Although the concept of green infrastructure co-benefits is widely recognized, few studies document how transdisciplinary governance mechanisms sustain multipurpose outcomes over several decades. This paper examines how sustained collaborative governance enabled the Asahata Reservoir in Shizuoka City, Japan, to transition from conventional flood infrastructure to multipurpose green infrastructure, providing environmental, social, and cultural benefits over four decades. Using project documents, government reports, and field visits, the study analyzes the governance arrangements and institutional mechanisms that maintained stakeholder engagement and achieved measurable outcomes in flood protection, biodiversity conservation, and social inclusion. The findings show that the transdisciplinary governance platform, the Nature Restoration Council, established in 2004, effectively coordinated multiple stakeholders, including government agencies, civil society organizations, healthcare institutions, and community groups, to support integrated management. The reservoir has maintained its flood storage capacity while supporting 214 bird species and 21 endangered plant species and providing vocational training and therapeutic programs for people with disabilities and older adults. This long-term collaborative approach created complementary relationships among flood protection, ecological restoration, and social services. The case study advances theoretical understanding of transdisciplinary governance by presenting concrete institutional mechanisms for sustained stakeholder coordination in managing multipurpose infrastructure. It offers policymakers transferable lessons for operationalizing collaborative approaches: establish stable multi-stakeholder platforms early in project development, combine public funding with community contributions to support long-term sustainability, and design adaptive management systems that address changing urban needs. The findings contribute to global discussions on aligning disaster risk reduction, climate adaptation, and social inclusion through innovative governance of green infrastructure.

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