利用航空攝影測量探討有勝溪河道地形變化與斷流之研究

為恢復臺灣櫻花鉤吻鮭的歷史棲地，雪霸國家公園管理處自2009年開始於有勝溪上游的羅葉尾溪進行域外放流，卻因下游頻繁發生斷流，直接衝擊國寶魚的生存。為探討有勝溪部分河段發生斷流的機制，本研究利用歷史航照影像以及無人飛行載具（Unmanned Aerial Vehicle，UAV）空拍影像產製出正射影像以及數值地表模型（Digital Surface Model，DSM），比較2010年至2022年間河道變遷情形，並搭配水文資料歸納出斷流的成因。結果顯示：2012年7月的大豪雨事件後使右岸的廢耕地遭受侵蝕，部分河段堆高1公尺，此後斷流頻繁發生，河道堆高導致地下水位遠離地表是斷流的主因，除此之外，本研究河段為失水河段，部分河段因側蝕加寬導致入滲量增加，則間接加劇了斷流的發生。雨量為驅動河道高程變化的主因，當最大時雨量大於47.5毫米時（大事件），則造成河道側蝕且最低點堆積；最大時雨量小於20.5毫米且有地表逕流產生時（小事件），則造成河道最低點下切。從2018年後，本研究區多為小事件造成的下切作用，然而下切程度有限，平均一年僅下切0.08公尺，至今部分河段的河道高程仍與2010年相差1公尺以上，加上每4~5年可能有大事件造成的側蝕與堆積，有勝溪在短期內似乎無法避免斷流的命運。

關鍵字

斷流；河道地形；地形變遷；極端降水事件；地表水與地下水交互作用

並列摘要

In order to restore the historical habitat of the Formosan landlocked salmons (Oncorhynchus masou formosanus), the offspring have reintrodueced to the Louyewei Creek which is the upstream of the Yousheng Creek by Shei-pa National Park Headquarters since 2009. However, the habitat expansion was hampered by the frequent flow disruption in the upsteram of the Yousheng Creek. In order to investigate the mechanism of flow disruption, this study used historical aerial photographs and unmanned aerial vehicle (UAV) aerial images to produce orthophotos and digital surface models (DSM) to reveal the morphological changes from 2010 to 2022. Besides, hydrological data were supplemented to summarize the causes of flow disruption. The results showed that after the heavy rainfall event in July 2012, the abandoned agricultural land on the right bank was eroded and part of the channel elevated by more than 1 meter, leading to frequent flow disruption afterwards. The groundwater table under the channel became deeper below the riverbed surface owing to sediment deposition, which was the main cause of the flow disruption. In addition, the study stream reach featured losing reach. Lateral erosion, resulting in wider channel, enhanced the amount of infiltrated stream water to the riverbed and indirectly aggravated the flow disruption. Rainfall intensity was the main driving force of the morphological change. When the maximum hourly rainfall was greater than 47.5 mm (major event), the channel was laterally eroded and the thalweg was elevated. When the maximum hourly rainfall was less than 20.5 mm and surface runoff occurred (minor event), the thalweg was incised. After 2018, the study area was mostly incised by minor events, and the extent of incising was around 0.08 m per year. However, the channel elevations of flow disruption reaches were still >1 m higher than that in 2010. At the given condition that major events might occur every 4 to 5 years, it seems difficult to incise the channel to the condition in 2010 and therefore to avoid flow disruption of the Yousheng Creek in the near future.