the fact that the fire was still Burning.
Angelenos who live several miles from the burn area, or who live near the burn area at higher elevations, are not at risk. But those who live near burn areas and at lower elevations may be at risk, especially near affected canyons or flood control channels or streets that frequently flood.
Some precautions, such as placing sandbags and boarding up sliding glass doors, could fortify their homes against more flooding and smaller mudslides, but mudslides are not the same.
exist natural controlAuthor and geology enthusiast John McPhee Describing what happened when an unexpected mudslide crashed into a California home: "The black stuff...was not just filled with boulders; it was filled with the mudslide. It was packed with cars like dough mixed with raisins." As it drives along Pinecone Road, it changes from driveway to street The road sucked the car away. There was a terrible explosion of shattering safety glass as it crashed into the Ginofels' house. Get in, keep going up." Everyone survived, but just barely. They should have evacuated.