A Socio-Hydrological Framework for Nature-Based Stream Restoration: Integrating Engineering Safety and Social Narratives in Taiwan

Implementing Nature-based Solutions (NbS) in Taiwan’s steep, rapid-flow river environments faces a unique governance paradox where high-level policies encourage ecological restoration yet frontline engineering practices remain constrained by strict flood safety liabilities. This governance challenge is particularly pronounced in the Gangkou River watershed, a biologically rich region adjacent to the Kenting National Park at the southern tip of Taiwan. Notably, the demonstration site for this study was not unilaterally selected by the research team but was identified by local stakeholders advocating for restoration. The conflict between local demands for water recreation and flood safety versus rigid concrete check dam designs necessitates scientific mediation.

To echo the social inclusivity emphasized by NbS and promote its practical implementation in Taiwan, the key challenge lies not in the lack of hydraulic modeling tools but in the absence of a mechanism to translate qualitative Stakeholder Narratives into adaptive engineering language to assist in assessing the multiple benefits of NbS in disaster prevention and ecology. Therefore, this study proposes a Socio-Hydrological Framework integrating Participatory Modeling (PM).

Drawing on the bi-directional translation and iterative spirit of the Story-And-Simulation (SAS) approach, we attempt to establish an intuitive process for mapping qualitative needs to adaptive schemes and utilize two-dimensional hydraulic simulation as the quantitative calculation method. First, through Upward Translation, key needs proactively raised by locals (including biological, security, and cultural demands) are directly mapped and translated into model parameter settings for NbS adaptive planning scenarios. Next, through simulation calculations, Downward Translation converts physical data into visualized trade-off indicators (such as Habitat Suitability and Flood Risk maps) which are fed back to stakeholders for Social Iteration.

Through this systematic translation process, the study aims to establish a bottom-up, iterative decision-making pathway that supports community consensus-building and provides a scientific reference for advancing Nature-based Solutions in ecologically sensitive and high-conflict river environments in Taiwan.

This framework was applied to the Ba-Yao Bridge reach to evaluate three scenarios: Baseline, Ecological-oriented (Full Removal), and Integrated NbS (70% Height Reduction with Regraded Banks). Preliminary simulation results indicate that the Integrated NbS Scenario demonstrates typical advantages of nature-based solutions, generating significant hydraulic synergy: lowering the existing check dam height by approximately 70% increased flow velocity while effectively reducing local water levels. This hydraulic margin, facilitated by feasibility discussions with right-bank landowners regarding regraded banks, allowed for the design of vegetated slopes that enhance longitudinal connectivity without causing flood risks for adjacent farmlands to exceed safety thresholds. Furthermore, the design reduced flow disconnection time by nearly 80% during low-flow periods, effectively addressing stakeholder concerns regarding eutrophication, shallow water depths, and the desire to restore childhood memories of water accessibility.

Ultimately, this study preliminarily validates that the conflict between flood safety and ecological restoration often stems from rigid engineering constraints. By systematically translating social values into NbS design parameters and aligning with local policies like 'Sediment-Water Inundation Zones' and regraded embankments, we establish a bottom-up iterative decision-making process to provide a solid scientific foundation for promoting Nature-based Solutions in high-conflict, ecologically sensitive areas.