Lacustrine sediment records provide considerable information on the environmental fluctuations in the surrounding catchments. Particularly, sediment records obtained in deltas provide precious earth surface information in the catchments of rivers forming the deltas. The long core called BDP-99 was obtained in 1999 at the margin of the Selenga Delta, Lake Baikal (in glaciated zone), which was formed by the Selenga River. The sedimentation rate is large in cold (glacial and stadial) periods while mineral grain size is comparatively large in warm (interglacial and interstadial) periods. Results suggest that erosible (transportable) materials were limited in the surface of catchments in warm periods because of large vegetation cover (humid climate) despite occasionally large discharge. A similar phenomenon is demonstrated by the core (BDP-98) from the Academician Ridge of the same lake although the sedimentation rate is much lower. By contrast, sedimentation rate and mineral grain size are both large in warm periods in non-glaciated zone (Lake Biwa, central Japan; humid temperate climate); however, only slight differences are observed between the warm and cold periods.

Microtopography of a debris-flow fan is a key to understanding the behavior of debris flow on it. However, recognizing the microtopography of debris-flow fans in urban areas is difficult due to modification by human activity. This study proposes a method to visualize the microtopography by creating a DEM that assumes the gradient from the fan apex to the fan toe is uniform (hereafter, approximate conical surface) and calculating the difference between the elevation of approximate conical surface (Z_S) and the real elevation based on airborne LiDAR DEMs (Z_DEM). This method was tested in two debris-flow fans where debris flow occurred in 2014 in Asaminami Ward, Hiroshima City. The shallow valleys in the urbanized debris-flow fans were clearly recognized as the areas with low values of Z_DEM – Z_S. In addition, the major debris flows in the two fans run out on the roads along these shallow valleys. This analysis can be applied to any area with airborne LiDAR DEM, and must be a useful tool for estimating disaster risk on debris-flow fan.