IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

3W_3G-P10

Date/Time: July 23 Poster

Lahar characteristics and erosion measurements using multiparameter recording
stations and DEMs in the Gendol catchment after the 2010 Merapi eruption
Jean-Claude Thouret1 , Akhmad Solikhin2 , Sandy Budi Wibowo3 , Danang Sri Hadmoko4 , Philippe
Grandjean5 , Pascal Allemand5 , Christophe Delacourt6
1Laboratoire Magmas et Volcans et ClerVolc, Université Blaise Pascal Clermont-Ferrand 2, France,
Center for Volcanology and Geological Hazard Mitigation - Geological Agency, Indonesia, 3 Institut deGéographie, Université Paris 1 Panthéon-Sorbonne, France, 4 Research Center for Disaster and Facultyof Geography, Gadjah Mada University, Yogyakarta, Indonesia, 5 Laboratoire de Géologie de Lyon,
Université Claude Bernard Lyon 1, France, 6 Laboratoire Domaines Océaniques, UMR6538, IUEM,
Université de Brest, France2

E-mail: aksolikhin@vsi.esdm.go.idLahars, including debris flows and hyperconcentrated streamflows, represent the most frequent natural hazardaround the Merapi volcano because they can be triggered during and after, or even without eruptions. Eachyear lahars with a discharge range between 200 and 2000 m³/s are triggered during the rainy season in several
rivers which drain the west, SW and south flanks. We carry out an experimental method to measure hydraulicand physical characteristics of lahars in the Gendol River channel on the south flank of Merapi. This valley washeavily impacted by Pyroclastic Density Currents (pyroclastic flows and surges) during the October-November
2010 eruption and flooded by lahars, which have started to remobilize the PDC deposits a few months later. Six
months after the 2010 large (VEI 4) eruption, small areas in Gendol down-valley 20 km from summit have been
affected by subsequent over-bank and avulsed lahars. The over-bank process can be attributed to low-gradient(0.04 m/meter), meandering rivers (sinuosity index of 1.25) across the low-angle (<2°) ring plain and the limited
capacity (200-300 m²) of river channels.

The method encompasses: (1) hydraulic and geophysical in situ measurements of flows with sensors locatedat the valley bottom and on the edges, (2) high-resolution (decimetric) Digital Elevation Models (DEM) of thevalley channel before and after a lahar, aiming at measuring the processes of aggradation and degradation in thecatchment, (3) remote sensing analysis of erosion processes such as lahars and fluvial transport remobilizing
the 2010 thick aprons of pyroclastic deposits (∼30 million m³). We use two experimental stations located on twocheck-dams ∼250 m apart for in situ measurements along the middle course of the Gendol River at about 700 melevation. The stations include 2 seismometers, 2 geophones, 2 load cells, 2 pore pressure sensors, 1 radar gauge,
2 rain gauges, a barometer and cameras. We measure discharge, sediment concentration, arrival and surface
velocities, and dynamics features at the flow surface. The sediment concentration is measured simultaneously
using buckets in the lower station every five minutes during the flow. We then compute the volumes of transported
sediment allowing us to understand the flow dynamics, the processes of entrainment, and the parameters fordescribing the rheology of the lahar material. We finally evaluate the processes of erosion and sedimentation of
two channel segments using multi-temporal DEM (before and after lahars) derived from low altitude stereoscopic
images acquired by UAV (Unmanned Autonomous Vehicle). The final goal is to track the time-related propagationof the flows down valley and calibrate the input parameters of two numerical models (Titan2D, VolcFlow), which
will be used to outline lahar-prone areas in the heavily populated ring plain of Merapi.

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