Analyses on forest road damages over the past 19 years using rainfall evaluation with return periods in Gunma prefecture, Japan

Developing forest road networks in Japan is necessary to enable more efficient and stable timber extraction from forest resources, as well as sustainable forest management, considering the perspective of public functions such as land and watershed conservation and climate change mitigation. Recently, heavy rainfall has been increasing once in 100 years, and the forest road damages will increase because of the sediments from the landslides induced by heavy rainfall. This study acquired governmental subsidized recovery costs from forest road damages over the past 19 years between 2001 and 2019 in Gunma prefecture. Then, this study analyzed correlation coefficients between return periods of rainfall intensity in relation with soil water index as well as three-layer water tanks and recovery costs. The study sites were four AMeDAS stations such as Kanna, Fujioka, Nishinomaki, and Tashiro which had relatively large governmental subsidized recovery costs in 2001, 2007, and 2019. Correlation coefficient between return period of third-layer water tank and recovery costs per forest road length was positive with the 5% significance for all data in these study sites. Correlation coefficients between return periods of rainfall intensity in relation with soil water index as well as all three-layer water tanks and recovery costs were also positive with the 1% or 5% significance in Kanna stations, especially that of the third-layer water tank was the strongest. It was implied that large forest road damages could occur due to deep rainwater penetration because correlation coefficients between water tanks and recovery costs per forest road length tended to increase according to the deeper tanks.