洪水風險頻率分析模型及堤防效益之分析探討

Abstract

摘要 台灣四面環海、地形變化大，且地理位置位於熱帶、副熱帶季風氣候區交界處，年雨量以及瞬間降雨強度都屬世界之最，再加上眾多的人口及高度的經濟發展程度，使得洪災來臨時，經常產生巨大的洪災損失。為了避免洪災事件的發生，經常以修築堤防系統作為主要的防災的手段。 而在以往堤防系統設計之初，多以定常性的風險概念為基礎來決定保護標準，並未考慮到氣候變遷導致極端水文事件頻率增加，以及堤防保護對社會經濟、人口發展之影響，有可能造成堤防系統所提供的保護越好，卻在洪災發生時，造成更多的洪災損失之情形發生。也因為如此，從前以風險效益分析，來考量堤防系統的經濟效益最佳狀況，有可能產生參數評估不符合實際狀況的失誤，導致決策的結果不佳。 本研究提出一套考慮了堤防效應以及環境變遷，並以卜松隨機損失為基礎的堤防系統檢視方法：以分析設計回歸週期( )來劃分堤防系統發揮效用的時間區間，以保護標準回歸週期( )來進行堤防保護程度的折減與修正，以定常性的替代風險( )簡化非定常性現象對洪災的影響，最後再以提高堤防保護的邊際防洪效益( )來判斷保護標準變動所帶來的影響。應用以上方法，期望能夠修正以往在期望平均損失概念，應用在堤防系統設計、分析中的可能錯誤，避免堤防保護的加強反而造成額外更多洪災潛在損失的狀況發生，並且使堤防系統的設計與分析能夠更接近符合狀況，以最佳的成本發揮最大的效益。 關鍵字：堤防效應、氣候變遷、堤防系統分析、回歸週期

Abstract Taiwan is in the subtropical and tropical monsoon climate zone with heavy precipitation. As a highly developed country with large population, flooding is always causes serious economic losses. In order to minimize impact, levee systems are a common implementation used to protect to cities. Engineers usually design levees based on concepts of risk assessment, but the non-stationarity has not been considered before. Under nonstationary condition, risk of extreme hydrological events increases and continuous social development could also reduce the effectiveness of the flood prevention system. For this reason, previous flood control projects based on risk-benefit analysis may be inaccurate in describing flood events, resulting in poor decision making by the management. This study re-examine the frequency analysis methods and risk model based on the Poisson random process, while taking into account both climate change and levee effect. The time interval of levee benefit can be determined by using the “analysis return period (Td)” while the reduction level of protection standard can be found using the “protection standard return period (Ts)”. The phenomena of unsteady condition are simplified using “substitute risk (Psus)”, and finally, marginal benefits of levee protection are used to determine the effects of variation in protection standard. Applying these methods, this study aims to revise past methods that are based on expected average loss to prevent situations where an increase of levee protection actually causes higher losses in an event of a flood, and most importantly, to make the design of levee systems more in line with reality and provides maximum benefit at a minimum cost. Key words: Levee effect, Climate change, Levee protecting system analysis, Return period