EFFECTS OF PERMEABILITY ON THE COMBINATION OF FLUVIAL EROSION AND SLOPE FAILURES AS THE MAIN PROCESS OF EROSION IN THE DEVELOPMENT OF EXPERIMENTAL LANDFORM

Permeability of the sand mound in uplift-and-erosion experiments, which is considered to affect the amount of surface runoff and the vulnerability to slope failures, is an important factor controlling the development of experimental landform. When the same material (a mixture of fine sand and kaolinite, 10:1 by weight) is used, the degree of compaction determines the mound permeability and additionally shear strength. High compaction is expected to yield low permeability and high shear strength, and low compaction high permeability and low shear strength. This time, four runs listed below, which have different combinations of permeability and uplift rate, are reported. All four runs are in the “steady state phase” of Ouchi (2015).

run --- permeability -------- uplift rate ------precipitation

32 --- 1.84 x 10^-4cm/s ---- 0.36 mm/h ----- 70-80 mm/h

36 --- 2.09 x 10^-4cm/s ---- 2.0 mm/h ------ 70-80 mm/h

37 --- 1.95 x 10^-3cm/s ---- 2.0 mm/h ------ 70-80 mm/h

38 --- 1.53 x 10^-3cm/s ---- 0.36 mm/h ----- 70-80 mm/h

Experimental landforms develop with uplift and erosion, which consists of fluvial peocesses and slope failures, as in the previous runs. Fluvial erosion with the development of a valley system dominates in the first stage of experiment, when a flat-topped square sand mound emerges from the ground level. Slope failures then take over after slopes ready to collapse develop. Channels become conduits of sediments produced by slope failures by this time, and average height becomes rather stable changing around a certain height. In runs with low permeability and high shear strength (runs 32 and 36), a significant amount of surface runoff and slopes less vulnerable to failures tend to make fluvial erosion the main erosion process. Because fluvial erosion is rather slow to lower the whole surface opposing to the rise by uplift, a high table-mountain develops with fast uplift (run 36), while relatively low and fine landform develops with slow uplift (run 32). The runs with high permeability and low shear strength (runs 37 and 38), on the other hand, show similar landform development of slope failure domination, despite the large difference in uplift rate. Slope failures, which occur in a short time, seem to effectively lower the surface against the uplift by changing the interval of concentrated occurrence corresponding to the uplift rate.