Sediment Conditions and Frequency of Debris Flows in and around Asaminami-ku and Asakita-ku, Hiroshima City

Abstract

Torrential rainfall occurred around Hiroshima City, in August 2014, leading to large scale debris flows that traveled via mountain streams. The weathered granitic rock in the Hiroshima region is likely to erode further during heavy rainfall, causing similar sediment disasters. Believing that it is important to elucidate the frequency and impact of previous disasters, and to assess the probability of future ones, we conducted a field survey. We observed multiple layers of paleo debris flow deposits along streams in this area and used radiocarbon dating techniques on carbonized materials in the deposits to determine when the debris flow occurred. We found evidence of at least 7 debris flow events based on the results of dating and historical documents. The occurrence cycle of debris flow is estimated at 150-400 years．