3D modeling and analysis of lavaka phenomenon in Madagascar  Andrea Raveloson1, Ferenc Visnovitz1, Beatrix Udvardi2, Gábor Molnár1, Balázs Székely1,3 1Department of Geophysics and Space Sciences, Eötvös University,2Lithosphere Fluid Research Lab, Department of Petrology and Geochemistry, Eötvös University, 3Institute of Photogrammetry and Remote Sensing, University of Technology, Vienna ISZA 2012 Raveloson, 2011

Raveloson, 2011

Objectives Presenting the lavaka phenomenon and its impact in Madagascar Understanding the cause of its formation Field surveys and documentation 3D modeling different types of lavaka Sedimentological analysis Observing lavaka’s features DEM, geomorphological studies Specify the link between weathered substance and erosion Raveloson, 2011

I. Lavaka Malagasy: hole , international terms: gully erosion Widespread in the highlands of Madagascar (<30/km2) Amphitheater shaped in deeply weathered bedrock Steep to vertical walls with tiny outlet V~100.000 m3 2 main types: 1. Mid-slope lavaka 2. Toe-slope lavaka Raveloson, 2011 Raveloson, 2011

Development Development stages of lavaka: Raw patches on the hillsides Rabarimanana et al., 2001 Raveloson, 2011 Development stages of lavaka: Mulder and Idoe, 2004 Raw patches on the hillsides Deep incision, active erosion Initial infilling with inactive, partly overgrown lower walls V-shaped lavaka U-shaped, Inactive lavaka Wells and Andriamihaja, 1991 Visnovitz, 2011

- + Impact Soil erosion High sediment concentration in rivers Silting up reservoirs and irrigation lands Undermine roads and buildings + Special micro climate Good soil Appropriate features for rice cultivation Protection=prevention (stabilisation needs long-term commitments and funding)

II. 3D modeling 3D models of 2 different types of lavaka were displayed with different softwares Toe-slope lavaka: Width: 72 m Length: 61 m Mid-slope lavaka: Width: 40 m Length: 103 m Google Earth, 2011 Tsiafahy, Madagascar Google Earth, 2011

Toe-slope lavaka’s model: Rózsi , 2011 Agisoft Photoscan Hypr3D Photosynth

Mid-slope lavaka Hypr3D Matlab-based semi-automatic model

IV. Sedimentological analysis Field surveys and sampling Mineral composition: Stereomicroscopy ATR-FTIR spectrometry X-ray diffraction Grain size distribution: Wet sieving Laser grain distribution Evaluating hydraulic conductivity: Zamarin formula Jáky formula Hansen formula „Yellow saprolite” „Red saprolite” Raveloson, 2011

Mineral composition IR-spectrometry Xray-diffraction Laterite Red saprolite Xray-diffraction Yellow saprolite Raveloson, 2011 Udvardi, 2011

Grain size distribution and hydraulic conductivity Wet sieving Laser grain distribution V% m% cm µm Hydraulic conductivity Legend: L: laterite sample Sz1: Red saprolite Sz2: Yellow saprolite

V. Conclusion and Summary Lavaka formation is due to the specific climate and lithological features Lavaka’s modeling can be achieved with cheap photogrammetrical methods MATLAB based program is better for complex structure, and when georefered model is needed Lavaka’s material consist of laterite topsoil and saprolite. Saprolite can be divided into 2 new parts a reddish and a yellowish saprolite layer. These layers seems to be the key to lavaka’s inner structure Further studies will concentrate on improving the models and continue the sedimentological studies. Our aim is to study lavakas from their formation till they get to the inactive, stage 5. Different stages of the same lavaka should be modeled to see more clearly. Studying lavaka’s behavior in rainy season is also important to understand their formation.

Thank you for your attention! The authors owe thanks to Judith Mihály (Chemical Research Center), Zoltán Szalai (Research Centre for Astronomy and Earth Sciences), Johary Raveloson and Roland Rozsi. We would like to thank Csaba Szabó, István Kovács and the members of Lithosphere Fluid Research Lab. Finally a sincere thank to Gábor Timár and Gábor Kovács for comments.

Semi-automatic modell Fényképek összepárosítása Adatbázis építés DEM pontok meghatározása Korrekciók kiegyenlítő számítással DEM értékelése Raveloson, 2011 Félautomatikus: manuális pontválasztás, automatikus illesztés