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

Abstract

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 () 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.