The duration of the soil-depth recovery needed for reoccurrence of shallow colluvial landslides at a given site in humid regions is much longer than the return period of rainfall needed to generate sufficient pore water pressure to initiate a landslide. Knowledge of the rate of change in soil depth in landslide scars is therefore necessary to evaluate return intervals of landslides. Spatial variation in sediment transport at the Kumanodaira landslide scar in central Japan was investigated by field observations. Spatial distribution of the rate of change in soil depth was estimated using sediment transport data and geographic information system (GIS) analysis. Observations revealed that the timing of sediment transport differed for shallow and deep soil layers. Near-surface sediment transport (mostly dry ravel and some shallow soil creep at depths ≤0·05 m) measured in sediment traps was active in winter and early spring and was affected by freezing-thawing; soil creep of subsoil (i.e. >0·05 m), monitored by strain probes, was active in summer and autumn when precipitation was abundant. Near-surface sediment flux was estimated by a power law function of slope gradient. Deeper soil creep was more affected by relative location to the landslide scar, which influences soil depth, than by slope gradient. Our study indicated that the rate of soil-depth recovery is high just below the head scarp of the landslide. Abrupt changes in the longitudinal slope topography immediately above, within and just below the head scarp became smoother with time due to degradation proximate to the landslide head scarp and flanks, as well as aggradation just below the head scarp.

Mountain hillslopes are generally shaped by a combination of various types of sediment transport processes (e.g. surface erosion, soil creep, and dry ravel), which can occur simultaneously in the same area. Since sediment transport is generally affected by multiple microclimatic factors, such as heavy rainfall and changes in ground temperature, the types of predominant sediment transport processes vary by season. We conducted field observations in the southern Japanese Alps in the period from 2009 to 2013, using sediment traps and time lapse cameras, to investigate the seasonal changes in the type and flux of hillslope sediment transport on steep mountains in which both rainfall and diurnal freeze–thaw triggered sediment transport. In winter and early spring, sediment transport via diurnal freeze–thaw (soil creep and dry ravel) was highly active, whereas rainfall‐induced soil creep and selective transport by overland flow were active in summer and autumn when precipitation is abundant (average sediment flux of 0.033 and 0.074 kg m−1 day−1, respectively). Sediment flux was spatially variable and was affected by the form of the slope; sediment flux on the concave slope was higher than on the ridge‐shaped slope during both freeze–thaw and rainfall periods. Sediment flux on an old landslide exceeded that in second‐growth forest regardless of the slope shape. Temporal changes in the sediment flux were not completely synchronized among monitoring plots and were affected by slope shape, grain size, and episodic sediment supply events such as release of sediment from woody debris. Copyright © 2016 John Wiley & Sons, Ltd.

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