Regional characteristics of driftwood runoff: Focusing on geotectonic subdivision in Japan

Abstract:

This study aims to classify geotectonic characteristics based on differences in driftwood runoff in Japan.

The damages of driftwood are reported at recent heavy rain disasters in Japan. Considering the fact that the number of reports of driftwood damage varies from region to region, and that there is no mention about correlation between driftwood runoff and geotectonic characteristics, it is meaningful to evaluate the variability of driftwood runoff affected by geotectonic characteristics.

We divide Japanese archipelago into 3 parts: the north side of Median tectonic line in Southwest Japan (Inner zone), the south side of the same line in Southwest of Japan (Outer zone) and Northeast Japan based on geotectonic characteristics. Southwest Japan and Northeast Japan are dived by Itoigawa-Shizuoka tectonic line. Driftwood runoff is measured by the amount trapped in the dam reservoir. As target dams, 3 dam catchments are selected from each geotectonic subregion and we analyze 9 catchments in total.

In each catchment, the annual driftwood runoff per unit area is compared with 4 different precipitation durations (1hour, 24hours, 1month and 1year). The correlation coefficient is calculated to evaluate the correlation between driftwood per unit area and 4 different precipitation durations.

From the comparison of the results among 3 geotectonic characteristics, it is indicated that Inner zone and Outer zone have positive correlation between 24hours precipitation, and annual total precipitation respectively with driftwood runoff per unit area. In addition, Outer zone tends to have less driftwood runoff compared to Inner zone. On the other hand, Northeast Japan shows weaker correlation compared to Inner zone and Outer zone. Geotectonic feature of Northeast Japan that easily leads to debris flow disaster will be a more influential factor than precipitation.

In the presentation, we will show the result of the whole analysis, which considers the influence of geotectonic backgrounds in each of the 3 parts of Japanese archipelago.