Quantifying the impact of landform modifications on flood hazards and exposure: Event-based numerical simulations using reconstructed historical terrain

Study Region: This study examines Marumori Town, Japan, situated within a 273.46 km 2 catchment basin drained by the Abukuma River. Over the past century, this historically flood-prone landscape underwent extensive anthropogenic modifications, with meandering channels straightened and confined by levees to support urban and agricultural development. Study Focus: This research quantitatively evaluates how historical landform modifications and urban expansion have altered rainfall–runoff behavior and flood exposure. A high-resolution hydrologic model simulated 35 actual rainfall events on both a reconstructed historical (1908) terrain and the present engineered landscape, alongside spatial analyses of changes in built-up flood exposure. New Hydrological Insights for the Region: Simulations reveal a rainfall- and terrain-dependent transition in flood response. During high-intensity events, modern levees reduce fluvial inundation by confining overbank flow. Conversely, under lower-intensity rainfall, these structures impede lateral floodplain drainage, trapping runoff and exacerbating localized pluvial flooding. Empirically derived intensity thresholds delineate this transition between pluvial- and fluvial-dominated regimes. Furthermore, structural measures modestly reduce overall catchment inundation and decrease mean built-up flood depth by 10%–15%. However, this incentivized a 78% expansion of urban development into active floodplains. Consequently, built-up flood exposure more than doubled across most events, demonstrating the “safe development paradox.” Finally, during extreme design-exceedance events like the 2019 Typhoon Hagibis, infrastructure benefits rapidly diminish, producing widespread inundation comparable to the unmodified historical landscape.