Post-Fire Landslide Problems Likely to Worsen: What Can Be Done?
By Lee MacDonald
Several weeks after a series of wildfires blackened nearly 500 square miles in Southern California, a large winter storm rolled in from the Pacific. In most places the rainfall was welcomed and did not cause any major flooding from burned or unburned hillslopes.
But in the town of Montecito, a coastal community in Santa Barbara County that lies at the foot of the mountains blackened by the Thomas Fire, a devastating set of sediment-laden flows killed at least 20 people and damaged or destroyed more than 500 homes. In the popular press these flows were termed "mudslides," but with some rocks as large as cars these are more accurately described as hyperconcentrated flows or debris flows, depending on the amount of sediment mixed with the water.
Why did these deadly flows happen? To what extent were these flows caused by the fire, the extreme burst of rainfall, or a combination? And what can we do to reduce similar risks in the future?
Causes of Post-Fire Erosion
Some national newspapers are saying that these post-fire flows are caused by the loss of vegetation, but as a scientist studying the effects of fires on soils, runoff and erosion, I can say this is not completely accurate. While many wildfires do burn trees and shrubs, the loss of this overlying vegetation canopy only slightly increases the amount of rainfall that reaches the soil surface and the kinetic energy delivered by the raindrops to the ground surface.
The far more important effect of high and moderate severity wildfires is that they can burn off all the surface litter and ground vegetation, leaving a layer of easily removed ash on top of otherwise bare soil.
Photo Credits: NASA Earth Observatory
In certain vegetation types, like chaparral and coniferous forests, fires will vaporize organic compounds found in the leaf litter. Some of these compounds are driven downward by the heat where they condense on cooler soil particles just below the soil surface. In sufficient quantity and especially in coarser-textured soils, the resulting water-repellent layer impedes the normal downward flow of water. Higher-severity fires also can consume some of the shallow soil organic matter that helps bind together larger soil clumps.
When winds and the first rains arrive, they quickly wash the ash away, and the impact of raindrops on the bare soil can detach and disperse small soil particles to create a surface seal or crust.
The net result is that after a high or moderate severity fire the ability of the soil to absorb water decreases from around several inches per hour to perhaps just one-third of an inch per hour. Any additional rainfall becomes surface runoff, and a rainstorm of only one inch per hour can generate 1.5 million cubic feet of runoff per square mile.
The raindrops and surface runoff can easily erode and transport the unprotected soil on the hillsides. The resulting accumulation and concentration of flow and sediment in stream channels can rapidly mobilize massive amounts of rocks and soil.
The resulting mixture of water, eroded soil and rocks can quickly bulk up to a concentrated mix of water with 10 to 40 percent sediment, or an even more concentrated and deadly debris flow moving at up to 20 miles per hour. Once these flows reach flatter areas or encounter obstacles, the velocity decreases and the rocks and mud are deposited. The potential for such flows is exacerbated in much of Southern California because the mountains are steeper than normal due to rapid uplift along regional faults.
With population growth there are ever more houses and other developments at the base of the mountains. Of even greater concern is the placement of houses and other structures on the alluvial fans where streams have been depositing sediment and rapidly changing their course over many thousands of years.
No Easy Fixes
When hillslopes are denuded of their surface soil cover by fire or other processes, the resulting increase in runoff and erosion is nearly inevitable. Mulch or other surface cover can be applied to help protect the soil from the raindrop impact, but mulching is difficult and expensive to quickly apply over large areas, and is progressively ineffective on steeper slopes or during more intense storms that produce more surface runoff.
Debris basins can be constructed to capture the runoff and sediment, but there are problems of finding locations and sizing their capacity for extreme events, as well clearing them prior to large storms.
In Montecito an exceptional storm cell developed over a severely burned area, with nearly an inch of rain in just 15 minutes and over half an inch of rain in just five minutes. Montecito is particularly at risk as the hillslopes above town are oversteepened by faulting and rapid uplift, and much of the town is built on deposits laid down by previous floods.
The Dunsmir Sediment Basin in Los Angeles County, CaliforniaFEMA
Some debris basins were in place, but they were quickly overtopped by the hundreds of thousands of cubic yards of water and sediment. While high post-fire runoff and erosion rates could be expected, it was not possible to accurately predict the exact location and extreme magnitude of this particular storm and resulting debris flows.
There is a long history of comparable post-fire debris flows in the Los Angeles area with a greater loss of life. Comparable events also have occurred elsewhere in the western U.S., but in most cases the consequences have been much less tragic because wildfires more commonly occur in much less populated areas. This means the resultant effects are often limited to degraded water quality, loss of aquatic habitat and excess reservoir sedimentation.
Looking to the future, it is very clear that the problem is only going to get worse.
First, climate change is increasing the length and severity of the fire season by reducing snowpacks and increasing temperatures. Warmer temperatures increase fire risk as well as the capacity of the atmosphere to hold water, which is increasing rainfall intensities.
Second, a policy of suppressing wildfires has increased the amount and density of vegetation in some areas. This greater fuel load can result in higher severity fires and more denuded hillslopes. Future wildfires are inevitable, and when there are high temperatures, high winds, low humidity and large fuel loads, it is not possible to safely fight or control a large wildfire.
Nor is it possible to stop the subsequent hillslope runoff and erosion. Debris basins or diversion structures can be built to reduce damage, but these are expensive and often do not have sufficient capacity for extreme post-fire storm events. While we are getting much better at predicting the risk of sediment-laden flows after wildfires through improved modeling and weather forecasting, the starting point has to be stricter zoning rules to minimize construction in vulnerable areas. And once an area does burn, residents must heed the calls for evacuation when post-fire rainstorms are predicted.
On the positive side, most burned areas generally revegetate within two to four years. Once there is less than about 30-35 percent bare soil, there is a greatly reduced risk of high runoff and erosion rates.
In Montecito the rapid delineation of risk zones led to the evacuations that undoubtedly helped save many hundreds of lives. What is now important is that the lessons from the Thomas Fire are applied in other areas to help minimize future losses of life and property. This is true in both the short term as more rain falls on the recently burned areas, and over the longer term given our increasingly fire-prone future.
Reposted with permission from our media associate The Conversation.
By Karen L. Smith-Janssen
Colette Pichon Battle gave a December 2019 TEDWomen Talk on the stark realities of climate change displacement, and people took notice. The video racked up a million views in about two weeks. The attorney, founder, and executive director of the Gulf Coast Center for Law & Policy (GCCLP) advocates for climate justice in communities of color. Confronted with evidence showing how her own South Louisiana coastal home of Bayou Liberty will be lost to flooding in coming years, the 2019 Obama Fellow dedicates herself to helping others still reeling from the impacts of Katrina face the heavy toll that climate change has taken—and will take—on their lives and homelands. Her work focuses on strengthening multiracial coalitions, advocating for federal, state, and local disaster mitigation measures, and redirecting resources toward Black communities across the Gulf South.
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By Stuart Braun
"These are not just wildfires, they are climate fires," Jay Inslee, Governor of Washington State, said as he stood amid the charred remains of the town of Malden west of Seattle earlier this month. "This is not an act of God," he added. "This has happened because we have changed the climate of the state of Washington in dramatic ways."
'These Aren't Wildfires'<p>Sam Ricketts, who led climate policy and strategy for Governor Jay Inslee's 2020 presidential campaign, tweeted on September 11 that "These aren't wildfires. These are #climatefires, driven by fossil fuel pollution."</p><p>"The rate and the strength and the devastation wrought by these disasters are fueled by climate change," Ricketts told DW of fires that have burnt well over 5 million acres across California, Oregon, Washington State, and into neighboring Idaho. </p><p>In a two-day period in early September, Ricketts notes that more of Washington State burned than in almost any entire fire season until now, apart from 2015. </p><p>California, meanwhile, was a tinderbox after its hottest summer on record, with temperatures in Death Valley reaching nearly 130 degrees Fahrenheit, according to the U.S. National Weather Service. It has been reported as the hottest temperature ever measured on Earth.</p>
<div id="29ad9" class="rm-shortcode" data-rm-shortcode-id="8346fe7350e1371d400097cd48bf45a2"><blockquote class="twitter-tweet twitter-custom-tweet" data-twitter-tweet-id="1306969603180879872" data-partner="rebelmouse"><div style="margin:1em 0">Drought-parched wetlands in South America have been burning for weeks. https://t.co/pjAKdFcKPg #Pantanal https://t.co/ImN2C5vwcp</div> — NASA Earth (@NASA Earth)<a href="https://twitter.com/NASAEarth/statuses/1306969603180879872">1600440810.0</a></blockquote></div><p>As evidenced by Australia's apocalyptic Black Summer of 2019-2020, fires are burning bigger and for longer, with new records set year-on-year. Right now, Brazil's vast and highly biodiverse Pantanal wetlands are suffering from catastrophic fires.</p>
#climatefires Started in Australia<p>Governor Inslee this month invoked the phrase climate fires for arguably the first time in the U.S., according to Ricketts.</p><p>But the term was also used as fires burnt out of control in Australia in late 2019. In the face of a 2000km (more than 1,200 miles) fire front, and government officials and media who <a href="https://www.dw.com/en/trump-climate-change-denial-emissions-environment-germany-fake-heartland-seibt/a-52688933" target="_blank">played down the link to climate change</a>, Greens Party Senator Sarah Hanson-Young and a friend decided that reference to bushfires was inadequate. </p><p>"We both just said, we've got to start calling them climate fires, that's what they are," the Australian Senator told DW.</p><p>Hanson-Young says scientists have been warning for decades that these would be the effects of global heating. "We've been told these kinds of extreme weather events and destruction is what climate change would look like, and it's right here on our doorstep," she said from her home state of South Australia — where by early September fire warnings had already been issued.</p><p>"Calling them climate fires was making it absolutely crystal clear. It is essential that there's no ambiguity," she said </p><p>Having deliberately invoked the term, Hanson-Young soon started to push it on social media via a #climatefires hashtag. </p>
How to Talk About the Urgency of Global Heating<p>The need to use more explicit language when talking about extreme weather events linked to climate change is part of a broader push to express the urgency of global heating. In 2019, activist Greta Thunberg tweeted that the term "climate change" did not reflect the seriousness of the situation. </p><p>"Can we all now please stop saying 'climate change' and instead call it what it is: climate breakdown, climate crisis, climate emergency, ecological breakdown, ecological crisis and ecological emergency?" she wrote. </p><p>"Climate change has for a long time been talked about as something that is a danger in the future," said Hansen-Young. "But the consequences are already here. When people hear the word crisis, they understand that something has to happen, that action has to be taken."</p><p><span></span>Some terms are now used in public policy, with state and national governments, and indeed the EU Parliament, declaring an official climate emergency in the last year. </p>
Words That Reflect the Science<p>But while the West Coast governors all fervently link the fires to an unfolding climate crisis, U.S. President Donald Trump continues to avoid any reference to climate. In a briefing about the fires, he responded to overtures by Wade Crowfoot, California's Natural Resources Secretary, to work with the states on the climate crisis by stating: "It'll start getting cooler. You just watch." Crowfoot replied by saying that scientists disagreed. Trump rejoined with "I don't think science knows, actually." </p><p>It was reminiscent of the anti-science approach to the coronavirus pandemic within the Trump administration, <a href="https://www.dw.com/en/donald-trump-admits-playing-down-coronavirus-risks/a-54874350" target="_blank" rel="noopener noreferrer">at least publicly</a>. Fossil fuel companies are also benefiting from his disavowal of climate science, with the Trump administration having <a href="https://www.dw.com/en/opinion-trumps-paris-climate-accord-exit-isnt-really-a-problem/a-51124958" target="_blank" rel="noopener noreferrer">pulled out of the Paris Agreement</a> and reopened fossil fuel infrastructure like the Keystone XL pipeline. </p><p>But the science community has responded, with Scientific American magazine endorsing Trump's Democratic presidential challenger Joe Biden, the first presidential endorsement in its 175-year history. </p><p>Hanson-Young says the use of explicit language like climate fires has also been important in Australia due to the climate denialism of politicians and the press, especially in publications owned by Rupert Murdoch. As fires burnt out much of Australia's southeast coast, they were commonly blamed on arson — a tactic also recently used in the U.S.</p>
Climate Rhetoric Could Help Decide Election<p>The language of climate has begun to influence the U.S. presidential election campaign, with Democratic nominee Joe Biden labelling President Trump a "climate arsonist."</p><p>Biden is touting a robust climate plan that includes a 2050 zero emissions target and a return to the Paris Agreement. Though lacking the ambition of The New Green Deal, it has been front and center of his policy platform in recent days, at a time when five hurricanes are battering the U.S. Gulf Coast while smoke blanketing the West Coast spreads all the way to the East. </p><p>People are experiencing the climate crisis in a visceral way and almost universally relate to the language of an emergency, says Ricketts. "They know something is wrong."</p>
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