莫拉克颱風在楠梓仙溪誘發之洪水與土石流作用及其溢淹範圍的含意

臺灣地形陡峻，山崩、土石流、洪水等地形作用活躍，除了少數例外，多由大規模、低頻率的豪雨或地震所誘發。2009年莫拉克颱風挾帶的超大豪雨，造成高屏溪沿線－從上游到河口、海岸地區－生命財產的嚴重損失。此高達200年重現期的降水和洪水所造成的地形作用，值得進一步探究。本研究以楠梓仙溪（旗山溪）為研究區，從地形判讀著手，套疊近百年的圖資影像，以了解研究區各河段在莫拉克颱風時的地形變動，並比對洪水溢淹範圍與近自然河道範圍的異同，討論其在洪氾區域劃設上的含意。所採用圖資中最早為20世紀初期的臺灣堡圖，最近期則為2009年莫拉克颱風後的正射影像，其中包含1940年代末期拍攝的航空照片。航照和正射影像判釋結果顯示，莫拉克颱風在楠梓仙溪旗山和民生間的河谷，誘發22處溪溝型土石流作用，主要分布在較上游河段；主流沿線洪水溢淹情形普遍，愈往下游淹水範圍愈大，但各河段所淹沒的地形單元，不盡相同；洪水溢淹的範圍，皆為20世紀前半葉河岸工事興修前，河道的流路或河岸低地。換句話說，隨著土地開發與河川工事興修，河道流路範圍逐漸縮減，莫拉克颱風之200年重現期洪水溢淹區域，並未超出50年前的近自然狀態的河道範圍。20世紀是臺灣快速向中上游河谷開發的時期，主要河川多經歷河川工事興修與河川地開發的過程，在臺灣地區極端降雨事件頻率日漸提高的隱憂中，實有必要檢視河流沿岸洪水溢淹的潛在風險，以降低受災風險。

關鍵字

地形變遷；氾濫；土石流；航照判讀；洪水區域劃設

並列摘要

Taiwan is subject to landslide, debris flow and flooding hazards which are mainly induced by major typhoon and earthquake events. A very recent case is that Typhoon Morakot caused overwhelming life and property losses from upstream valley to coastal area in the Kaoping River basin, August 2009. Geomorphic effects triggered by this major event are worth to do further research. This study aims to understand the spatial distribution of various geomorphic processes, and discuss the implication of flooded area induced by Typhoon Morakot, within the study area of the Nanzisian Stream (Chishan Stream). Geomorphic interpretation and mapping are completed by using historical maps, aerial photographs and orthomaps from early 20th to 2009 (after the typhoon). Results show (1) 22 stream-type debris flows mainly occurred in the upper sections and flooding were widespread in the lower sections, and (2) the flooded area were limited within the active-channels zone identified from 1948 aerial photos and historical maps. Over last several decades, along with the reclamation of farmlands, the Nanzisian Stream has been significantly channelized. This process is not unique but has experienced for most major rivers in Taiwan. Occupation of former active-channel zones is particular worrying when it is suspect that the frequency of extreme rainfall events is rising.