The research team, led by Xiaoyang Li of the University of Tokyo’s Institute of Industrial Science together with Kei Yoshimura and Hironori Fudeyasu of Yokohama National University, built a national-scale impact-based flood-damage assessment framework around the Integrated Land Simulator, or ILS. This platform couples two established models: MATSIRO, a physical land surface model that handles soil, canopy, and runoff processes, and CaMa-Flood, a hydrodynamic river routing model that simulates floodplain inundation using the Stabilized Local Inertial Equation, an adaptive time step scheme, and an explicit levee and reservoir scheme. The modeling chain was anchored in high-resolution topography from the MERIT digital elevation model, allowing inundation depths computed at roughly 1.5-kilometer resolution to be downscaled to about 30 meters, fine enough to resolve individual buildings and neighborhoods.

The atmospheric side of the experiment relied on the Weather Research and Forecasting Model, version 4.3.3, run in a one-way nested configuration with an outer domain at 15-kilometer resolution and an inner domain at 5 kilometers, both covering the Japanese archipelago and surrounding seas with 48 vertical layers. Initial and boundary conditions came from the Japanese 55-year Reanalysis, and the physics suite included the RRTMG radiation schemes, the WSM6 cloud microphysics parameterization, the Kain-Fritsch cumulus scheme, and the Yonsei University planetary boundary layer scheme. The crucial twist was the perturbation method: the researchers systematically shifted the reanalysis fields longitudinally in increments of 0.2 degrees, spanning 11 degrees westward to 5 degrees eastward. In total, 81 plausible versions of Typhoon Hagibis were simulated, each following a slightly different east-west path across Japan.

Before running these scenarios, the team validated the control simulation against observations. The WRF control run reproduced the spatial distribution and intensity of accumulated rainfall recorded by the Japan Meteorological Agency’s radar network during the storm’s passage, with heavy rainfall concentrated in central Japan and along the Pacific coast, exactly as observed. Downstream, the ILS control simulation matched observed maximum river discharge and river water levels from the ministry’s gauging network remarkably well, with correlation coefficients of 0.90 for discharge and 0.80 for water level. The validation also revealed something important: even the small westward and eastward biases in the simulated storm track produced corresponding shifts in simulated rainfall and river response, a first hint of how sensitive the entire flood system is to the precise position of the typhoon.

The headline finding is stark. A longitudinal shift of just 0.2 degrees, roughly 20 kilometers at Japan’s latitude, produced an average difference in total flood damage of 83.3 billion yen between adjacent scenarios. The response was highly nonlinear. In one dramatic case, moving the storm from the w012 to the w014 scenario, a shift of only 0.2 degrees westward, increased national damage by 956.8 billion yen, because the altered track steered intense rainfall and severe inundation directly onto the densely populated Osaka and Nagoya metropolitan areas. Across the full ensemble, total damage generally rose as the storm moved eastward from the far-west scenarios, with two pronounced peaks: 1,966.3 billion yen in the e010 simulation and 1,522.6 billion yen in the w014 simulation, after which damage fell off rapidly.

The geography of worst-case tracks differs sharply by region, and that is where the study becomes a planning tool rather than a curiosity. The single most damaging scenario for Japan as a whole, e010, placed the typhoon’s landfall over the Kanto Plain rather than the mountainous interior, unleashing intense rainfall and widespread inundation across greater Tokyo, the country’s most populous metropolitan area. In that scenario, Kanto alone suffered 1,571.7 billion yen in damage. For the Kinki region around Osaka, the worst track was w016, producing 911.4 billion yen, while for Chubu around Nagoya it was w004, producing 642.2 billion yen. In the control run, total damage reached about 1,181.1 billion yen, with 724.1 billion yen concentrated in Kanto. A modest 0.2-degree westward shift cut the national total to 897.9 billion yen but multiplied Kinki’s damage more than eightyfold, from 9.3 billion to 752.5 billion yen, as the rainfall belt migrated toward Osaka, Chugoku, and Shikoku.

Most of the simulated damage, like most of the real damage, occurred in floodplain areas along the Pacific coast of Kanto, Chubu, Kinki, Shikoku, and Tohoku, with Kyushu’s damage concentrated instead along the Japan Sea coast. This pattern reflects the fact that 96.8 percent of Hagibis’s actual flood damage came from inland river overflows rather than coastal storm surge, which justified the study’s focus on riverine flooding. Because these floodplains coincide with Japan’s highest population densities, the authors argue that enhanced flood protection infrastructure and impact-based early warning systems are urgently needed there. The framework also quantified the value of hard infrastructure: in levee-height sensitivity experiments ranging from a 20 percent decrease to a 20 percent increase, total damage fell from 1,189.1 billion yen to 1,152.9 billion yen, an average reduction of about 9.1 billion yen for every 10 percent increase in levee height.

Technically, the damage estimates were built from the ground up. Building areas came from the global three-dimensional building footprint dataset, and asset values per unit floor area were drawn from Japan’s 2018 Housing and Land Survey, set at 37.05 thousand yen per square meter for structural assets and 91.78 thousand yen per square meter for contents. Downscaled inundation depths were converted into damage rates using the empirical depth-damage relationships in the ministry’s Manual for Economic Evaluation of Flood Control Investment. The authors are careful to note the limits: land values and indirect losses such as supply-chain disruptions, business interruptions, and long-term recovery costs were excluded, so the figures represent a lower bound on the true economic impact. The study is also explicitly retrospective and scenario-based, not an operational real-time forecasting system for evacuation decisions.

Even so, the implications reach well beyond one storm. The researchers point out that forecast errors of a similar magnitude actually occurred with Hagibis, with a notable westward bias in track forecasts roughly three days before landfall, meaning the kind of uncertainty explored here is not hypothetical. As the climate warms and the hydrological cycle accelerates, extreme rainfall events across East Asia are expected to intensify, while aging populations in Japan, China, and South Korea make evacuation harder; 77 percent of Hagibis’s victims, 67 of 87 people, were aged 60 or older. By translating small meteorological uncertainties into concrete yen-denominated damage figures for specific regions and cities, the framework gives decision-makers a way to weigh investments in levees, warning systems, and preparedness against the plausible worst cases. It also fills a genuine gap: national-scale impact-based flood forecasting has been demonstrated in East Africa and the Philippines, but never before in East Asia, and the authors suggest this integrated approach of atmospheric modeling, hydrological simulation, and socioeconomic exposure data could become a template for flood-risk management across the region.

Subject of Research: Sensitivity of national-scale flood damage in Japan to typhoon-track shifts and levee height during Typhoon Hagibis (2019)

Article Title: Impact-based flood-damage assessment in Japan under longitudinal typhoon-track perturbations and levee-height variations during Hagibis (2019)

Article References: Impact-based flood-damage assessment in Japan under longitudinal typhoon-track perturbations and levee-height variations during Hagibis (2019). (n.d.). https://doi.org/10.1038/s43247-026-04077-8

Image Credits: AI Generated

DOI: 10.1038/s43247-026-04077-8

Keywords: Typhoon Hagibis, flood damage, Japan, typhoon track, levees, impact-based forecasting, hydrology, inundation modeling, WRF, disaster risk, Kanto, climate extremes

The study, conducted by researchers at the International Research Institute of Disaster Science (IRIDeS) at Tohoku University together with colleagues at Meijo University, was published in the Journal of Hydrology: Regional Studies on September 14, 2026. The work was carried out under a comprehensive partnership agreement between the two institutions. Its central question was deceptively simple: has the flood-prevention infrastructure built over the past century actually made the residents of Marumori Town safer? Answering that question rigorously, however, required the team to overcome a long-standing methodological obstacle in flood hazard research.

The difficulty lies in the fact that multiple drivers of flood risk have changed simultaneously over the past hundred years. Rainfall patterns have shifted, river channels have been modified with embankments and other structures, and urban growth has transformed the landscape. Disentangling the contribution of man-made structural changes from the effects of a changing climate and expanding settlements is extremely challenging with observational data alone. To isolate the specific impact of engineered modifications to the terrain, the researchers turned to historical cartography. By digitizing and analyzing old maps from the Meiji era, they reconstructed the original, largely untouched natural landscape of the region as it existed before modern river engineering began, and used this as a natural baseline for comparison.

“By looking back at conditions in the Meiji era as a natural baseline, we can use models to determine whether the flood prevention infrastructure we installed is helping,” says Nilo Lemuel J. Dolojan of IRIDeS. “Surprisingly, we discovered that these ‘upgrades’ might have made things worse in some cases.” The simulations compared flood behavior across two virtual worlds: one built on the reconstructed historical terrain and one representing the present-day landscape with its modern levees and modified channels. Rainfall scenarios were then run through both versions of the catchment, allowing the team to measure precisely how the engineered changes altered flood volumes, depths, and the extent of areas exposed to inundation.

The results revealed a pattern that may sound counterintuitive but is well grounded in hydraulics. Levees are walls constructed along rivers to prevent swollen channels from spilling over their banks during heavy rain and extreme weather. During intense storms that produce widespread flooding, the simulations showed that flood volumes did indeed decrease in the modern landscape, confirming that the levees perform their intended function when water levels are high. But during lighter rainfall events, when flooding would historically have been scarce or mild, the very walls designed to protect communities trapped rainwater inside the neighborhoods, preventing it from spreading across the wider floodplain and draining away naturally. The consequence was worse inland flooding today than occurred in the same area more than a century ago under comparable light-rain conditions.

Further analysis uncovered something even more valuable for planners: a tipping point. The researchers found that there is a threshold beyond which modern terrain modifications transition from mitigating flood hazards to exacerbating them. Below that threshold, the engineered landscape reduces flood depths and volumes; above it, the same modifications intensify the hazard. The precise location of this tipping point varies for each body of water, depending on channel geometry, catchment characteristics, and the scale of the modification. Identifying where that threshold lies, the authors argue, should be a key step in deciding whether flood-prevention infrastructure is genuinely necessary for a given reach of river, or whether it may inadvertently create new hazards during more common, moderate rainfall events.

“In general, the levees are still effective during heavy rainfall. But even then, we still found exceptions,” says Shuji Moriguchi of IRIDeS. “During the unexpectedly strong 2019 Typhoon Hagibis, modern flood-control structures were completely overwhelmed.” Typhoon Hagibis, which struck eastern Japan in October 2019, produced rainfall totals that exceeded the design capacity of many flood defenses across the country, and the simulations for Marumori Town confirmed that the modern structures could not contain the extreme flows. The lesson is sobering: levees cannot keep communities completely safe under every weather condition, and their apparent reliability can itself become a source of risk when it encourages development in exposed locations.

That is precisely where the Safe Development Paradox becomes visible in the data. Although the levees were not failproof, the researchers found that nearby communities had behaved as if they were. By comparing the built-up area distribution recorded in the Meiji-era maps with present-day settlement patterns, the team documented a striking 110 percent growth in settlement area exposed to flooding. In other words, over the course of a century of infrastructure development, the amount of human development situated in locations that could be inundated more than doubled. The protective structures reduced the frequency of flooding, but the resulting confidence drew development into the floodplain, so that when defenses are exceeded, far more people and property stand in the path of the water than would have been the case in the historical landscape.

The significance of the study extends well beyond a single town in Miyagi Prefecture. Because the modeling framework separates the effects of man-made terrain modifications from the effects of climate change and urban growth, it allows for quantitative verification of the Safe Development Paradox rather than relying on anecdotal or correlational evidence. The same approach, combining reconstructed historical terrain with event-based numerical simulations, can be applied by researchers and planners in other river basins to evaluate whether existing or proposed flood defenses are delivering net benefits across the full spectrum of rainfall intensities. The method offers a way to test infrastructure decisions against the landscape that existed before engineering, providing a baseline that is often missing from modern hazard assessments.

The researchers hope to continue refining their model, and they emphasize that the ultimate goal is practical: to spark informed conversation about urban planning that reduces hazard exposure and keeps people safe. The findings suggest that flood management should not be judged solely by how it performs during catastrophic storms, but also by how it reshapes both the hydrology of moderate events and the behavior of the communities it is meant to protect. As climate change alters rainfall patterns and populations continue to concentrate in river valleys, understanding the full consequences of terrain modification, including the unintended ones documented in Marumori Town, will be essential for designing flood-risk strategies that do not quietly manufacture the disasters they were built to prevent.

Subject of Research: Quantifying how historical landform modifications, rainfall, terrain, and urban growth affect flood hazards and exposure

Article Title: Past vs. modern flood risk: Impacts of rainfall, terrain, and urban growth

Article References: Past vs. modern flood risk: Impacts of rainfall, terrain, and urban growth. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: flood risk, levees, Safe Development Paradox, flood simulation, urban growth, terrain reconstruction, Meiji era, Marumori Town, Miyagi Prefecture, Typhoon Hagibis, Tohoku University, Journal of Hydrology