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El Niño: Rain This Winter Plus Water Repellent Soil May Result In Mountain Flooding

Sep 29, 2015

Weather in the Sierra Nevada can change on a whim. The sun will be out in full array and then all of a sudden a thunderstorm will let loose. With such parched and fire scorched land around many mountain communities in California that could mean a greater chance of flash floods this fall and winter. FM89’s Ezra David Romero reports on how one scientist is working to prevent mountain flooding in the region.

Fires are burning through the Sierra Nevada at a rapid pace leaving behind large patches of scorched and barren earth. In many cases when rain falls the ground won’t immediately take in moisture. It’s almost as if the dirt is scared of water. That’s a phenomenon called hydrophobia. And dirt that can’t absorb want can lead to flooding and mudslides.

"I think we're looking at a new normal with temperature increases, larger fires." - Alan Gallegos, US Forest Service

“Its soil that I would say is susceptible to erosion and with that erosion you can get a lot of sediment in the channel. All of that leads to potential for debris flow," says US Forest Service Geologist Alan Gallegos based in the Sierra National Forest.

Gallego's brought me to an area burned by the 5,500 acre Willow Fire near the community of North Fork earlier this summer.  As a soil scientist Gallegos is a dirt expert. He scrapes the ground to find some of this water repellent soil.

“If I had a shovel or a knife we would expose the soil like this and we’d want to get it deeper at the end here," Gallegos says.

He sprinkles water over dirt charred by the fire.

“This is called the drop test," says Gallegos. "You see the portion right through here, the water is still standing, it’s beading. We’re just barely into the mineral soil. We’re probably maybe a half of an inch down. It was probably beading for about 30 seconds.”

Fire has left large swaths of charred of forest in the Sierra Nevada.
Credit Ezra David Romero / Valley Public Radio
"Fire is being influenced by our past history, the fact that we've removed fire from our system. It's been influenced by the current climate trends that we're seeing right now. On top of that bark beetles are coming in and taking advantage of that. And then you can bring water into the picture." - Marc Meyer, US Forest Service

When dirt is hydrophobic it takes consistent rain to build up the moisture profile of that soil.  Gallegos says it takes two winters for the threat of flooding to go away after a blaze. He says when fire burns through forest, sap and oil oozes from brush covering dirt making it impervious to water.

“The chaparral has got those oils in them," Gallegos says. "It’s like a waxy substance that coats the soil particles and it’s the waxy substance that causes the hydrophobicity.”

The US Forest Service installs large pipes to divert water in flood prone areas in case of a storm event.
Credit Ezra David Romero / Valley Public Radio

It’s not just fire that causes dirt to repel the water California’s dry forests need. The absence of rain for long periods of time creates soil that is water immune. Earlier this summer a thunder storm broke open over the Oakhurst and Bass Lake region. The forest couldn’t soak up the rainfall fast enough.  

“We saw evidence of some debris flows and some flooding just from that same storm that we got this summer," says Gallegos.

But the blame for flash flooding in the Sierra Nevada can’t only be put on burned forest and dirt that isn’t able to absorb water. US Forest Service Ecologist Marc Meyer says the bark beetle has sucked the life out of so many pine trees that if a rain event takes place then roots and branches can’t pull in moisture. With so many dead trees and such large burn areas Meyer says the probability of more fires and winter flooding is high.  

“If there’s a double threat it’s actually the impact of fire when it overlaps areas recently impacted by beetles because those are the areas where you have very readily accessible dry fuels that can create much more severe effects of fire," Meyer explains. 

"Fire is being influenced by our past history, the fact that we've removed fire from our system. It's been influenced by the current climate trends that we're seeing right now. On top of that bark beetles are coming in and taking advantage of that." - Marc Meyer, US Forest Service

He also says the root cause for the massive size of fires witnessed in the Sierra Nevada and the potential flooding dwindles down to forest management and warming temperatures.

"Fire is being influenced by our past history, the fact that we’ve removed fire from our system," says Meyer. "It’s been influenced by the current climate trends that we’re seeing right now. On top of that bark beetles are coming in and taking advantage of that. And then you can bring water into the picture.”

But even with all these threats and their causes, Meyer says that consistent precipitation is the only way to answer California’s thirst for water. Once again US Forest Service Geologist Alan Gallegos.

“I think we’re looking at a new normal with temperature increases, larger fires," says Gallegos. "Look at the Rough Fire on the Sierra. We’ve never had a fire that big.”

And with the Rough Fire almost fully contained, scientists like Gallegos are trying to minimize mountain flooding by preparing that area for these so-called massive storms brought on by El Niño.