莫拉克風災河道淤塞及變遷 －以荖濃溪流域為例

Abstract

台灣島是由菲律賓海板塊碰撞歐亞板塊所造成，形成複雜的地形，山多坡陡，河川短促，邊坡脆弱。又因位於副熱帶以及熱帶季風氣候區內，每年約有3~4個颱風侵襲台灣，每逢颱風豪雨，山區容易發生山崩、地滑、土石流等災害，近年因全球氣候變遷，天然災害更加頻繁。2009年8月莫拉克颱風極端強降雨以及長延時的特性，在台灣中南部造成許多生命財產的損失，為台灣近50年來最嚴重的颱洪災情之一。 荖濃溪為高屏溪最大支流，全長137公里，流域面積1373平方公里，莫拉克颱風於本地區降下超過2000mm之雨量，最大時雨量超過100mm/hr，如此豪雨所引致之崩塌土砂，隨著雨水流入河道，造成河床淤積或河岸沖刷，造成本溪流域內嚴重坡地災害。 從前調查河道中泥沙的淤塞現象多以人力直接到現場進行勘查，若以此方式勘查，有時可能會因為交通中斷、地形限制、或位於偏遠地區等情況而無法到達。近年來航、遙測科技的蓬勃發展，影像解析度及品質大幅提升，遙測技術可以提供大範圍的影像資料，提供人員無法到達的區域資訊。此外，輸砂量雖可以利用降雨量、河川流量、懸浮質等數值來估算，但無法了解大型土石運移的情況。透過分析所建置數值地形模型（Digital Terrain Model, DTM）更容易了解集水區內山崩、土石流以及河流中沉積物的運移模式。 本研究是利用農林航空測量所拍攝之荖濃溪沿岸航空影像，分別製作莫拉克颱風前後的數值地形模型，使用的影像為數位製圖相機（Digital Mapping Camera, DMC）所拍攝之風災前98年4月影像，以及空載多光譜掃描儀（Airborne Digital Scanner 40, ADS 40）所拍攝之風災前97年10月與98年1月影像，以及風災後98年8月影像。發現風災造成河道側向侵蝕情形嚴重，尤其以上游玉山國家公園內及中游地區側向侵蝕情形非常嚴重，中下游地區河道邊坡侵蝕現象較不明顯。同時風災也造成上游河道堆積深度較中游及下游深，可能是因為莫拉克颱風使上游崩塌土砂隨雨水流入河道，而崩塌量過大造成土砂就地淤積在上游，而中游地區的土砂量也過大，亦使土砂堆積於中游地區。下游地區因為河道邊坡侵蝕與崩塌土砂較少，使得河道侵蝕量略大於淤積量。

Taiwan results from ongoing oblique collision between the Philippine Sea Plate, and the Eurasian Plate. Due to the arc-continental collision, complex topography of high relief has been developing and also creating steep mountains, rapid rivers, and fragile slopes . Also, Taiwan is located in the climate zone of subtropical and tropical monsoon area. On average, 3 to 4 typhoons hit Taiwan every year. When the typhoon comes, it bring heavy rains to cause landslides and debris flow easily in the mountain areas. In recent years, global climate changes dramatically, natural disasters take place more frequently. Typhoon Morakot brought extreme and long-time rainfall for Taiwan in August 2009. It further caused huge loss of life and property in southern Taiwan. As a result, Typhoon Morakot marks the worst flooding situation of Taiwan in the past 50 years. Laonong River is the largest tributary of Gaoping River. It’s length is 137 km, and the basin area is 1373 km2. More than 2000mm rainfall brought and maximum rainfall exceeded 100mm/hr in the region by Typhoon Morakot in Aug, 2009. Its heavy rains made many landslides and debris flew into the river and further brought out accumulation and erosion on river banks of different areas. It caused severe slope disasters within the Laonong River drainage. In the past, the study of sediment blockage of river channel usually relies on field investigation, but sometimes survey cannot be completed due to inconvenient transportation, topographical barriers, or located in remote areas, etc.. In recent years, the rapid development of remote sensing technology improves image resolution and quality significantly. Remote sensing technology can provide a wide range of image data, and receive information in the places where personnel cannot reach. Furthermore, although the amount of sediment transportation can be estimated by using data such as rainfall, river flux, and suspended loads, the situation of large debris migration can not be studied via those data. However, landslides, debris flow and river sediment transportation model in catchment area can be evaluated easily through analyzing the digital terrain model (DTM) . The purpose of this study is to investigate the phenomenon of river migration and to evaluate the amount of migration along Laonong River by analyzing the DEM before and after the typhoon Morakot. The DEM is built by using the aerial images taken along Laonong River by the Aerial Survey Office. They are images before the storm as digital mapping camera (DMC) taken in April 2009, airborne digital scanner 40 (ADS 40) taken in October 2008 and January 2009, and the images after the storm as airborne digital scanner 40 taken in August 2009. The results show that lateral erosion of the Laonong River caused by the typhoon seriously, especially in Yushan National Park, and midstream region. However, lateral erosion in downstream region is not so obvious. Meanwhile the siltation depth resulted from the Typhoon Morakot is larger in upstream region than in midstream and downstream regions. Perhaps the amount of landslide material created by Typhoon Morakot was too excessive to be transported. Material just siltated in the upstream in place, while the middle region also is. Because of the amount of river slope erosion and sediment collapse in the downstream region is less than in upstream and midstream region, the amount of river erosion slightly larger than the amount of river siltation.