The aftermath of intense wildfires such as the Palisades and Eaton fires in California brings an impending danger in the form of debris flows. These hazardous flows, composed of sediment, burned trees, cars, and boulders, can reach speeds of about 56 kilometers per hour (35 miles per hour). They traverse vast distances through valleys, canyons, and stream channels, posing significant risks to affected areas. The Palisades burn area, characterized by its gently sloping terrain and watersheds flowing towards the ocean, remains particularly vulnerable.
Debris flows are triggered by precipitation and the risk remains elevated for two to five years post-fire, contingent upon precipitation patterns. The National Weather Service (NWS) plays a crucial role by issuing watch alerts to residents' mobile devices 12 to 36 hours prior to an anticipated storm, if forecasted rainfall could incite debris flows and flash flooding. The urgency of such alerts is underscored by the fact that debris flows can commence within minutes of a rainstorm.
“It literally rained four days into our deployment,” – Jeremy Lancaster
In regions moderately affected by fire, up to 80 percent of surface litter is incinerated, with fine roots near the surface potentially burned and killed. In areas of high burn severity, no litter or near-surface roots remain, severely impacting the soil's ability to absorb water and stabilize slopes. Consequently, debris basins and dams at the bases of some watersheds serve as vital infrastructure to catch debris flows before they wreak havoc on communities.
The threat of debris flows is most acute in areas with moderate to high burn severity. Teams have already documented field observations in USGS computer programs, generating maps to indicate where debris flow hazards are greatest. Jeremy Lancaster emphasized the rapid response necessitated by the wet season.
“We assume that it’s going to rain soon because we’re in the middle of wet season, so we deploy rapidly.” – Jeremy Lancaster
Jayme Laber from the NWS elaborates on the process of forecasting potential debris flow events.
“These rainfall thresholds were developed based on lots of field work and observations of the types of rain intensities that caused or triggered soil movement and erosion, and which turned into debris flows,” – Jayme Laber
He further highlights the immediacy required when dealing with such events.
“It’s either happening now or it’s about to happen,” – Jayme Laber
The cautionary tale of Montecito, California in 2018 lingers in the minds of many. A month after being scorched by the Thomas fire, the region experienced deadly debris flows that claimed 23 lives and destroyed over 100 homes.
“Either you’ve lost your home or you’re just returning to your home after being evacuated from the fires, and then here you’re being told to evacuate again,” – Jayme Laber
The NWS has established rainfall thresholds that signal when debris flows are probable. This development is rooted in extensive fieldwork and observations of rain intensities that have previously triggered soil movement and erosion.
“Around 12 to 36 hours in advance of an incoming storm, the NWS may issue a watch alert to residents’ mobile devices if the forecasted rainfall could trigger debris flows and flash flooding.” – Jayme Laber
The slow recovery of vegetation in burn areas further complicates the situation.
“It’s been really dry, so it might take a couple more years before we get enough rain where we get the vegetation coming back, where it’s really starting to recover.” – Jayme Laber
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