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 (ZS) and the real elevation based on airborne LiDAR DEMs (ZDEM). 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 ZDEM – ZS. 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.
