Any good introductory article to the hazards of lahars ought to begin with a definition of just what they are. A lahar can be defined as a volcanically induced water-based debris flow. If that seems a little complicated, breaking it down will help: volcanically induced means it is from or caused by a volcano (emptying crater lake, flash-melted glacial cap, displacement of nearby water bodies by debris flows, etc…); water-based means that it is extremely fluid and mostly water (as opposed to other types of debris flows which are clast-based and mostly debris). Lahars are extremely important to volcanologists and the people who live near active volcanoes because they are common and can be very deadly.

Perhaps the best known example of a lahar is Nevado del Ruiz, Colombia. This event marked a turning point in volcanic hazards strategies as it illustrated a horrific example of how lack of communication failed to protect the people and how dangerous these floods can be. On November 12, 1985, the town of Armero was sort of an average Colombian town, lying in a valley next to the edge of a tall and narrow gorge. Sitting in the west of the country and 73km from the nearest volcano, the residents didn’t realize they had anything to fear. There were no international volcanologists present and, at the time, no mobile volcanologist teams that could be dispatched there. The night of November 13 was stormy and so no-one was able to see the volcano erupt- the only warning was an earthquake that was picked up by a seismograph 9km away. No-one saw the wall of mud, water, and debris rushing into their town. It was moving at 50km/hr and was up to 40m high in some places (130ft). The result was utter devastation. Over 23,000 people died that night and in the following days.

In this article we are going to look at several aspects of this hazard. The first deals with the mechanics: how they form, how they travel, what influences them, and what they leave behind. The second is a look at the specific dangers these floods pose to people and structures in the way. The article will end by looking at various strategies for mitigating and reducing these hazards, how they have worked in the past, and how they may work in the future.

Where what when why how?

Above, I stated the definition of a lahar so it won’t be repeated, but the manner in which they occur is of vital importance. There are a number of manners in which a lahar might be formed. Most common is the flash-melting of summit glacial caps during the course of an eruption (or shortly preceding). Remember that most stratovolcanoes are tall enough to have permanent glacial caps at their summits, and so during an explosive eruption, that ice is flash melted in a matter of minutes. The resulting cascade of boiling water mixes with mud and debris, devastating anything in its path. This type of lahar is typically the most devastating as it usually releases the largest volumes of water in the shortest periods of time. A second method of formation is the rapid displacement of a lake, either in a crater or nearby, by volcanic debris. This may occur through the expulsion of water from a crater lake during an eruption, or by the displacement of a nearby lake by other debris flows (lahar, pyroclastic flow, etc…). Moreover, debris flows may cut across rivers, damming them. While flooding behind them is usually not too destructive, the water may continue to rise until the dam is breached, resulting in a catastrophic release of water in a matter of minutes or hours. Similarly, a thoroughly contained crater lake may continue to rise until it breaches the crater rim. Once this breach occurs, a channel will form and will scour itself toward the base, resulting in a catastrophic release of water over the course of hours or days. There is another method of formation which, in my own opinion, stretches the definition of lahar (though it is widely used); this is simply a landslide from the slopes of a volcano caused by prolonged hydrothermal activity and/or rain. Where hydrothermal activity is prevalent, acid rich waters and gasses seep through the hard rocks and soil of a volcano, transforming it to a weak altered clay. This clay is easily eroded and becomes quickly water-logged during wet weather. While the lahar is not volcanically induced in this case, it is still composed of the same volcanic material and is thus technically a lahar.

It is important to note, as well, that the composition of lahars varies according to the conditions of formation as well as the area through which they travel. Lahars may be mostly water or they can grade towards a dry debris flow. One thing that all lahars have in common, however, is that they are all gravity driven. They flow downslope and if they encounter a basin or lake, that is pretty much the end of things. There is quite a bit of science to lahars, however…it’s not just rushing water.

When you see a lahar, it’s this massive torrent of raging water and debris and it can be easy to think that everything going on is pure chaos, but it’s not. Lahars actually have a fairly well-defined structure, even if it’s not immediately obvious. To begin, let’s look at what happens during the formation of a lahar:

Where the lahar begins as mostly water, you have a number of transitions occurring. Instances of this would be the displacement of a lake or the flash melting of a glacier. Here the lahar begins its life as almost pure water, but this stage is extremely short lived. The slopes of stratovolcanoes are dominantly composed of ash and clay, which is easily eroded. The raging water cascades down the steep slopes of the volcano, eroding channels into the sides and picking up tons of debris (mud, gravel, trees, etc…). The transformation is rapid and the lahar reaches the base of the volcano in a matter of seconds or minutes. The lahar follows lowlands: river valleys and canyons, places where people frequently live. The torrent of water, confined by the local topography, continues to erode at the banks of the river and from the base. Depending on how confined the lahar is, the flow may quickly dissipate or continue for dozens of miles. The only true way for a lahar to stop is for it to reach flat land or a lake. As the lahar gets further from the volcano, the slope of the land typically becomes smaller and the lahar loses some of the energy that was driving it. Moreover, the flow has picked up enormous amounts of debris which is inhibiting the flow of the water (logs, boulders, and mud are slowing the flow through friction). As the lahar loses its energy, it slows and begins to redeposit the sediment and debris it has picked up. Eventually it will enter a lake or dissipate to the point at which it no longer remains a significant hazard. The amount of time it takes for this to happen, however, depends on the confinement of the lahar; if it is in a canyon, the flow may continue indefinitely (such as the case at Armero).

There is more to a lahar than just this, however. During the debris-rich erosional phases of the lahar, the head or front is often higher than the rear portions. The cause is friction. The front of the lahar is the most active erosional part as this is where it first encounters the lose debris which it “picks up.” This addition of debris, however, dissipates energy through friction, which causes it to slow down. The lahar further back, however, is not picking up debris at the same rate and is thus traveling faster than the front. This results in a sort of pile-up at the head of the lahar making this the most dangerous section of the flow. This, then, would also be the most debris-rich section. Further back, the amount of debris being eroded decreases meaning it is more water-rich. At the back, where the lahar is the weakest, it behaves less like a debris flow and more like a flood. On the other hand, the more water-rich a flow is, the more rapidly it erodes the channel. This is an interesting situation to comprehend: the head of the flow carries the most debris, but as the amount of material it carries increases, the amount it can continue to hold or erode decreases. While the rear portions of the lahar are more water-rich and continue to erode the channel, the activity is not so pronounced because the energy is not so great here! The water is not moving so fast, which means it can’t pick up the amounts of sediment the head can. Thus, the fact that the rear is watery proves that it is not as swift.

The process through which a lahar erodes its channel is fairly simple: debris is cut away from the banks and from the base. Undercutting of the banks can cause enormous problems in populated areas, as seen in the lahars of Mt. Pinatubo during 1990’s. Channels can suddenly widen from fifteen or twenty feet to over one hundred feet; this causes enormous damage to neighborhoods built along these channels. Moreover, excavation of the channel bottom causes it to deepen and increases confinement of the lahar, allowing it travel more rapidly. As the flow dissipates, however, the action of the lahar reverses as it redeposits the debris it has picked up. The largest/heaviest objects drop out first, as these require the most energy to carry. As the flow continues to lose energy, progressively smaller and smaller debris drop out until the lahar has weakened itself out of existence. The weakening process, however, can be thought of as a conservation of energy. With the larger debris removed, the flow can continue to move while dissipating less of it’s gravitational energy to friction. Thus, a mud flow can continue to move long after the boulders have dropped out, so long as confinement is continued. The structure of a lahar is not purely chaotic either, which may be expected if you look at the raging river of mud that typically makes up a lahar. Friction at the base of a lahar, caused by the interaction of debris, water, and the channel bottom, causes this section to move more slowly than the top. This results in a behavior, similar to the treads of a tractor or tank. A similar situation is taking place along the sides of the lahar, where friction is slowing the flow where it contacts the banks of the channel. Again, this results in a circulating motion along the edges, similar to the treads of a tractor, but in this case, sideways. When you combine these actions, we can see that the top-center of the flow is fastest and becomes slower outward and downward. Consequently debris is pushed in toward the center and larger pieces are forced to drop out toward the edges or bottom. In addition, this can facilitate erosion of the channel as the larger debris scours out the banks. Finally, this tread-like action helps to conserve energy in the flow and keep it moving.

And if you understood half of that, congratulations: it’s a confusing subject!!

The Deadly Impact

    Warning: This section contains extremely graphic descriptions and so is not for the faint of heart.

As you may have figured out, lahars are extremely devastating to the environment and to communities in their paths. As far as structural damage goes, buildings and bridges are frequently leveled due to the impact of debris. Bridges may catch the debris passing under like a sieve resulting in the destruction of the bridge and complications in the flooding. Frequently, entire communities may be buried in mud and debris so that seeking shelter in-doors is futile. Comparisons may be drawn to instances where a man-made dam has broken and downstream communities have been impacted. In Armero, however, we see a much better example…rather, it was much much worse. The following contains quotations from the article “Impacts of Eruptions on Human Health” by Peter J. Baxter.

Up to 85% of the town of Armero was buried by 3-4 meters (10-13 feet) of mud and debris. The total death toll from lahars that day was estimated to be roughly 22,942 people with 4470 injured survivors. The head of the lahar was extremely water-rich and cascaded through town, overturning cars and sweeping people away. The first wave of the main lahar rose 30 meters high, traveled at a rate of 12 m/s and lasted 10-20 minutes. “Survivors clung to moving pieces of debris or were miraculously swept along on top of the mud. Overall, the inundation of mud lasted about 2 hours, with two slower moving major pulses and several smaller pulses over this period.”

“The head of the lahar would have been in turbulent motion and contain cobbles and boulders. Most people would have been killed immediately by the severe trauma caused by the collapse of buildings, flying debris, and burial by the slurry mass. As well as the risk of being engulfed, bodies would have been driven against stationary objects, or contorted and crushed by entrained debris such as trees and collapsed parts of buildings, resulting in mutilation and fractures of limbs and skull bones. Stones and other sharp objects would cut into the skin, causing deep lacerations. Mud was inhaled as it forced its way into the eyes, mouth, ears, and open wounds. The pressure of the mud against the chest would have inhibited breathing in those buried to the neck and caused some deaths by traumatic asphyxia…The commonest lesions in the hospitalized patients were lacerations, penetrating wounds, and infections. Many of the wounds were infected on admission. Most of the deaths in hospitals were due to infections including gas gangrene, tetanus, and a form of necrotizing fasciitis, which occurred almost exclusively in victims rescued after being in the mud for 3 days. This dreaded complication was due to normally nonpathogenic soil organisms replicating in wounds in the absence of oxygen, and is resistant to medical treatment” [in this condition, the body’s tissue literally destroys itself]. To make matters worse, the mud left behind by lahars is often extremely fluid and similar to quicksand, thus making rescue extremely difficult. Adding to the problem is the fact that there may be additional waves coming, which will spell disaster for rescuers wading through the thick mud as well as for survivors trying to escape. Overall, the entire experience bears the very definition of nastiness and so it goes without saying that we volcanologists do everything within our power to prevent these disasters from happening.

Predict and Prevent

After the events in Armero, the volcanic community decided never to allow a disaster of this magnitude to happen again. Shortly thereafter, the USGS formed the VDAP (Volcano Disaster Assistance Program), which is designed to be a mobile disaster response team capable of reaching distant locations in a matter of hours. One of the primary hazards volcanologists now look for are lahars, which can strike with devastating force at distances far from the volcano. While it remains difficult to estimate the size of future lahars, it is very possible to forecast their existence and paths they may follow. What we can do is create hazard maps, depicting the affected areas for lahars of different volumes. For example, there will be a map showing surface relief and drainages around the volcano. Sections will be highlighted for areas that will be impacted by lahars; each color represents a different volume. Thus, you may have red representing a 500,000 cubic meter lahar, yellow showing 1,000,000, blue showing 2,000,000 etc… This can also be viewed as a probability map as smaller lahars are more likely to occur than larger ones. For most of our history, lahar prediction has been based on educated guess work. Today, however, we have automated computer simulations that can create hazard maps, like those I’ve described above, such as the new LAHARZ program. With these programs, a digital elevation map (DEM) is inputted to a computer [a DEM is a relief map where each pixel represents a specific elevation]. After inputting a specific lahar volume, the map then predicts flood height along each selected drainage and estimates the area affected. That region is colored a specific color (selected by the programmer) and then a new region or volume is selected. After all is said and done, an accurate and reliable lahar hazards map is available for use in mitigating future hazards. This means taking action to either prevent the lahar from occurring or moving people from high risk areas.


Evacuating large numbers of people can be a domestic nightmare in disaster situations. Due to the rarity of lahars in most areas, local governments almost never remove residents in danger areas before the situation becomes urgent. When the danger finally becomes apparent there may only be days or even hours before it is too late. Once the lahar has begun, there may be only seconds for residents to flee to higher ground. On the other hand, after an evacuation notice has been issued, a lahar may not ever form leading to public distrust of government officials and scientists. This makes the entire situation precarious at best. Scientists must play a delicate balance between public opinion and public safety. In the end however, it is the job of the government to issue evacuation orders; scientists simply advise.

Preventing lahars from occurring is not a new idea, but these efforts have only recently become effective. There have been a number of methods used in this effort, all with various degrees of success. Crater lake lahars are the easiest to mitigate as the size of the lahar depends on the water level within the crater. Hence, by simply draining the lake, a potential lahar can be eliminated. In 1919, an eruption of the Indonesian volcano Kelut displaced roughly 40 million cubic meters of water, causing the deaths of over 5000 people. The Dutch colonial government recognized the problem and undertook to drain the lake permanently. However, when engineers began work four years later, the lake had already reached 38.5 million cubic meters. Over several decades, a series of drainage tunnels were built into the crater walls and the lake was essentially drained. Subsequent eruptions have shown that lahars will not be produced if the lake level is kept below 1 million cubic meters. Here we see a preventative measure that worked extremely well for this style of lahar, but most lahars don’t occur in this manner. These events require different preventative measures.

Rain instigated lahars require other methods. At Kelut, a series of dams were built along the slopes to block water and, more importantly, trap debris. The problem with building dams is that the debris will build up and essentially render the dam useless. It is therefore an expensive and temporary measure. Moreover, the dams must be well built to withstand potential forces during an eruption (including earthquakes). An extremely low number of people died during the eruption of Mt. Pinatubo in 1991 due to the excellent work of the VDAP team deployed there. The rain-based lahars that occurred afterwards were absolutely devastating and caused a number of fatalities (the eruption actually occurred during a typhoon). At this time, the government made the poorly planned decision that permanently abandoning towns in the affected area would be more expensive than trying to stop the lahars. Most of the preventative measures the government has taken have failed. One such measure was the building of levees and dikes to divert the flow of lahars…however, these structures were built with previous lahar deposits! It should not be too hard to figure out how this is a bad idea: the structures were and continue to be quickly eroded by the very forces they are trying to control. Moreover, the deposits are permeable meaning that they essentially become “soggy” in rain and thus less stable. Even concrete dams have failed in this situation due to the pressure of the flow or the scouring of loose rock from underneath the dams. A more effective measure that was taken was the blasting of channels to stabilize a local lake that frequently served as a source of lahars. The artificial channels divert water to unpopulated areas when the lake level rises while the lake itself serves to trap debris flowing down the slopes. Every year, however, lahars continue to take lives during the rainy season there and will continue to do so for years to come. Japan has been at the forefront of lahar reduction technology. To detect lahars, engineers stretch wires across channels that, when tripped in succession, trigger an alarm. To reduce the hazard, engineers have built several series of metal nets that act to remove debris from cascading water, thus limiting the destructive power of the flow. The main drawback to this method of mitigation is that the nets eventually fill with debris and begin to act as dams, which can amlify problems later.

I don’t want the reader to get the idea that lahars are unstoppable and inevitable, though. Efforts to prevent lahars have shown mixed results and attempts to reduce them are, more often than not, at least temporarily successful. Moreover, new techniques in forecasting lahars and their paths have led to disaster plans and community preparedness. People living in the path of a lahar should not feel doomed, but be prepared and knowledgeable. The most important thing for people living in the path of lahars (which includes people in the Seattle area) is to know if you live in a danger area and where you should go in the case of a lahar. If you are potentially in the path of a lahar, you should also be able to recognize the warning signs of such an event (as public notice may be late in coming). Volcanic activity near your area should be a HUGE warning. If you see the water-level in streams and rivers around your house rising together with nearby volcanic activity, run for high ground! People who have experienced lahars first hand report hearing a loud roar for minutes before the flow reaches them. Know where the high ground is and the quickest way to get there. Taking these simple preventative measures can keep you safe in the event of a disaster.


Hopefully after reading this, you have a much better understanding of what goes on with volcanic floods. To those of us in the volcanic community, this hazard is a very real threat that endangers the lives of hundreds of thousands of people across the world. Far from a chaotic and unpredictable rush of water from the slopes of a volcano, a lahar is a predictable and manageable event that we can actually forecast years in advance. New strategies in prediction and mitigation, together with new technologies, give us the ability to manage disasters and prevent catastrophes. For more information on the other hazards present at or around active volcanoes, check out the rest of the site.