302 / 2024-02-29 12:02:08

Optimal Arrangement of Small-Scale Countermeasures in Densely Populated Areas against Combined Inundation Caused by Debris Flows and River Flood

numerical calculation,debris flow,river flood,multiple inflow,1-D and 2-D combined model

In Japan, the concurrent occurrence of debris flows and river flood has become more frequent owing to an increase in the duration and intensity of heavy rainfall caused by climate change. During these events, securing shelters and evacuation routes with a low risk of flood damage is difficult. In contrast, historically, damage caused by debris flows or river flood was mitigated through urban development strategies such as elevating buildings and erecting stone walls. These structures can be considered as disaster prevention countermeasures that coexisted within populated areas. However, these structures have been disappeared with the modernization of cities.

We investigated the arrangement of small-scale countermeasures to decrease the flood risk around shelters and evacuation routes during combined inundation by debris flows and river flood in a populated area adjacent to mountains and rivers. The study areas were the Izushi district of Hyogo Prefecture and the Chizu district of Tottori Prefecture, Japan. We considered a temporary road-closure facility, temporary levee, and temporary farmland or park reservoir as the small-scale countermeasures without damaging the existing urban functions and landscape. The simulated inundation results under various inflow conditions for the both target districts under existing conditions (without small-scale countermeasures) suggested that the inflows from the main- and sub-rivers have a small impact on flood damage in both districts, whereas debris flow inflows from mountain streams have a large impact on flood damage in both districts. In addition, the inundation area expanded along roads, affecting the inundation of the central districts, which was far from the inflow points of the debris flows. Thus, the numerical simulation of the combined inundation caused by river floods and debris flows on the digital topography considered the locations of houses and road network in the target areas revealed an uneven distribution of flood damage risk. Next, we conducted a numerical simulation of the combined inundation on the digital topographies installed small-scale countermeasures. The simulation results with the temporary road-closure facility eliminated flood damage in the center of the district, making it possible to evacuate local residents and tourists at these points. However, the temporary levee and temporary reservoirs failed to eliminate flood damage simulated under no-facility condition. These results suggest that the use of the countermeasures and drainage channels together with the existing road network and new culverts for the rapid discharge of floods and debris flows is effective in eliminating flood damage.