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Lahar-Chasers in Indonesia

30 April 2006

Semeru Volcano
Semeru volcano (3676 m) in east Java, Indonesia, during one of its hourly eruptions in March 06.

The internationally-used word for volcanic rock slides and mud flows, “lahar”, comes from Java in Indonesia. For good reason: Java hosts many active volcanoes, including the deadly cones of Merapi and Semeru. On these mountains, lahars can be a daily occurrence in the wet season. Hence, what better place to study the processes and internal dynamics of lahars while we wait for one to happen at Ruapehu? Suzy Cole, a PhD student from the Institute of Natural Resources, Massey University has recently spearheaded a new angle in our research on lahar processes – the geophysical imaging of active flows.

During March-April 06, Suzy, along with her supervisors, Dr Shane Cronin, (INR-Massey), and Dr Vern Manville (GNS Science, Wairakei), went lahar-chasing at Semeru volcano in east Java. They were joined by PhD student Jon Procter (INR Massey) who is trying to relate mathematical lahar models to reality, and by colleagues Prof. Jean-Claude Thouret from the Université Blaise Pascal (France) and Dr Indyo Pratomo from the Indonesian Ministry of Energy and Mineral Resources. The group aim was to test new monitoring systems for these types of highly destructive flows at Ruapehu volcano. This research forms part of the Marsden Funded program of “Capturing the secrets of a life-sized lahar” that Cronin and Manville lead, and is also supported by FRST programmes at both Massey and GNS Sceince, along with Earthquake Commission funds.

Lahar
A small lahar in the Lengkong channel at Semeru, Suzy Cole is standing beneath a tarpaulin in the pouring rain videotaping and recording the passage of a moving lahar on a broad-band seismograph at Semeru in Indoensia.

Semeru, like its better-known sibling Merapi, is in a constant state of eruption, with viscous lava oozing out slowly in its craters to form a dome. Regular collapses from the growing domes produce incandescent avalanches of rock, some of which race up to 11 km down the volcano to destroy villages along their paths. Most of the flows are, however, small and rock debris piles-up on the lower volcano flanks during the dry season, only to be eroded and swept downstream along river channels in a series of lahars during the rainy season.

The internal dynamics of lahars are still poorly understood by scientists, because they flow with such violence and such a large content of sediment that normal flood monitoring instruments are destroyed. In addition, since they often occur suddenly, not many scientists have been able to mobilise in time to observe and sample them in flow. Hence we hope that the automatic measurements taken from the Semeru flows will enable us to understand how lahars impact on structures in their paths and how sediment is carried within them. The team took a broadband seismometer with them that allows the imaging of ground vibrations in three directions and across a huge range of frequencies. Suzy is using this instrument to try to identify the “signature” vibrations of different types of lahars, particularly to understand the different ways sediment is carried within them – ie., is it sliding, rolling, bouncing or floating? She ran a digital video camera throughout her recording period, to be able to correlate flow conditions with the seismograph signal.

Lahar Vibrations
The passing of a lahar appears as these wiggly-lines in an overview of the seismic signal collected for three directions. Rain onset is shown by the smooth to low vibrations. The lahar onset is shown by the higher-amplitude vibrations.

The arsenal of equipment was completed by a water depth probe and a load-pressure plate, along with two “acoustic flow monitors” (AFMs). The intention was to bury the water pressure probe and load cell below the river bed in time for the first lahar. These instruments are designed to weigh the active lahar as it passes over them, capturing its evolution. Unfortunately it was impossible to bury the sensors deeply enough in the bouldery river-bed before a tropical downpour triggered a lahar. However, 20 minutes of flow was captured before the small and relatively sediment-poor flow scoured out and broke the sensor cables. Shane and Vern spent the next day scouring the shops of Lumajang for appropriate wires, tubes, glue and pipes to repair the instruments with typical kiwi ingenuity. Returning to the fray, the two buried the instruments deeper and with a special scour-proof design that lasted 50 minutes in the next, larger lahar, before a change in the flow path again uprooted them and swept them 10 m downstream. Despite this set-back, it was proven that buried instruments can successfully record the passage of a lahar above them: next time Shane and Vern are going to invest in a mechanical excavator rather than picks and shovels!

Between lahars, the ever-changing channel dimensions were mapped by Jean-Claude and Jon using high-precision real-time-kinetic GPS equipment from Massey. Once the lahars started flowing, the pair sprung into action and along with Indyo they collected a series of bucket samples of the lahars as they went by to determine the changing sediment concentration in the flow.

Dr Vern Manville
Dr Vern Manville (GNS Science) at Semeru with our lahar-damaged equipment, the water-pressure sensor in his right hand and the load cell (aka “frying-pan”) in his left.

The trip was ostensibly to record data from any flow which occurred, but also allowed for testing of the effectiveness and reliability of prototype equipment intended for installation on Ruapehu. It highlighted a few problems with installation and the equipment used – namely a greater depth of burial for the channel-centered instruments, due to the extremely erosive nature of the flows encountered. An additional off-the-cuff experiment was conducted using a magnetic induction loop in an attempt to utilise the magnetic properties of the rocks passing in the flow. The experiment did not yield any real results but further adaptation of the idea may be possible and could form a new detection system.

Armed with this experience and with a new set of equipment the team are undeterred in their aims and will return to the area in January 2007 in time for the highest-intensity part of the rainy season and most frequent lahars. The recent activity at Mt Merapi means that this may also be targeted for study, however, it may not be safe to observe the larger flows expected there.