The role of soil creep and slope failure in the landscape evolution of a head water basin: field measurements in a zero order basin of northern Japan

Field measurements of soil creep and slope stability were conducted on a nose, side-slope and hollow in a zero order basin near Sapporo, Hokkaido, northern Japan, and the preferential location of soil creep and slope failure was determined. Soil creep was continuously measured by the strain probe method at three sites from 1994 to 1995, and was compared with soil moisture conditions and ground temperature. In the summer, active soil creep occurred only when rainfall led to large soil moisture changes and a near-saturated condition, which was most likely induced by shrink–swell activity of soil. In the winter, soil creep was caused by seasonal frost, although the mass transport was limited because of the insulation provided by snow cover. These results indicate that the soil moisture change and soil moisture content during a rainfall event in the summer are the major factors controlling soil creep in this basin. Soil moisture conditions were further measured by a tensiometer at 16 sites in the rainy season in 1994. On the nose and side-slope, active soil-moisture changes took place during rainfall-events. The hollow tended to maintain higher soil-moisture conditions than the nose and side-slope, because subsurface flow was concentrated in the hollow. Thus the soil-moisture variation that encourages soil creep rarely occurred in the hollow. From these results, sediment transport rates caused by creep were estimated to be 207.0, 159.5 and 9.0×10−3 m3/yr on the nose, side-slope and hollow, respectively, and the resultant mass balances were calculated at −207.0, +25.1 and +172.9×10−3 m3/yr, respectively. These results clearly show infilling in the hollow and denudation on the nose. Slope stability was analyzed by the infinite slope model. The potential of slope failure was evaluated from the relationship between critical water depth Hcr and soil thickness D. The analysis revealed that an increase in D causes a marked decrease in Hcr on the side-slope, indicating the high potential of slope failure on the slope. In contrast, both on the nose and in the hollow, the decrease in Hcr for the same increase in D was lower than that on the side-slope. However, slope failure on the side-slope and soil creep on the nose infill material into the hollow. Thus, the increase in D in the hollow is higher than that on the other slopes; leading to an increase in slope failure potential. These results indicate that soil creep and slope failure act as infilling and evacuating processes of the zero order basin with differing intensities depending on slope form: soil creep removes soil materials from the nose and deposits them in the hollow, whereas slope failure removes materials from the side-slope and deposits them in the hollow. When infilling develops a sufficiently thick soil accumulation in the hollow, slope failure evacuates the hollow.