**5. Conclusions**

In this study, we statistically analyzed the characteristics of each topographic type at the points where failures occurred. Specifically, forest road segments were classified into four categories: stream crossings, streamside, zero-order basin, and others, and comparisons were made regarding the length of forest road failure, the relative probability of occurrence, repair costs, and induced rainfall intensity in each category. We were able to obtain the "Forest Road Facility Failure Assessment Document" prepared in Nagano Prefecture between 2006 and 2010. Based on this document, an inventory of forest road failures was compiled. The analysis in this study covers the 526 failures that occurred along these 207 routes. There are various types of failure to forest roads, and it is necessary to construct an evaluation model of failure risk and prioritize countermeasures according to the cause and risk of failure. Streamside forest road segments accounted for only 15% of the total length of forest roads analyzed, but 42% of the road segments that were damaged. Furthermore, the relative risk of the streamside forest road segment was about 6.0 times higher than that of the other categories of forest road segments, indicating that it was the most likely terrain type to be damaged in this analysis. From the perspectives of both the length of failure and the relative risk, it is clear that the most important issue in the target area is the prevention of failure on the streamside forest road segments. The results showed the importance of responding to forest road failures along streams, which was revealed by the analysis based on a large amount of data for a wide area of the prefecture.

**Author Contributions:** Conceptualization, M.W. and M.S; formal analysis, M.W.; resources, K.T. and H.S.; data curation, K.T. and H.S.; writing—original draft preparation, M.W.; writing—review and editing, M.W., M.S., K.T. and H.S.; funding acquisition, M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Japan Society for the Promotion of Science (JSPS) KAK-ENHI (Grant Number: 22H03800, 21K05665, 21H03672, 19H02991) by M.S.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data are not publicly available due to confidentiality reasons.

**Acknowledgments:** We would like to thank Nagano Prefecture and the municipalities in Nagano Prefecture for providing valuable data for this study.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


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