Mt. Merapi's Lahars...

On January 21st of this year a slurry-river of volcanic debris (a lahar) covered Magelang Road just north of Jogjakarta. The flow destroyed dozens of houses and businesses, but thankfully left none dead. In its wake the flow left thousands of cubic meters of rocks, sand, ash, and other volcanic materials to a depth of up to 7 meters. I noticed the remnants of the lahar last week on the way to the Dieng Plateau. Although the road, which is a main thoroughfare for the region, was cleared relatively quickly, a significant amount of debris remains. I wanted to know more about the lahar and its effects on people, so I rented a motorcycle and made my way back to Jumoyo to talk with residents and have a walk around. Several folks were kind enough to share their experiences with me and tell me about how the lahar has altered the local economy.

The people there told me that the flow was 7 meters high in some cases, or enough to completely engulf a two-story building. The main flow was 30 meters wide, and it flowed as quickly as a rapidly moving river (newspaper accounts estimate 14 kilometers/hour). The lahar destroyed 44 houses in this area and quite a number of businesses, as you can see from the pictures. Nothing has been rebuilt and many of the displaced residents are still living in a nearby shelter. In the wake of the lahar a new industry has emerged: breaking up and hauling off the volcanic material. As it turns out, the volcanic debris that comprises the flow makes an excellent building material when mixed with water. Hundreds of trucks have come from as far away as Semarang (200km away) to load up the stuff and cart it away. To meet the demand a number of enterprising folks have set up shop chipping the material out of the flow to load onto trucks. The owners of the property covered by the lahar have the rights to the material, and so the diggers contract with them to mine the material. A digger can fill up to 2 trucks a day, at 180,000 rupiah per truck (around $21). The owner's share is 50,000 rupiah per load. Many of the larger rocks are taken away to make stone carvings, but the largest ones are left as they lay. The people in the area believe that some of the rocks have spirits, and so they don't want to move them. One rock in particular has become something of a local celebrity; the story goes that it was crying, and the belief is that it is inhabited by the spirit of a local resident who is waiting for her husband to return. The mining and loading operation only employs a few people, though; others have taken to selling drinks and pictures to the tourists who have come to see the lahar and the weeping stone. Still others our completely out of work; I had a sort of conversation with an old mute lady that was previously employed as a dishwasher in one of the restaurants destroyed by the lahar. She has literally nothing left. Unfortunately in situations like this it is frequently those least able to cope that are hit the hardest. The residents told me that there was some relief early and a significant amount of involvement from NGOs, but support has dropped off rapidly. The residents are still waiting on payments from the government to rebuild. It's hard to grasp the shear magnitude of the debris flow from the pictures, but look at the truck and steam-shovel loading material for comparison. One of the most astonishing things is that this spot is more than 20 kilometers away from the volcano. I took the video below to give you an idea of the scale of the disaster.

After my visit to Jumoyo I headed into the countryside to visit some more lahar sites. I stopped briefly at the dam featured in the following video, further up the slopes of Merapi. This structure was constructed across the small canyon of the Kuning River (Kali Kuning) about 5 years ago specifically to stop lahar flows. As you can see from the pictures it has functioned as advertised; on the upstream side of the dam there is a 12-meter accumulation of material. The material actually overflowed the dam, as you can see from the picture to the left (note the broken guardrail). According to the folks around the dam, the government has moved some of the material out, but they don't know if the trucks and other heavy equipment will be back to finish the job. The folks hawking drinks, food, and pictures here were all working to make ends meet after the lahar destroyed their livelihoods. They told me they had no warning before the lahar came barreling down the mountain. And although they were all struggling, they made it a point to express how fortunate they felt to be alive; they said it was thanks to Allah. I was stunned that the dam structure is able to withstand the tremendous weight of the accumulated material.

Next I continued up the hill towards the villages of Rangkah and Kinah Rejo as I'd been told that these places had been stricken by lahars as well. I wasn't prepared for the absolute and complete devastation I saw here. These villages were almost completely obliterated by lahars that moved through along with the latest eruptions of Merapi last October. 33 people were killed and the local economy, which was based on cattle, was completely upended as the debris flows followed poisonous gas clouds through the area. This area as well has turned into a tourist destination, and the people of the area have partnered (1) with the Department of Tourism for the Jogjakarta region to collect an entry fee of 5000 rupiah (65 cents or so) per vehicle. The folks on duty told me that there are 35 attendants that work rotating shifts of 10 days or so (after 10 days the whole shift is replaced). This seemed like a creative way to provide temporary employment, but see the note below.

As I walked around the site I talked to some of the residents. One gentleman took a break from scavenging the rebar and other metal from his collapsed home to talk to me. He told me that before the disaster he had 5 cows, but they were all killed. The government provided a replacement payment of 8 million rupiah for each large cow and 3.5 million for each young cow, but this was below the market rate. He told me he had received no aid to rebuild his home, as he showed me the temporary structure he built for his family. Another resident told me that the government had promised 5000 rupiah per person per day to meet daily needs, but the money had not yet materialized. Other folks were busy cons tructing ponds to raise leleh fish as part of a government recovery plan. I didn't quite under stand the logic of raising fish in the mountains, and no one could really explain it to me. The picture to the left is of a painting by local artist Ki Joko Wasih that was painted after the disaster. I think it speaks for itself.

What is a lahar, exactly?

A lahar (sometimes also referred to as a "debris flow" or "mudflow") is a rapidly flowing mixture of rock debris and water from a volcano. The word "lahar", which has entered the technical lexicon of geography and geology, is from Javanese and loosely translates to "mudflow", but this is misleading because they don't contain much silt, sand, or clay, which is what makes mud muddy. Lahars are interesting from a geographic perspective because they arise from a combination of volcanic and climatic process. They occur on both active and extinct volcanoes and can be devastating due to the extreme force of the flow; lahars have the consistency of wet concrete and flow as fast as rivers. One lahar killed 23,000 people in 1985 after a relatively minor eruption of the Nevado del Ruiz volcano in Colombia. Lahars can also displace thousands of people, which increases their vulnerability to hunger and disease. Lahars are different from other mass movements (like landslides and rock falls) because of the materials involved, the way the material moves down slope, and the triggering mechanisms. They are very mobile and can cover up to 20 times the area of an avalanche with the same amount of material. There are several different types of lahars, but they can be loosely lumped into two categories: "hot" lahars and "cool" lahars.

Hot Lahars (also called "primary lahars") are those associated with volcanic eruptions. Lahars require water, which can come either from convective rainfall associated with the air rising from the heat around the volcano or from melted glacial ice in the case of alpine volcanoes. In the tropics hot lahars often occur when an eruption coincides with the rainy season. Around Mt. Merapi hot lahars also happen when pryoclastic material and other debris mixes with one of the many rivers that drain the mountain. On average this happens once every two years or so in some channels. The lahar that destroyed Kinah Rejo was a hot lahar.

Cool Lahars ("secondary lahars")don't coincide with an eruption. Rain erodes away accumulated lahar material, which is very unstable and unconsolidated to begin with. Around Merapi this most frequently occurs during the rainy season, which lasts from November to April. According to the government's geological agency, they are most likely to occur when there is more than 40 millimeters (around a foot and a half) of rain in a 2 hour period, but in reality this varies depending on slope, type of rainfall, elevation, and a number of other factors. The lahar that covered Magelang Road was a cool lahar.

In addition to the obvious impacts around Merapi, it's been suggested (see Walter et al reference below) that lahars can amplify earthquake damage. This is because the material is loose compared to solid bedrock, and so volcanic vibrations loosen the material and cause it too shift and settle. It's kind of like what happens when you shake a cereal box. Thus structures built on old lahar deposits are more likely to be damaged. Human actions can also exacerbate the damage done by lahars. Deforestation and conversion to farmland reduces the hydraulic roughness of volcano flanks and leads to more severe lahar events.

Mass Movements?

Lahars are an example of what geomorphologists (2) call "mass movements". This technical term is pretty self explanatory. Another example would be the landslides I described in the Dieng Plateau post. Mass movements are typically the result of some sort of slope failure. A very simplistic way to think about this is to imagine the Earth as a dynamic system that is trying to level itself out. It does this by using rain, waves, wind, and seismic activity to move material from high places to low places. However, most of the time the forces that are trying to tear mountains and hills down are balanced by those trying to hold the mountain up. Slope failure happens when these forces are temporarily out of balance, and the resulting mass movement only stops when a new equilibrium is reached. There are all sorts of mass movements, but they are generally classified on the basis of the type of movement, the speed, and how much water is involved. I've included a diagram I took from the textbook I use to teach Introduction to Physical Geography at UH (3). I'll write more about the specifics of mass movements in future posts, but this is a good place to start.

Acknowledgments:

Many thanks to my friends Wanto and Mike at Wisma Bahasa for giving me directions and recommendations for this post.

(1) According to the people working the gate, they get paid 14000 rupiah a day, just under two dollars. However, when I was there there were dozens of visitors. The remaining money goes to the department of tourism for the province of Yogyakarta. I'm not sure why this is or where that money goes.

(2) A geomorphologist is a geographer that studies landforms and their life cycles.

(3) Do any of these types of mass movements look familiar to you? What types of slope failures are most common in Hawai'i? Can you use the internet to find some newspaper stories about different

References:

Christopherson, Robert. Every couple of years or so. Geosystems: An Introduction to Physical Geography. New York: Pearson/Prentice Hall. A bunch of pages.

Kerle, Norman, and Clive Oppenheimer. 2002. Satellite Remote Sensing as a Tool in Lahar Disaster Management. Disasters 26:2, pp140-160

Lavigne, Franck. 1999. Lahar Hazard Micro-Zonation and Risk Assessment in Yogyakarta City, Indonesia. GeoJournal 49: pp173-183

Rodolfo, Kelvin. 1989. Origin and Early Evolution of Lahar Channel at Mabinit, Mayon Volcano, Philippines. Geological Society of America Bulletin 101, pp414-426.

Walter, TR, and 14 coauthors. 2008. The 26 May 2006 Magnitude 6.4 Yogyakarta Earthquake South of Mt. Merapi Volcano: Did Lahar Deposits Amplify Ground Shaking and Thus Lead to the Disaster? Geochemistry, Geophysics, Geosystems 9:5.