"Santa Barbara County Fire search dog Reilly looks for people trapped in the debris left by devastating mudslides in Montecito, California." Credit: Mike Eliason/Santa Barbara County Fire
As we keep all those affected by the mudslides and flooding in California in our thoughts, we should note that post-fire flooding can impact islands in the Pacific, as well. The characteristics that lead to these events are similar whether in California or in Hawaii.
From the Source:
"Also called post-fire debris flows, these mudslides form when water rushing down slopes picks up dirt, burnt trees, rocks, and other debris (like cars), reaching speeds of more than 35 miles per hour. “When you mix a lot of mud, water, and boulders, it certainly can be quite catastrophic,” says Dennis Staley, a scientist with the US Geological Survey Landslide Hazards Program. The slurries can start with almost no warning after as little as a third of an inch of rain in just 30 minutes — especially on slopes scorched by fires. After fires blazed across more than half a million acres this fall in California’s worst fire season on record, it’s not hard to find burnt land."
Fire makes slopes more susceptible to mudslides for a few reasons, according to climate scientist Daniel Swain’s Weather West blog. For one thing, flames can strip hillsides of plants that would otherwise anchor the dirt in place. Extreme fires that burn through thick vegetation can also physically change the soil — leaving behind a layer of water-repellant dirt near the surface. That layer acts like a raincoat, slicking off water that can then form mudslides, according to the USGS California Water Science Center.
Plus, without plants to slow the rain before it reaches the dirt, the soil can’t absorb as much water — leaving more to race down hillsides as runoff. Imagine the soil as coffee grounds in a filter: if you pour your boiling water slowly, it will soak into the grounds and drip through into your cup. But if you dump your boiling water all at once, a watery, muddy slurry will overflow. That’s what’s happening on the bare slopes of Southern California right now."