What is a Debris Flow?

Landslide in Guatemala

Landslides constitute one of the most destructive geological hazards in the world today. One of the main reasons for this is because of the high speeds that slides can reach, up to 160 km/hour (100 mph). Another is the fact that these slides can carry quite a bit of debris with them that serve to amplify their destructive force.

Taken together, this is what is known as a Debris Flow, a natural hazard that can take place in many parts of the world. A single flow is capable of burying entire towns and communities, covering roads, causing death and injury, destroying property and bringing all transportation to a halt. So how do we deal with them?

Definition:

A Debris Flow is basically a fast-moving landslide made up of liquefied, unconsolidated, and saturated mass that resembles flowing concrete. In this respect, they are not dissimilar from avalanches, where unconsolidated ice and snow cascades down the surface of a mountain, carrying trees and rocks with it.

Images of a Debris Flow Chute and Deposit, taken by the Arizona Geological Survey (AZGS). Credit: azgs.com
Images of a Debris Flow Chute
and Deposit, taken by the Arizona Geological Survey (AZGS). Credit: azgs.com

A common misconception is to confuse debris flows with landslides or mudflows. In truth, they differ in that landslides are made up of a coherent block of material that slides over surfaces. Debris flows, by contrast, are made up of “loose” particles that move independently within the flow.

Similarly, mud flows are composed of mud and water, whereas debris flows are made up larger particles. All told, it has been estimated that at least 50% of the particles contained within a debris flow are made-up of sand-sized or larger particles (i.e. rocks, trees, etc).

Types of Flows:

There are two types of debris flows, known as Lahar and Jökulhlaup. The word Lahar is Indonesian in origin and has to do with flows that are related to volcanic activity. A variety of factors may trigger a lahar, including melting of glacial ice due to volcanic activity, intense rainfall on loose pyroclastic material, or the outbursting of a lake that was previously dammed by pyroclastic or glacial material.

Jökulhlaup is an Icelandic word which describes flows that originated from a glacial outburst flood. In Iceland, many such floods are triggered by sub-glacial volcanic eruptions, since Iceland sits atop the Mid-Atlantic Ridge. Elsewhere, a more common cause of jökulhlaups is the breaching of ice-dammed or moraine-dammed lakes.

Debris flow channel in Ladakh, NW Indian Himalaya, produced in the storms of August 2010. Credit: Wikipedia Commons/DanHobley
Debris flow channel in Ladakh, near the northwestern Indian Himalaya, produced in the storms of August 2010. Credit: Wikipedia Commons/DanHobley

Such breaching events are often caused by the sudden calving of glacier ice into a lake, which then causes a displacement wave to breach a moraine or ice dam. Downvalley of the breach point, a jökulhlaup may increase greatly in size by picking up sediment and water from the valley through which it travels.

Causes of Flows:

Debris flows can be triggered in a number of ways. Typically, they result from sudden rainfall, where water begins to wash material from a slope, or when water removed material from a freshly burned stretch of land. A rapid snowmelt can also be a cause, where newly-melted snow water is channeled over a steep valley filled with debris that is loose enough to be mobilized.

In either case, the rapidly moving water cascades down the slopes and into the canyons and valleys below, picking up speed and debris as it descends the valley walls. In the valley itself, months’ worth of built-up soil and rocks can be picked up and then begin to move with the water.

As the system gradually picks up speed, a feedback loop ensues, where the faster the water flows, the more it can pick up. In time, this wall begins to resemble concrete in appearance but can move so rapidly that it can pluck boulders from the floors of the canyons and hurl them along the path of the flow. It’s the speed and enormity of these carried particulates that makes a debris flow so dangerous.

Deforestation (like this clearcut in Sumatra, Indonesia) can result in debris flows. Credit: worldwildlife.org
Deforestation (like this clearcut in Sumatra, Indonesia) can result in debris flows. Credit: worldwildlife.org

Another major cause of debris flows is the erosion of steams and riverbanks. As flowing water gradually causes the banks to collapse, the erosion can cut into thick deposits of saturated materials stacked up against the valley walls. This erosion removes support from the base of the slope and can trigger a sudden flow of debris.

In some cases, debris flows originate from older landslides. These can take the form of unstable masses perched atop a steep slope. After being lubricated by a flow of water over the top of the old landslide, the slide material or erosion at the base can remove support and trigger a flow.

Some debris flows occur as a result of wildfires or deforestation, where vegetation is burned or stripped from a steep slope. Prior to this, the vegetation’s roots anchored the soil and removed absorbed water. The loss of this support leads to the accumulation of moisture which can result in structural failure, followed by a flow.

Sarychev volcano, (located in Russia's Kuril Islands, northeast of Japan) in an early stage of eruption on June 12, 2009. Credit: NASA
Sarychev volcano, (located in Russia’s Kuril Islands, northeast of Japan) in an early stage of eruption on June 12, 2009. Credit: NASA

A volcanic eruption can flash melt large amounts of snow and ice on the flanks of a volcano. This sudden rush of water can pick up ash and pyroclastic debris as it flows down the steep volcano and carry them rapidly downstream for great distances.

In the 1877 eruption of Cotopaxi Volcano in Ecuador, debris flows traveled over 300 kilometers down a valley at an average speed of about 27 kilometers per hour. Debris flows are one of the deadly “surprise attacks” of volcanoes.

Prevention Methods:

Many methods have been employed for stopping or diverting debris flows in the past. A popular method is to construct debris basins, which are designed to “catch” a flow in a depressed and walled area. These are specifically intended to protect soil and water sources from contamination and prevent downstream damage.

Some basins are constructed with special overflow ducts and screens, which allow the water to trickle out from the flow while keeping the debris in place, while also allowing for more room for larger objects. However, such basins are expensive, and require considerable labor to build and maintain; hence why they are considered an option of last resort.

Aerial view of debris-flow deposition resulting in widespread destruction on the Caraballeda fan of the Quebrada San Julián. Credit: US Geological Survey
Aerial view of the destruction caused by a debris-flow in the Venezuelan town of Caraballeda. Credit: US Geological Survey

Currently, there is no way to monitor for the possibility of debris flow, since they can occur very rapidly and are often dependent on cycles in the weather that can be unpredictable. However, early warning systems are being developed for use in areas where debris flow risk is especially high.

One method involves early detection, where sensitive seismographs detect debris flows that have already started moving and alert local communities. Another way is to study weather patterns using radar imaging to make precipitation estimates – using rainfall intensity and duration values to establish a threshold of when and where a flows might occur.

In addition, replanting forests on hillsides to anchor the soil, as well as monitoring hilly areas that have recently suffered from wildfires is a good preventative measure. Identifying areas where debris flows have happened in the past, or where the proper conditions are present, is also a viable means of developing a debris flow mitigation plan.

We have written many articles about landslides for Universe Today. Here’s Satellites Could Predict Landslides, Recent Landslide on Mars, More Recent Landslides on Mars, Landslides and Bright Craters on Ceres Revealed in Marvelous New Images from Dawn.

If you’d like more info on debris flow, check out Visible Earth Homepage. And here’s a link to NASA’s Earth Observatory.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Sources:

A Dark and Dusty Avalanche on Mars

Mars may be geologically inactive but that doesn’t mean there’s nothing happening there — seasonal changes on the Red Planet can have some very dramatic effects on the landscape, as this recent image from the HiRISE camera shows!

The full extent of the 1000-meter-long dusty landslide (NASA/JPL/University of Arizona)
The full extent of the 1000-meter-long dusty landslide (NASA/JPL/University of Arizona)

When increasing light from the springtime Sun warms up the sides of sheer cliffs made from countless layers of water and carbon dioxide ice near Mars’ north pole, some of that CO2 ice sublimes, sending cascades of loose soil and dust down to the terraced base below. This uncovered material stains the frost-covered polar surface dark, outlining the paths of avalanches for HiRISE to easily spot from orbit. (See the original HiRISE image here.)

Circling Mars since March 2006, the HiRISE camera aboard NASA’s Mars Reconnaissance Orbiter has even captured some of these polar landslides in action.

The rust-colored avalanche shown above has fallen hundreds of meters from the middle of a layered ice deposit, spreading nearly a kilometer across the frozen ridges at the base of the cliff. The view was acquired on Sept. 13, 2013.

Check out a video explaining this view and the processes that created it below, narrated by Phil Plait (aka the Bad Astronomer).

Mars’ seasonal polar caps are composed primarily of carbon dioxide frost. This frost sublimates (changes from solid directly to gas) in the spring, boosting the pressure of Mars’ thin atmosphere. In the fall the carbon dioxide condenses, causing the polar caps to reach as far as ~55 degrees latitude by late winter. By learning about current processes on a local level we can learn more about how to interpret the geological record of climate changes on Mars. (Source)

What is an Avalanche?

Have you ever noticed how the snow packs on a car windshield after a heavy snowfall? While the temperature is cold, the snow sticks to the surface and doesn’t slide off. After temperatures warm up a little, however, the snow will slide down the front of the windshield, often in small slabs. This is an avalanche on a miniature scale.

On the other hand, a mountain avalanche in North America might release 229,365 cubic meters (300,000 cubic yards) of snow. That’s the equivalent of 20 football fields filled 10 feet deep with snow. However, such large avalanches are often naturally released. They are primarily composed of flowing snow but given their power, they are also capable of carrying rocks, trees, and other forms of debris with them.

In mountainous terrain avalanches are among the most serious objective hazards to life and property, with their destructive capability resulting from their potential to carry an enormous mass of snow rapidly over large distances.

Classification:

Avalanches are classified based on their form and structure, which are also known as “morphological characteristics”. Some of the characteristics include the type of snow involved, the nature of what caused the structural failure, the sliding surface, the propagation mechanism of the failure, the trigger of the avalanche, the slope angle, direction, and elevation.

Loose snow avalanches (far left) and slab avalanches (near center) near Mount Shuksan in the North Cascades mountains. Credit: wikipedia
Loose snow avalanches (far left) and slab avalanches (near center) near Mount Shuksan in the North Cascades mountains. Credit: Thermodynamic/Wikipedia Commons

All avalanches are rated by either their destructive potential or the mass they carry. While this varies depending on the geographical region – – all share certain common characteristics, ranging from small slides (or sluffs) that pose a low risk to massive slides that come that pose a significant risk.

An avalanche has three main parts: the starting zone, the avalanche track, and the runout zone. The starting zone is the most volatile area of a slope, where unstable snow can fracture from the surrounding snowcover and begin to slide. The avalanche track is the path or channel that an avalanche follows as it goes downhill. The runout zone is where the snow and debris finally come to a stop.

Causes:

Several factors may affect the likelihood of an avalanche, including weather, temperature, slope steepness, slope orientation (whether the slope is facing north or south), wind direction, terrain, vegetation, and general snowpack conditions. However, weather remains the most likely factor in triggering an avalanche.

During the day, as temperatures increase in a mountainous region, the likelihood of an avalanche increases. Regardless of the time of year, an avalanches will only occur when the stress on the snow exceeds the strength either within the snow itself or at the contact point where the snow pack meets the ground or the rock surface.

An avalanche east of Revelstoke in 2010 Credit: Canadian Avalanche Center
An avalanche east of the town of Revelstoke, BC, in 2010 Credit: Canadian Avalanche Center

Although avalanches can occur on any slope given the right conditions, in North America certain times of the year and certain locations are naturally more dangerous than others. Wintertime, particularly from December to April, is when most avalanches will occur with the highest number of fatalities occurs in January, February and March, when the snowfall amounts are highest in most mountain areas.

Deaths Caused by Avalanches:

In the United States, 514 avalanche fatalities have been reported in 15 states from 1950 to 1997. In the 2002–2003 season there were 54 recorded incidents in North America involving 151 people.

In Canada’s mountainous province of British Columbia, a total of 192 avalanche-related deaths were reported between January 1st, 1996 and March 17th, 2014 – an average of roughly ten deaths per year. During the winter of 2014, avalanche concerns also forced the closure of the Trans-Canada highway on a number of occasions.

Avalanches on Other Planets:

Not too surprisingly, Earth is not the only planet in the Solar System to experience avalanches. Wherever their is mountainous terrain and water ice, which is not uncommon, there is the likelihood that material will come loose and cause a cascading slide to take place.

On February 19th, 2008, NASA’s Mars Reconnaissance Orbiter captured the first ever image of active avalanches taking place the Red Planet. The avalanche occurred near the north pole, where water ice exists in abundance, and was captured by the MRO’s HiRISE (High Resolution Imaging Experiment) camera completely by accident.

Images taken by the MRO's HiRISE camera show at least four Martian avalanches, or debris falls, taking place near the north pole. Credit: NASA/JPL
Images taken by the MRO’s HiRISE camera show at least four Martian avalanches, or debris falls, taking place near the north pole. Credit: NASA/JPL

The images showed material – likely to include fine-grained ice dust and possibly large blocks – detaching from a towering cliff and cascading to the gentler slops below. The occurrence of the avalanches was spectacularly revealed by the accompanying clouds of fine material (visible in the photographs) that continue to settle out of the air.

The largest cloud (shown in the upper images) was about 180 meters (590 feet) across and extended about 190 meters (625 feet) from the base of the steep cliff. Shadows to the lower left of each cloud illustrate further that these are three dimensional features hanging in the air in front of the cliff face, and not markings on the ground.

The photo was unprecedented because it allowed NASA scientists to get a glimpse of a dramatic change on the Martian surface while it was happening. Despite seeing countless pictures that have detailed the planet’s geological features, most appear to have remained unchanged for several million years. It also showed that terrestrial events like avalanches are not confined to planet Earth.

We have written many articles about the avalanche for Universe Today. Here’s an article about the Mars avalanche predicted by geologists, and here’s an article about the volcanic tuff.

If you’d like more info on avalanche, check out NASA Science News: Avalanche on Mars. And here’s a link to the American Avalanche Association Homepage.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 51: Earth.

Sources: