荖濃溪勤和地區全新世河流地形演育

Holocene Landscape Evolution along the Chin-ho Area of Lao-nong River Southwestern Taiwan

陳立淳(Li-Chun Chen)

指導教授 ： 謝孟龍

國立中正大學/理學院/地球與環境科學系地震研究所/碩士(2013年)

摘要

2009年莫拉克颱風的豪大雨引發台灣西南部廣泛的山崩、土石流，並造成若干主流河床顯著的堆積。在荖濃溪勤和地區，這河床的堆積幅度可達三十公尺，阻絕了交通，掩埋了「曾文溪越域引水」工程的引水隧道，並帶走十多條人命。為了解這大規模堆積事件的再現週期，本研究調查了引水隧道口上游五公里內的河階剖面，試圖重建該河段河流堆積、下切的歷史。本研究將出露的沉積層分為主流礫層、主流砂層、與支流礫層，並得到54筆碳十四年代(41筆落於0-2 ka)。根據這些資料，推論近兩千年來至少有9次大規模的河床堆積事件，其中6次幅度均不小於2009年者；最大的事件(發生於1.4-1.5 ka cal BP)其幅度甚至超過一百公尺。此外，至少5.5-5.7 ka及8.2-8.4 ka還有兩次堆積事件，其中5.5-5.7 ka事件幅度超過一百五十公尺。這些堆積事件，如同2009年者，都和支流大規模土石流的活動有關(在支流口形成土石扇)，惟各支流發生土石流事件的時間(或時段)並不相同。注意：研究區本就侵蝕旺盛(只有較大事件的證據才可能被保存)，且各堆積事件之間河流的下切量不明。因此，本研究所推估的堆積事件，無論是頻率或幅度，都是最保守的。換言之，雖然研究區長期以下切為主(反應地殼隆升)，短時間當頻繁發生災變性的堆積。以上河流特性當作為土地利用的參考。

關鍵字

河流地形變遷 ； 土石流 ； 「曾文溪越域引水」計畫 ； 荖濃溪

並列摘要

Studies of the active mountains of Taiwan have been focused on the efficiency of erosion processes to balance the rapid tectonic uplift. However, by triggering widespread landslides/debris flows, 2009 Typhoon Morakot (7–10 August) brought aggradation of 10–30 m along much of the Lao-nong River in southwestern Taiwan, killing 14 people and causing enormous economic losses.
To understand the Holocene evolution of this river, we undertook a stratigraphy-based geomorphic study along a 5 km-long reach near Chin-Ho, in the midstream of the river (drainage area: 540 km2; mean channel gradient: 0.019; current uplift: ~1 cm/yr). This river reach exhibits 6 terraced fan systems at tributary mouths. The sediments exposed along these terraces, up to 220 m thick, were classified into 3 facies: tributary gravel, trunk-river gravel, and trunk-river sand, based on their distinct roundness, composition and/or grain size.
Totally 54 radiocarbon dates, < 8.4 ka cal BP, were obtained. Given these, we identified 8 aggradation events (with > 30 m-thick sediment deposition) in the past 2 ka. The greatest event in this period, occurring ~1.4 ka, had elevated the trunk-river bed by 100 m. More aggradation events were found around 5.5 and 8.2 ka cal BP, which had elevated the trunk-river bed by at least 150 m and 80 m, respectively. We conclude that the Lao-nong River is intrinsically subject to episodic aggradation, in association with the development of tributary-fan systems, which has greatly affected the morphology and incision of the river in response to the active tectonic uplift.

並列關鍵字

River Evolution ； Tseng-wen Reservoir Transbasin Diversion Project ； Debris flows ； Lao-nong River

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