Nature-Based Solutions (NBS) are increasingly promoted for river restoration and flood risk management. However, most studies focus on large-scale interventions such as floodplain reconnection or channel remeandering, which often necessitate extensive land acquisition and long implementation periods. The hydraulic effectiveness of smaller, reach-scale NBS measures in confined torrential rivers remains poorly quantified. This study quantifies the hydraulic response of localized NBS-based restoration measures in the Yoshio River, a tributary of the Kuma River in Kumamoto Prefecture, Japan. Using the two-dimensional hydraulic model iRIC Nays2DH, unsteady flow simulations were conducted for baseline conditions and four restoration scenarios: channel widening, widening with increased roughness, widening with spur dikes, and widening combined with riparian vegetation. Results show reductions in water surface elevation of up to 1 m at certain sections. Water levels were lower than baseline conditions along 98% of the reach under the widening scenario and 57% under the vegetation scenario. Peak travel time increased by approximately 25–75%, with vegetation producing the largest delay. Vegetation-based restoration also increased spatial flow variability compared with other scenarios. Simulated water levels showed strong agreement (<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(R^2 = 0.89\)</EquationSource> </InlineEquation>) with results from a previously calibrated hydrodynamic model of the same river. These findings indicate that localized, reach-scale NBS measures can measurably reduce flood stages and delay peak propagation in confined tributary systems, providing quantitative evidence for their hydraulic effectiveness in climate-adaptive river management. Graphical Abstract This graphical abstract summarizes the conceptual framework, study area, modeling approach, and key findings of this study, which moves beyond large-scale river restoration to evaluate reach-scale nature-based solutions in the Yoshio River (a tributary of the Kuma River system, Japan). Post-flood channel geometry was used as the baseline condition, and flood propagation was simulated under unsteady flow using the two-dimensional hydraulic model iRIC Nays2DH. Multiple restoration scenarios were examined, including channel widening alone and channel widening combined with upstream roughness enhancement, spur-dike installation, and riparian vegetation. The results demonstrate that, contrary to conventional expectations, localized and small-scale restoration measures can significantly reduce flood water levels and peak discharge while delaying flood peak timing. Vegetation-based and upstream roughness–based interventions provide the greatest attenuation, whereas spur-dike–based restoration exhibits comparatively lower effects. Overall, the findings indicate that locally feasible, reach-scale nature-based solutions can contribute to flood-risk reduction during extreme events, even in steep and spatially confined river reaches.

Nature-based Solutions at the Reach Scale: A Case Study of Flood Mitigation in the Yoshio River

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2026年6月30日

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Sayed Hashmat Sadat,
Yuichi Kayaba

摘要

Nature-Based Solutions (NBS) are increasingly promoted for river restoration and flood risk management. However, most studies focus on large-scale interventions such as floodplain reconnection or channel remeandering, which often necessitate extensive land acquisition and long implementation periods. The hydraulic effectiveness of smaller, reach-scale NBS measures in confined torrential rivers remains poorly quantified. This study quantifies the hydraulic response of localized NBS-based restoration measures in the Yoshio River, a tributary of the Kuma River in Kumamoto Prefecture, Japan. Using the two-dimensional hydraulic model iRIC Nays2DH, unsteady flow simulations were conducted for baseline conditions and four restoration scenarios: channel widening, widening with increased roughness, widening with spur dikes, and widening combined with riparian vegetation. Results show reductions in water surface elevation of up to 1 m at certain sections. Water levels were lower than baseline conditions along 98% of the reach under the widening scenario and 57% under the vegetation scenario. Peak travel time increased by approximately 25–75%, with vegetation producing the largest delay. Vegetation-based restoration also increased spatial flow variability compared with other scenarios. Simulated water levels showed strong agreement ( \(R^2 = 0.89\) ) with results from a previously calibrated hydrodynamic model of the same river. These findings indicate that localized, reach-scale NBS measures can measurably reduce flood stages and delay peak propagation in confined tributary systems, providing quantitative evidence for their hydraulic effectiveness in climate-adaptive river management.

Graphical Abstract

This graphical abstract summarizes the conceptual framework, study area, modeling approach, and key findings of this study, which moves beyond large-scale river restoration to evaluate reach-scale nature-based solutions in the Yoshio River (a tributary of the Kuma River system, Japan). Post-flood channel geometry was used as the baseline condition, and flood propagation was simulated under unsteady flow using the two-dimensional hydraulic model iRIC Nays2DH. Multiple restoration scenarios were examined, including channel widening alone and channel widening combined with upstream roughness enhancement, spur-dike installation, and riparian vegetation. The results demonstrate that, contrary to conventional expectations, localized and small-scale restoration measures can significantly reduce flood water levels and peak discharge while delaying flood peak timing. Vegetation-based and upstream roughness–based interventions provide the greatest attenuation, whereas spur-dike–based restoration exhibits comparatively lower effects. Overall, the findings indicate that locally feasible, reach-scale nature-based solutions can contribute to flood-risk reduction during extreme events, even in steep and spatially confined river reaches.