Physical Principles of River Basin Flood Control: Decoding Hydrograph Deformation and Active Flow Management

As climate change intensifies extreme rainfall, Nature-based Solutions (NbS) have gained prominence, yet engineering frameworks remain largely empirical and underdeveloped. We propose ten fundamental physical principles of river-basin flood control, grouped into runoff control and inundation-flow control: cover, root reinforcement, infiltration, evaporation and interception, storage, resistance, reverse flow and dispersion, filtering, deflection, and blocking, and we clarify their mechanisms, representative cases, and engineering significance within co-created, basin-wide risk reduction. Analyses of the 2020 Kuma River flood, complemented by evidence from the 2019 East Japan Typhoon (Hagibis) in the Zenpukuji River basin, indicate that the basin-scale runoff-regulating capacity is maintained even under record-breaking rainfall, with no evidence of reaching a plateau. To quantify this behavior, we define the spatio-temporal effective rainwater density (\(\:{\rho\:}_{ster}={V}_{e}/\:\left(A{\bullet\:T}_{fd}\right)\)), where \(\:{V}_{e}\) is the time-integrated effective rainfall, \(\:A\:i\)s the catchment area, and \(\:{T}_{fd}\) is the flood duration. On average, peak discharge tends to be lower as \(\:{\rho\:}_{ster}\) decreases; this tendency is probabilistic. Because \(\:{V}_{e}\) is an integral, lowering \(\:{\rho\:}_{ster}\) hinges on enhancing basin loss processes (infiltration, interception, evaporation) and extending \(\:{T}_{fd}\) through physical delay, providing a physical definition of “slowing the flow” and grounding hydrograph deformation in first principles. While modulating \(\:{\rho\:}_{ster}\) reduces peaks on average, it cannot completely prevent flooding under climate change. Therefore, inundation-flow control technologies that dissipate the kinetic energy of floodwaters and conveyed materials—through resistance, reverse flow and dispersion, filtering, deflection, and blocking—are essential, and their effective implementation at scale requires co-creation, a bottom-up process that builds social consensus and pragmatically redefines river-system design boundaries.