Differential rock fragmentation in the Ohya-Kuzure Debris-flows (Japan): Inferences from UAV-based remote-sensing and uniaxial compression strength.

Abstract:

INTRODUCTION - The Ichinosawa catchment - Ohyakuzure-landslide, Shizuoka prefecture, Japan - sees several debris flows every year, which are fueled by a combination of heavy rainfall and an alternation of highly fractured sandstones and mudstones. One of the hypothesis that has been put to the forth is the potential role of those geological formation on the triggering of debris-flows. The hypothesis is that mudstones tend to break in smaller fragment and fines that can be part of the waxing of the debris-flow, while sandstones only are part of the dry-load.

OBJECTIVES AND METHODS - To test this hypothesis, the contributors compared the spatial and size distributions & the mechanical characteristics of sandstones and mudstones from the debris-flow fan deposit. We combined UAV-SfM photogrammetry constrained from GNSS-RTK control points. From the obtained orthophotographs, the size, shape and nature of a clasts > 10 cm were retrieved. The matrix was sampled for grain-size, and compression strength was obtained from Schmidt hammer.

RESULTS - The results demonstrate that the surface of the debris-flow fan is dominated by sandstone clasts, with a minority of mudstone clasts. Oversized mudstone clasts are more represented at the apex and near the toe of the fan. The matrix shows the opposite result. It is dominated by a median size 4-8 mm gravel size matrix in the interfluves, while the material in the gullies has a median size 0.25 mm to 1 mm (suggesting that there is a washing process of the fine material). The vertical distribution is dominated by either a mixture of sandstones in a coarse mudstone matrix, or a top layer of sandstones with a matrix layer only underneath. This was observed in the central part and the apex of the debris-flow fan. Regardless of the location, the matrix is always made of a layer of mudstones that can incorporate sandstones, but not the opposite. The Schmidt hammer results provided rebounds between 58 and 78 for sandstones and < 36 for mudstones.

INTERPRETATION AND CONCLUSION – The mudstone contributes mostly the matrix of the debris flows while the sandstones the clasts. The waxing due to the fines in the water suggests that monitoring mudstone availability at the source could help predict the triggering of debris flows. Furthermore, its distribution on the fan must also be a control on the stability.