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

This study examines Marumori Town, Japan, situated within a 273.46 km2 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. This research quantitatively evaluates how historical landform modifications and urban expansion have altered rainfall–runoff behaviour 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. 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 localised 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 incentivised 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.