Hisayoshi NONOGAKI

Elucidation of River Conditions during the Washout of the Old Nekohana Bridge on the Hime River Using Probabilistic Rainfall and River Inundation Simulation

Minjiao LU

The Hime River is a high-gradient Class A river flowing along the Itoigawa-Shizuoka Tectonic Line, a region historically prone to frequent hydrological disasters. Following the Meiji Restoration, the regional transportation network was modernized, replacing the mountainous Kuzuha Pass route with a new road along the Hime River to facilitate vehicular traffic. However, the new route on the east bank proved extremely vulnerable to natural hazards. In particular, the Nekohana Bridge suffered repeated washouts, eventually necessitating a return to the inland pass. Understanding the correlation between historical infrastructure and meteorological forcing is essential for regional disaster history; however, historical precipitation data for that period is unavailable. Therefore, this study quantitatively reconstructs the rainfall scales and river conditions during bridge washouts by integrating basin-specific probable precipitation with historical literature.
The methodology comprises three stages: field/literature surveys, precipitation estimation, and hydrodynamic simulations. Topographic conditions were validated through on-site inspections to align historical events with physical terrain features. Probable precipitation was calculated using the Nonomura method to reflect basin characteristics, from which two types of hydrographs—"short-duration concentrated" and "average"—were derived. Inundation simulations were performed using the iRIC framework, with upstream boundary conditions established via the Rational Method to optimize computational efficiency.
The analysis revealed a long-term increasing trend in hourly maximum precipitation within the basin. Simulations using a 2-year return period (44 mm/h hourly max; 125 mm/day total) demonstrated that short-duration concentrated rainfall produced a maximum water depth of 30.60 m at the Nekohana Bridge site, exceeding the defined structural failure threshold. Consequently, the frequent route changes of National Route 148 were attributed to the inherent vulnerability of the era's infrastructure to localized heavy rains occurring with a recurrence interval of approximately two years.