氣候變遷與地層下陷對台灣西南沿海地區淹水之衝擊評估

Abstract

近年來由於全球溫室效應的影響，造成地球表面溫度的升高，導致了異常的降雨變異，亦引發全球平均海水面的上升。此類氣候變遷對台灣所衍生的問題相當廣泛，以淹水災害為例，台灣具有特殊之地理位置、地形與天然氣候，原本就是高淹水潛勢地區，尤其是河川下游沿岸與沿海低窪地帶，而這些地區又恰好是人口聚集之所在，在在增加了淹水災情。有鑑於過去淹水災害之嚴重，台灣在1970年代起開始規劃興建許多重大防洪設施，其初期治理成果效益相當顯著，但當年的工程設計並未考量氣候變遷之因素，若加入氣候變遷所增大之極端降雨事件規模及愈形惡化的海水位極端變異程度，淹水災情必更形嚴重，其中又以深受地層下陷之苦的西南沿海低窪地區為最。因此，本研究以台灣西南沿海低窪地區之嘉義縣荷苞嶼排水系統為研究地區，蒐集氣象、水文、地文及水利設施資料，統合該地區歷史重大颱洪、豪大雨之災害損失統計與風險特性，研擬出一套以一維河川變量流模式與二維地表漫地流淹水模式為主體，其能掌握沿海低窪地區之水利特性，又能納入氣候變遷極端變異特性的評估方法，進行研究地區受極端降雨及海水位變異影響之淹水境況模擬。本研究並同時檢討氣候變遷對現階段台灣水利主管機關正在執行的「易淹水地區水患治理計畫(八年八百億)」的衝擊影響，探討其保護標準之差異。 在氣候變遷預設情境中，本研究以三種不同GCMs模式所模擬而得之降雨量結果之趨勢並不一致，有增加之情形，但亦有減少之情形，故仍有其不確定性存在，尚需進一步研究。而以不同GCMs模式所模擬而得之氣候變遷預設情境海水位結果發現，各模式間之變化趨勢較為相似，其趨勢均為上升，且其上升幅度之差異性不大。本研究以降雨變異與海水位變化均為增加之HADCM3模式A2情境為例，若同時考量未來10年地層下陷情境與氣候變遷引致極端降雨變化及海水位變化對水患治理計畫之衝擊結果發現，在短期與中期境況下會發生嚴重溢堤現象，而在長期境況下會有非常嚴重溢堤之現象發生，嚴重的威脅人民生命財產安全。

Recently, it has been confirmed that climate change has induced sea level rises and extreme rainfalls, resulting in more serious flood and inundation disasters, especially in the coastal lowlands in Taiwan. Climate change has great impact on the safety of the hydraulic infrastructure, which was built from 1970s. The main objective of this paper is to investigate the evaluation method for assessing the impact of climate change on flood hazard in the coastal lowlands of Taiwan. The paper selects Hebauyu watershed of Chiayi County as the demonstration area. The inundation-related information, such as geographic situation, hydrologic data, surface drainage system, storm sewer system etc., is collected. The newly updated DTM data in 2006 is used in the present study as well. An integrated numerical model is herein developed to assess the impact of climate change on flood hazard in the coastal lowlands, based on the extension of the 1-D river routing model and the 2-D inundation routing model. Next a series of inundation simulations of the coastal lowlands are conducted for various scenarios caused by the extreme rainfall event in the upstream watershed and the highest sea level rise in downstream, respectively. The inundation loss and risk for various scenarios are analyzed as well. Finally, the worst case of the climate-change-induced inundation scenarios is studied. The possible adaptation strategies, including the engineering and non-engineering measures, for alleviating the impact of climate change on flood hazard are proposed. In comparison with the results of three GCMs with various emission scenarios, it is found that the amounts of future precipitation calculated by the above three GCMs give different tendency, indicating that there exists some uncertainty in this topics. On the other hand, all of the three GCMs demonstrate the similar tendency of sea level rises with limited variation. Taking the A2 scenario of HADCM3 model as example, which results in the increases of precipitation and sea level rises, for the case considering both of climate change and land subsidence, the simulated results show that serious over-banking flow will both happen in 2010 to 2039 and in 2040 to 2069, and very serious over-banking flow will occur in 2070 to 2099.