磺溪流域洪水治理自然解方可行性評估

Abstract

現今人類環境與社會面臨日益複雜的挑戰，在水利工程領域，洪水治理之觀念逐漸迎來典範轉移（paradigm shift），由採用灰色基礎設施之傳統工程解方轉向多目標、更為永續、以生態為基礎的自然解方（nature-based solutions）。這個轉變不僅關乎個別流域的洪水風險，亦對全球氣候變遷的適應性產生影響。然而，儘管NbS的潛力已被廣泛討論，對於其具體應用和評估仍缺乏明確架構。本研究的主要目的在於建立通用且可靠的NbS導入與評估流程，並實際應用於磺溪流域，以評估自然解方在降低洪水風險、其它生態系服務價值等方面的效果，填補NbS缺乏系統性方法的空白。 為實現上述目的，本研究參考2020年IUCN發布的NbS全球標準，建立一個五階段的NbS導入流程以及工作細項。本研究以磺溪流域作為案例，盤點背景資料，結合利害關係人之意見訂定改善指標，並設計具體的濕地方案。運用多種工具，包括HEC-RAS、InVEST和自適應克利金可靠度分析，來評估基線與NbS方案之間的差異並描述NbS效益。最後，將可靠度分析成果轉化為工程師慣用的安全係數，考慮氣候和地理條件下的不確定性因素，科學地量化自然解方在特定防洪標準下之安全性。 效益評估的主要發現包括：濕地設計方案在50年重現期洪水情境下能夠減少磺溪下游土地淹水面積約9.86%，在2年重現期洪水情境下減少約3.95%；規劃濕地設計方案將提高磺溪舊河道之碳儲存量75.74%並減少實際土壤侵蝕量50.77%，棲地適合度將保持在相似水平；濕地設計方案有助於減低洪水風險，淹水發生機率降至3%，等同於安全係數1.37。 此外，本研究嘗試將IUCN的NbS自我評估工具應用於濕地設計方案，總分為30分（部分相符）。結果顯示，濕地設計方案在應對社會挑戰及基於包容、透明和賦權的治理過程方面具有優勢，但在經濟可行性、公平與平衡地權衡、適應性管理和主流化等標準方面尚未達到要求。自我評估工具有助於辨識行動方案與NbS全球標準的相符程度、釐清關鍵問題，並提供具體改善方向。 本研究建立標準的NbS導入及評估流程，並綜合評估NbS在不同方面的效果，作為NbS應用的示範案例，為水利工程領域的研究和實務提供重要支持和參考。

In response to the increasingly complex challenges faced by our environment and society, there's a paradigm shift in flood management practices within the field of hydraulic engineering. Traditional approaches using gray infrastructure are giving way to Nature-based Solutions (NbS), which prioritize sustainability and ecosystem-based approaches. Despite widespread discussions about NbS potential, there's a lack of a clear framework for its application and evaluation. The main objective of this study is to establish a universal and reliable process for the implementation and assessment of NbS, applied practically in the Huang River Watershed. This aims to evaluate the effectiveness of NbS in aspects such as flood risk and the value of ecosystem services, addressing the existing gap in systematic methods for NbS. By referencing the 2020 global NbS standards from the IUCN, the study creates a five-stage process and tasks for NbS implementation. To evaluate the effectiveness of the implemented measures, several methods such as hydraulic analysis using HEC-RAS 2D, ecosystem service assessment via Integrate Valuation of Ecosystem Services and Tradeoffs (InVEST), and risk analysis through reliability analysis are adopted. The key findings of the benefit assessment include: the designed wetland can reduce the flooded area in the downstream of Huang River by 9.86% for 1 in 50 year flood event and by 3.95% for 1 in 2 year flood event; the designed wetland will increase carbon storage in the old river channel of Huang River by 75.74% and reduce actual soil erosion by 50.77%, while habitat quality will be maintained at a similar level; the designed wetland contributes to lowering flood risk, reducing the probability of flooding to 3%, equivalent to a safety factor of 1.37. Additionally, this study applies the IUCN’s NbS self-assessment tool to the designed wetland, resulting in a total score of 30 (Partial). The results indicate that the designed wetland demonstrates strengths in addressing societal challenges and governance processes based on inclusiveness, transparency, and empowerment. However, it falls short in meeting the standards for economic feasibility, fair and balanced trade-offs, adaptive management, and mainstreaming. The self-assessment tool could help in identifying the alignment of the action plan with the NbS global standards, clarifying key issues, and providing specific directions for improvement. In summary, this study serves as a demonstration case for NbS application, providing a clear process for NbS implementation It comprehensively evaluates the effects of NbS across various dimensions. The outcomes of this study provide valuable support and reference for research and practices in the field of hydraulic engineering.