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February 2, 2021

USGS researchers respond to coastal hazards and wildfires: The intense storm that hit California’s coast between January 26 and 28, 2021, blew out a portion of Highway 1 near Big Sur. USGS works diligently to provide officials with science to help inform their decision making before, during, and after such events. Here, we present photos that describe this work.

View from the sky looks down on a roadway that runs along a coastal cliff, part of the road has washed away.
Rat Creek

 

Map of a coastal area with colored areas and dots to show various hazards.
Example of a debris-flow hazard map.

The coastal change team at the USGS Pacific Coastal and Marine Science Center in Santa Cruz, California sent a reconnaissance flight over the area on Friday, January 29th, and snapped dramatic photos above the Rat Creek drainage. The deluge from the atmospheric river storm of January 26-28 washed mud, boulders, ash, and trees downslope from areas burned by last fall's Doran Fire. The culverts at Rat Creek, which normally allow Rat Creek waters to flow easily under Highway 1 rather than over it, became clogged with the debris. Continued heavy rain was no match for the clogged culverts, causing water to flood out and eventually erode the cliffside to the point of catastrophic collapse.

A woman wearing a mask, hard hat, personal floatation device, and waders stands near a river.
Sampling mud from creeks near wildfire burn zones, for geochemical analysis.

USGS-produced hazard maps of the denuded, burned hillsides around Big Sur, California, are critical for planning and emergency agencies, like the National Weather Service, to help save lives and property. The maps help officials identify potentially dangerous conditions, so they can issue warnings and sometimes evacuation orders and close roadways before and during extreme weather events like the one that hit on January 26-28, 2021.

The USGS landslide team monitors and continues to update the hazard map models based on data collected in burn areas. This information improves future models and provides better hazard assessments used by officials for emergency response and decision making. Many of the steep hillsides burned and denuded in California fires repel water rather than soak it in. This condition, called hydrophobic soil and caused by the fire’s hot temperatures, causes water to flow over land instead of soaking in, picking up surface debris along the way with potentially devastating force once it reaches people, communities, or property.

USGS collects data about the contaminants found in water flowing off the burned areas. To do this, geochemists collect sediment samples right after a fire, before the rain, and periodically return for additional sampling to understand how contaminants move through the ecosystem.

Read more: “Bringing USGS Science Expertise Together for Natural Hazards in Big Sur, CA

Illustration showing how a forested watershed ability to absorb and filter precipitation changes after a fire
The 2020 fire season provided stark evidence that wildfires are changing the landscape of America. But when a forest burns, the impacts on water supply and quality last long after the flames go out. BEFORE A FIRE: Forests act like a sponge and a water filter, meaning that rainwater can recharge drinking water supplies and only needs minimal treatment before use. AFTER A FIRE: Forests respond to rainfall as if the ground is covered in a layer of plastic wrap. Water cannot penetrate into the soil. Instead, huge amounts of surface runoff from rainstorms carry ash, sediment and other pollutants downstream into streams and reservoirs, which often means that water treatment costs skyrocket.
A woman stands in a rut eroded by water, on a very steep hill surrounded by burned trees.
Example of the soil, forest, and runoff patterns after a wildfire and rainstorm.

 

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