荖濃溪晚第四紀階地與河流演育

Late Quaternary terrace formation and evolution of the Lao-nong River, southwestern Taiwan

崔秀國(Hsiu-Kuo Tsui)

指導教授 ： 謝孟龍

國立中正大學/理學院/地球與環境科學系應用地球物理與環境科學研究所/碩士(2015年)

摘要

在台灣，受控於長時間的地殼隆升，在大時間尺度下，河流必然下切。然而，台灣山區許多被山崩、土石流的礫石覆蓋的坡地(Hillslope)或堆積形成的階地(Alluvial terrace)的說明了災變性堆積事件的存在，而研究這些階地有助於我們了解河流的堆積、下切歷史，以及他們如何影響河流行為。2009年莫拉克颱風引發了台灣西南部山區廣泛的山崩土石流，造成荖濃溪縱谷段堆積數公尺至三十公尺不等，該風災也使荖濃溪產生了許多的剖面。為了解荖濃溪河流行為與演育歷史，本研究調查了荖濃溪眾多階地之(沉積層)剖面，將沉積層分為支流礫層、主流礫層、主流砂層。本研究含前人資料共收錄了98筆碳十四年代，其中39筆為第一次發表。根據野外調查及定年結果，研究區主要的堆積事件發生在：0.2-2.0、5.3-6.5、11.7-12.5 ka cal BP，且大多數階地都位於右岸。推論梅山全新世以來至少經歷四次大於莫拉克颱風的堆積期(事件)，規模最大者發生於大約3 ky BP，可高於2008年主流河床270 m，說明主流堆積、下切相當頻繁。利用研究區階地規模、年代、位置分布資料，推論不同支流流域的堆積時間、規模、頻率有所不同。本研究亦發現沖積扇的發育能迫使主流側蝕(以莫拉克颱風侵襲後的布唐布那斯沖積扇為例)，是一個山崩土石流影響河流行為的例證。將有出露基盤面的階地，利用其堆積年代計算下切速率，發現基盤下切速率在全新世可以很快(塔古夫庫拉，58.4 mm/yr)，也可以很慢(復興北，0.3 mm/yr)。然而由於研究區侵蝕旺盛(僅較大或較年輕的事件能被保存)，且各堆積事件之間的下切量無法估計。因此需要注意的是，本研究所推估的堆積事件，無論是規模或是頻率，亦或是下切基盤的最快速率，都是最保守的。

關鍵字

階地 ； 土石流 ； 荖濃溪 ； 碳十四定年

並列摘要

Although dominated by erosion over long term, the tectonically active mountains of Taiwan commonly contain thick landslide and debris-flow gravels capping hillslopes or forming alluvial terraces. These deposits and their associated landforms serve to study ancient mass-wasting histories and their controls on fluvial processes. The development of such alluvial terraces can be observed during 2009 Typhoon Morakot, which brought record-setting 2000 mm of rain. Triggered by this heavy rain, almost all the tributaries of the Lao-nong generated alluvial fans at their mouths and consequently caused up-to-30 m aggradation along the trunk river.

The Lao-nong has yielded 98 radiocarbon dates (all <20 ka). These data reveal that the formation of the observed alluvial terraces (from deposition of thick gravels to subsequent incision) could proceed very rapidly (<1 ky), and that the trunk river has been characterized by repeated aggradation and incision. These dates suggest the clusters of mass-wasting events around ~11.7–12.5 ka, 5.3–6.5 ka, and 0.2–2.0 ka. Using the distribution of magnitude, dating and the location of terraces suggests that most of the recorded mass-wasting events vary in time and magnitude among tributary catchments.

An illustration of landslides/debris flows control the river landscape process has observed in the the Pu-tun-pu-nas river. The formation of the tributary fan can limit the channel of the trunk river and force it to erode the bedrock of the bank. The bedrock incision rate can be very high (up to 58.4 mm/yr, Ta-gu-fu-ku-la) and also can be very slow (down to 0.3 mm/yr, Fu-shin north).

The frequency, magnitude and bedrock incision rate are always underestimated because: (1) The resolution of radiocarbon date (2) Fossil sampling points is not the top or bottom of terrace (3) The preservation of terraces.

並列關鍵字

terrace ； Debris flow ； Lao-nong River ； radiocarbon dating

參考文獻

陳立淳(2013)，荖濃溪勤和地區全新世河流地形演育。碩士論文，國立中正大學地震學研究所。

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