蘭陽溪颱風事件懸浮顆粒輸出與組成之時序變化與風化作用之交錯影響

Abstract

臺灣為島嶼型河川，山高水急，降雨之時間與空間分布不均，乾濕季明顯，加上臺灣地區受極端事件影響頻繁，平均每年有三至四個颱風侵襲，以及板塊運動影響產生地震、抬升等，加劇物理及化學風化機制，使得河川流域母岩性質脆弱易斷裂，造成臺灣河川每年攜帶大量之懸浮顆粒進入海洋後沉積，其單位面積的傳輸量甚至遠超過世界主要大河。 本研究係以台灣東北部蘭陽溪流域為研究對象，探討平時與颱風期間，河川輸出懸浮顆粒量的差異，分析顆粒物質主要元素組成，比較總懸浮顆粒TSM (total suspended matter)與總溶解性物質TDS (total dissolved solids)之元素組成差異，推測蘭陽溪流域上游岩層物理及化學風化機制，及其對懸浮顆粒之影響。 根據研究結果，蘭陽溪輸出之懸浮顆粒化性表現，與地層岩石化性表現相似，表示此流域輸出顆粒，多為物理風化的碎屑性(detrital)顆粒，在颱風時刻，不同岩層間，顯示碳酸岩類與鋁矽酸岩類的混合機制。 輸出總懸浮顆粒量部分，本研究期間(2007~2010)統計蘭陽溪流域平均懸浮顆粒輸出量為21 Mt/yr，每個颱風暴雨期(3~7日內)的懸浮顆粒輸出量為0.7~12 Mt不等，最大可超過年輸出量21 Mt/yr的一半。若以總量來看，四年共輸出了85 Mt，其中71 %為颱風所出輸出。顯示颱風暴雨期造成的懸浮顆粒輸出是臺灣小型河川顆粒態物質進入海洋的關鍵，若與世界河川相比，蘭陽溪每年單位面積向海洋之輸出量亦高於世界河川平均值。

Taiwan is one of the high standing islands (HSIs) which is frequently affected by extreme events which caused uneven rainfall in time and space. The amount of parti-cle per unit area is even far greater than that of the world major rivers. On average, three to four typhoons occur every year. In addition, plate movements causing earth-quakes and uplifts are usual in Taiwan. Both frequent chemical and physical weath-ering mechanisms easily break parent rocks in the river basin, and that is a key reason causing Taiwan's rivers export a large amount of particulate matter to the ocean every year. Therefore, Taiwan is a very special and suitable place to study the small river weathering. The aim of this study is to explore how much suspended particles are transported into the ocean in the small river, and of which elements in the particulate matter are mainly composed. We compared the amount of suspended particles and its chemical elements during typhoon and normal peroid in Lanyang River. The element composi-tion of TSM (total suspended matter) and TDS (total dissolved solids) are used to evaluate physical and chemical weathering mechanisms in the small river. Our results reflect that the output of suspended particulate matter in Lanyang River is higher during the typhoon period than that in the normal day. During the three days of typhoon period, the highest TSM is nearly 0.7~12 Mt and the average output is reaching about 21 Mt/yr, which is nearly half of annual output in the Lan-yang River. This phenomenon indicates that the typhoon event is a key factor pro-moting the export of small river particulate matter into the ocean in Taiwan. Moreo-ver, the annual output per unit area of the Lanyang River to the ocean is much higher than that of the world river. It shows the importance of chemical weathering of sus-pended particulate matter exported to the ocean in Taiwan's small rivers.