從水文空間關聯性探討地區減災責任──以鹽水溪流域為例

Exploring Flood Characteristics of Source and Impact Areas for Runoff Management - A Case of Yanshuei River Basin

隨氣候變遷極端氣候的加劇，以及災害風險不確定性與成本效益之考量，凸顯傳統防洪工程對耐災與減災的有限性，使得水土整合規劃與流域綜合治理成為韌性國土的重要理念，其中「逕流分擔」即為國土洪災調適的重要策略之一。逕流分擔係將超額逕流量轉由淹水潛勢地區負擔，然而在流域系統複雜的交互關係中，普遍認知河川地文與水體流動將暴雨逕流由上游帶至下游地區，以致多數都市地區屢見洪水積淹現象，此流域空間的連結性仍尚未被明確界定；且若淹水現象影響人類活動將構成洪水災害，在上、中、下游非均質發展的情形下，洪災責任在流域各地區間應如何分配以提升洪災治理效率實為一重要課題。
基此，本文透過流域空間關聯分析之研究設計，嘗試轉用投入產出概念於三維空間中，建立洪水逕流關聯矩陣，找尋流域各地區間地表逕流的互動關係，藉以界定洪水致災與受災區位特性，並納入「水文循環的公平性」與「洪災治理效率」兩觀點，考量都市發展程度與洪災暴露度，設計三種逕流分擔方案進行效率評估與比較，其中方案三以「致災比例」進行分配，將流域存在洪水積淹的空間不對等性及災害風險外部性等納入考量，較現行以「受災比例」的分擔原則更具減洪效率，同時也較具逕流責任分配的公平性。

Traditional projects of flood mitigation have been limited by the uncertainty of disaster risk and cost-effectiveness as climate change induces a rise in extreme rainfall events. Integrated flood management has become an important concept for resilience.“Runoff control”is the important strategy for reducing flood risk in the watershed.“Runoff control”is one of the strategies for flood adaptation, and the principle is that flood potential areas are responsible for excess runoff. Keeping the complex interaction of a basin in mind, we commonly recognized that most flooding in urban areas is due to the hydrologic connectivity of river flow bringing the ground runoff from upstream to downstream. However, the spatial connectivity in watersheds is still undefined. Therefore, it is a critical issue how to distribute the responsibility of runoff control to improve the disaster-mitigation efficiency in case of heterogeneity in a watershed.
This study applied the source-to-impact analysis to clarify runoff interaction between 39 regions in the Yanshuei River Basin and defined the spatial inequality of flood-induced and disaster-stricken areas, which demonstrated the external cost of flood risk. In terms of“the fairness of hydrological cycle”and“flood management efficiency”, this paper evaluated the flood protection efficiency of a different division of responsibilities in the watershed. The conclusions include (I) Empirically confirming the upstream is the main flood-induced region, and the downstream is the main disaster-affected region. (II) Assigning runoff responsibility by flood-induced ratio is a relatively acceptable principle for watershed management. The results aim to provide an implementation reference for the coordination of water and urban departments.