by Stephen M. Testa Friday, January 20, 2012
In 2010, California was struck by 6,656 wildfires, due in large part to the strong and extremely dry Santa Ana Winds that sweep through Southern California and northern Baja California in the late fall and winter. These fires often lead to debris flows after the winter rains set in.
The cycle of fires and debris flows has become a problem for Californians and other citizens of the American Southwest in the last several decades. Between 1900 and 1990, population in the Southwest increased by 1,500 percent; during the same period, population throughout the entire United States only increased by 225 percent. By the 1970s, prime flat land was built upon, and development had nowhere to go but up. So up we went. We started building for the landscape, for the view — the higher the better. We headed farther into dangerous territory: forested canyons and mountains, places where wildfires rage — and where winter rains can trigger muddy disasters on the denuded hillsides.
This has led to an important question: When such debris flows ensue, who is responsible for cleaning up the mess?
Fires and debris flows are not new to the American Southwest. Sediment cores from the Santa Barbara Channel off California show evidence of fires going back centuries; the fires were usually followed by increases in sediment production — runoff from denuded hillsides — with up to a 40-fold increase in sediment production during the first storm season following a watershed fire if high-intensity rainfall occurred.
Two processes can account for post-fire debris flows. Surface erosion caused by rainfall runoff is the most prevalent as the infiltration capacity of the soil is significantly reduced. The other culprit is land actually sliding, caused by infiltration of rainfall into the ground. With prolonged heavy rains, such as the ones California has been receiving lately, soil moisture can dramatically increase after a wildfire, causing slope failures. When an area is hit by prolonged heavy rains, these types of ground failures are commonly referred to as infiltration-triggered landslides.
For areas underlain by sedimentary or metamorphic rocks — which includes much of the Southwest — about 65 percent of an area must be burned at moderate to high severities to cause a debris flow, according to the U.S. Geological Survey. In addition, debris flows are produced most commonly from steep, tightly confined drainage basins with abundant stored materials, and are unlikely to extend beyond the mouths of basins larger than about 25 square kilometers. Translation: It may not be wise to build your home at the opening of a canyon unless you plan on being a candidate for the Darwin Awards.
In the Southwest, debris flows have reached as large as 300,000 cubic meters — enough to cover a football field with mud, rocks and debris to about 65 meters deep. Any storm with intensities greater than about 10 millimeters per hour could produce debris flows. These debris flows and subsequent landslides occur every year, often in very exclusive neighborhoods, such as the affluent Anaheim Hills in Orange County, the Hollywood Hills and the cliffs of Malibu.
This seasonal scenario plays out in California. In fall 2009, the Station Fire in Angeles National Forest burned 650 square kilometers of the San Gabriel Mountains, running right up to houses in Paradise Valley in the La Cañada-Flintridge area. In February 2010, the rains set in. Following the torrential rains, catch basins designed to hold the mud and water filled to capacity. It was all downhill from there. Tons of devegetated slopes took a road trip and ended up in people’s houses and backyards. In the aftermath, the impacted areas looked like homes on wheels literally stuck in the mud. Cars, two-ton concrete barriers, logs and boulders were thrown about like sticks. This same area was again hit hard in December when a Pineapple Express storm dropped copious amounts of rain on Southern California.
The earlier event raised a money issue. The impacted houses were on state land, but it was federal mud — the mud was derived from federal land in the national forest. Officials from the U.S. Forest Service said it wasn’t their problem: “This is not the federal government’s mud coming out of their hills; these are Mother Nature’s hills, trees and mud — there long before residents selected these locales to build homes.” That did not sit well with the locals who opined that Mother Nature alone was not responsible, and that scaling back firefighting efforts too early after the Station Fire broke out back in August 2009 was a strategic mistake. The mayor of La Cañada-Flintridge said the federal government should be responsible for cleaning up the mud, at the least. After all, he said, the federal government caused the mud to flow. A county supervisor added that the community was “suffering from the actions of the U.S. Forest Service.”
The situation got really bad before it got better, with much mud-slinging. The federal government finally agreed to designate five Southern California counties as disaster areas, which allowed homeowners access to low-interest loans to repair their properties. This was a fine solution for the time being; however, the story of wildfires and associated debris flows is an ongoing saga in the West. And in a depressed economic environment at the local, state and federal levels, this version of mud wrestling is sure to continue.
All I can say as we start another year in the Southwest is: Here we go again!
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