Quantcast

Post-Fire Landslide Problems Likely to Worsen: What Can Be Done?

Climate
Abdallah Issa / Flickr

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.

What Next?

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.

EcoWatch Daily Newsletter

Sen. Michael Bennet (D-CO) speaks during the North American Building Trades Unions Conference at the Washington Hilton April 10, 2019 in Washington, DC. Zach Gibson / Getty Images

Colorado senator and 2020 hopeful Michael Bennet introduced his plan to combat climate change Monday, in the first major policy rollout of his campaign. Bennet's plan calls for the establishment of a "Climate Bank," using $1 trillion in federal spending to "catalyze" $10 trillion in private spending for the U.S. to transition entirely to net-zero emissions by 2050.

Read More Show Less
Foto-Rabe / Pixabay

When Trump's Environmental Protection Agency (EPA) announced its replacement for the Obama-era Clean Power Plan in August 2018, its own estimates said the reduced regulations could lead to 1,400 early deaths a year from air pollution by 2030.

Now, the EPA wants to change the way it calculates the risks posed by particulate matter pollution, using a model that would lower the death toll from the new plan, The New York Times reported Monday. Five current or former EPA officials familiar with the plan told The Times that the new method would assume there is no significant health gain by lowering air pollution levels below the legal limit. However, many public health experts say that there is no safe level of particulate matter exposure, which has long been linked to heart and lung disease.

Read More Show Less
Sponsored
A crate carrying one of the 33 lions rescued from circuses in Peru and Columbia is lifted onto the back of a lorry before being transported to a private reserve on April 30, 2016 in Johannesburg, South Africa. Dan Kitwood / Getty Images

By Andrea Germanos

Animal welfare advocates are praising soon-to-be introduced legislation in the U.S. that would ban the use of wild animals in traveling circuses.

Read More Show Less
A tornado Monday in Union City, Oklahoma. TicToc by Bloomberg / YouTube screenshot

Extreme weather spawned 18 tornadoes across five states Monday, USA Today reported. Tornadoes were reported in Texas, Oklahoma, Kansas, Missouri and Arizona, but were not as dangerous as forecasters had initially feared, the Associated Press reported.

Read More Show Less
A woman walks in front of her water-logged home in Sriwulan village, Sayung sub-district of Demak regency, Central Java, Indonesia on Feb. 2, 2018. Siswono Toyudho / Anadolu Agency /Getty Images

A new study has more than doubled the worst-case-scenario projection for sea level rise by the end of the century, BBC News reported Monday.

Read More Show Less
Sponsored
Matt Cardy / Stringer / Getty Images

The Guardian is changing the way it writes about environmental issues.

Read More Show Less
Blueberry yogurt bark. SEE D JAN / iStock / Getty Images Plus

By Lizzie Streit, MS, RDN, LD

Having nutritious snacks to eat during the workday can help you stay energized and productive.

Read More Show Less
A 2017 flood in Elk Grove, California. Florence Low / California Department of Water Resources

By Tara Lohan

It's been the wettest 12 months on record in the continental United States. Parts of the High Plains and Midwest are still reeling from deadly, destructive and expensive spring floods — some of which have lasted for three months.

Mounting bills from natural disasters like these have prompted renewed calls to reform the National Flood Insurance Program, which is managed by Federal Emergency Management Agency and is now $20 billion in debt.

Read More Show Less