ACCUMULATION RATES OF ALLUVIAL CONES, CONSTRUCTED BY DEBRIS-FLOW DEPOSITS, IN THE DRAINAGE BASIN OF THE TAKAHARA RIVER, GIFU PREFECTURE, CENTRAL JAPAN

Abstract

The Takahara River and its tributaries dissect rugged mountains in central Honshu Island, forming steep mountain slopes along the rivers. Small precipitious streams deeply dissecting the mountain slopes usually attain gradients more than twenty degree. Debrisflows have occasionaly occurred from these streams, bringing heavy damages to inhabitants. The authors selected 271 streams from those in the study area and investigated the number of debris-flows occurred from each stream by the method of interviews to the inhabitants. It is revealed that the debris-flows occurred sixty times from the selectedd streams during the period from 1913 to 1979. The estimated average probability value of debris-flow occurrence from a stream, is accordingly calculated to be 3.3 × 10-3/year. The estimated return period of debris-flow occurrence is, therefore, determined to be 3.0 × 102 year. The distribution of stream numbers classified by the number of debris-flow occur rence are conformed to the poisson distribution with the same probability, except four large streams (Table 2) . Accordingly, it is considered that the occurrence of debrisflow has a character of a randam phenomena as a whole. The authors also measured the volumes of fifteen debris-flow deposits accumulated during the last seventy years, and found that the volume of each debris-flow deposit is explained as a function of the catchment area of each stream (eq. 2; Fig. 6). Six steps of fluvial terraces are distributed along the Takahara River and its tributaries; namely, Hongo, Miyahara, Nakagoe, Tono, Sakamaki and Miza terraces, from higher to lower. Hongo terrace was formed by the deposition of volcanic mudflows mainly transported by the fluvial process. The volcanic mudflow deposit covers the underlying Sukusaka terrace deposit with slight unconformity (Koike, 1978) . Sukusaka terrace is a dissected alluvial fan formed along the Jinzu River, the main river of the Takahara, and correlated with fluvial terraces distributed scatteredly around the margin of Northern Japan Alps (Fukai, 1957; Machida, 1979). As the age of these terraces is considered to be ca. 50, 000_??_60, 000y. B. P., the age of Hongo terrace is estimated to be slightly younger than 50, 000years. Miyahara terrace was mainly formed by the erosion of Hongo terrace deposit. An alluvial cone deposit intercalating the volcanic ash bed(DKP, 45, 000_??_47, 000y. B. P.) covers the terrace surface. Therefore, the age of Miyahara terrace is estimated to be ca. 47, 000y. B. P. Etchu Akatsuchi covers both Hongo and Miyahara terraces (Fig. 4). Nakagoe terrace was formed by fluvio-volcanic mudflow deposit, derived from Kachigane mudflow. The age of the terrace was estimated to be 11, 000y. B. P., from the 14C age of wood intercalated in Kachigane mudflow (Koike, 1978). This terrace is not covered by Etchu Akatsuchi, but by brown forest soil with clear B-horizon. Tono terrace was formed slightly eroding Nakagoe terrace deposit. This terrace is formed immediately after the formation of Nakagoe terrace. Sakamaki terrace is a strath terrace formed by usual fluvial process. This terrace is correlated with the buried terrace developed along the Ashiaraidani. The age of terrace deposit was measured to be ca. 5, 000y. B. P. with 14C-dating method by Fujii et al. (1974). Miza terrace is another strath terrace. The terrace deposit unconformably covers Nakao pyroclastic flow deposit, the age of which is reported to be 2, 500y. B. P. by Fujii et al. (1974). The age of this terrace is, therefore, estimated to be 2, 300y. B. P. (Table 1).