This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: As climate change is expected to increase the risks associated with extreme rainfall and flooding, managing floods requires approaches beyond conventional infrastructure such as dams and reservoirs. Recognizing this, Japan's Ministry of Land, Infrastructure, Transport and Tourism introduced the "River Basin Disaster Resilience and Sustainability by All" (RBDRSA) project, which takes a comprehensive approach to flood management across the entire river basin.
Implementing this basin-wide approach requires accurate modeling of water movement through the watershed. Small drainage channels can carry rainwater toward larger rivers, yet their role in mountainous and forested areas remains poorly understood. In these regions, drainage networks can extend far upstream, making it important to determine how far upstream the network needs to be represented for reliable predictions.
Accurately representing these networks could improve flood simulations and contribute to more reliable flood forecasts, providing useful information to support evacuation decisions. However, the present study did not directly evaluate early warning systems or evacuation decisions. To answer this question, researchers led by professor Yasuo Nihei from the Department of Civil Engineering at Tokyo University of Science (TUS), Japan, used data from a real flood in the Ichinomiya River Basin, where drainage channels extend into mountainous areas.
The team included Ryutaro Otsuka, a TUS alumnus who completed his master's degree in 2025, and Kengo Ohno, a second-year master's student. They simulated rainfall runoff, river flow and flooding under different drainage channel configurations and assessed how these configurations affected water levels, discharge and the basin's overall water balance. This paper was published in Journal of Hydrology: Regional Studies.
"In typical large-scale flood analyses, rivers are primarily modeled explicitly, while the influence of smaller channels and drainage pathways upstream is often represented indirectly through runoff parameters. However, since channels play a crucial role in draining rainwater into rivers, we believed that directly reflecting them in the model would allow us to reproduce water movement within the basin more closely to real-world conditions while reducing reliance on parameter adjustments," says Nihei. The Ichinomiya River Basin is a 203 km² (78 square miles) basin in Chiba Prefecture, Japan, that has experienced typhoon-related flooding.
Following severe flooding caused by Typhoon Bualoi in 2019, flood-management measures were developed for the basin under the RBDRSA project. In 2023, the basin experienced another flood caused by Typhoon Yun-yeung while these measures were still being implemented. The researchers modeled the 2023 flood to investigate how far upstream drainage channels need to be represented in a flood model.
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