Human impacts on infilling rates of hollows in landslide‐prone areas of western Japan: Estimation from radiocarbon dates and high‐resolution DEMs

The infilling rate of hollows is one of the primary factors affecting the recurrence interval of shallow landslides. Although many studies have focused on colluvium in hollows under various regional settings, few have directly estimated the infilling rates from radiocarbon dating and airborne LIDAR digital elevation models (DEMs) in shallow landslide scars. In this work, we analysed the radiocarbon dates of charcoal in the colluvium in the granitic mountains of Hiroshima and Hofu, where heavy rainfall has caused multiple shallow landslides in recent years. A total of 27 samples were collected from landslide scars in seven hollows, and the infilling rates were inferred from the calibrated radiocarbon ages and pre‐landslide depth of the samples estimated from the DEMs before the landslide event. Topographic parameters including local slope gradient and size of the source area were also measured using the DEMs. Our findings showed that the calibrated radiocarbon ages of colluvium in hollows ranged from about 350 to 1700 cal BP, and the infilling rates ranged from about 0.2 ± 0.2 to 5.4 ± 0.9 mm/y. In the two hollows with limited human impacts, the infilling rates were relatively low, and there is a trend of increase with increasing source‐area size and topographic curvature. In contrast, infilling rates were very high at two hollows with intensive human impacts where active surface erosion occurred from the 17th to the early 20th century. Three hollows located around the ruins of a medieval castle near the drainage divides of the source area also had relatively high infilling rates. For the hollows with intensive human impacts, the infilling rates were not controlled by topographic attributes such as source‐area size and curvature. Infilling rates in hollows were inferred from the calibrated radiocarbon ages of buried charcoal and pre‐landslide soil depth estimated from the LIDAR digital elevation model (DEM). Infilling rates were extremely high at two hollows with intensive human impacts from the 17th to the early 20th century. In two hollows with limited human impacts, infilling rates tended to increase with increasing topographic curvature.