California Burning: Photographs from the Los Angeles Examiner

Debris Flows

Some wildfires are so destructive that they leave nothing behind except burned ground. However, the destruction does not stop when the fire goes out. The surrounding community can also experience further damage during heavy rains. Like a chain reaction, the heat of the wildfire can sear the top layer of the soil to make it almost like wax, which accelerates the flow of mud and water during storms. Trees and roots destroyed by the fire can no longer prevent rocks and mud from flowing downhill, as depicted in the image of the Banning Fire in 1951. These mudslides are known as debris flows. Similar to fresh concrete, the debris consists of, in the words of writer John McPhee, “water mixed with a good deal of solid material, most of which is above sand size … Boulders bigger than cars ride long distances in debris flows. Boulders grouped like fish eggs pour downhill in debris flows.”[1]

Debris flows generally are not as destructive of life as of property, but they can cause casualties.[2] In February 1978, as rain fell over La Crescenta, California, Jackie Genofiles watched as “one big black thing coming at us, rolling, rolling with a lot of water in front of it, pushing the water, this big black thing.”5 The debris made a roar as loud as a jet engine, and it carried large tree trunks that crashed through houses. It was virtually impossible to stop the debris flow, and the only option was to evacuate everyone, including the Genofiles family. Debris flows can also cut off power and cut gas lines, causing explosions .On the morning of January 9, 2018, in the wake of the Thomas Fire in Santa Barbara, USC Professor Peter Westwick recalled how increasing heavy rain caused mudslides to hit the house. Luckily his family was safe, but the mudslide cut off their power, destroyed part of their house, and washed away their belongings, including their washer and dryer.[3]

In response to this experience with debris flows, families in Los Angeles built deflection walls, which could guide the debris through their garages to the street.6 However, debris flows continue to pose a significant hazard to communities in mountainous areas, after wildfires. Efficiently determining the downstream effects of anticipated debris flows requires a rapid hazard assessment which demands complex computational models. Such technology remains insufficient to understand and assess debris flows quickly nowadays.7 Infrastructure like debris basins needs better maintenance to ensure proper functioning because the government has not been actively emptying filled debris basins, like the photograph of the Eaton Wash Basin in 1954 [photograph to be uploaded].

[1] McPhee, John. “Los Angeles Against The Mountains,” The New Yorker (1988).

[2] Ibid.

[3] Ibid. 5
McPhee, John. “Los Angeles Against The Mountains-I”. The Controle of Nature, The New Yorker, 1988. 6 Ibid. 7
Gorr, A.N., McGuire, L.A., Youberg, A.M., and Rengers, F.K. “A progressive flow-routing model for rapid assessment of debris-flow inundation”. Landslides, Dordrecht, Vol. 19, Iss. 9, 2022.
 

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