林邊溪流域河川化學性質、輸砂量與山崩之關係

Abstract

地質材料的風化可以分為物理風化及化學風化。河水中的溶解載與輸砂量，可以視為集水區內的物理風化以及化學風化產物。因此研究河水中的溶解載與輸砂量特性，有助於我們了解集水區內的風化侵蝕作用。颱風會在台灣山區造成大量的山崩，為輸砂量的重要來源。本研究以屏東縣林邊溪為例，利用1982年至2013年的31年間之輸砂量、2005至2012年的5個颱風事件以及2013年3月至2014年4月的河水化學性質，來探討河川化學性質、輸砂量與山崩之間的關係。 從輸砂量的研究中發現，這個區域的年輸砂量為介於0.06至8.08 Mt，平均為1.40 Mt。5月至10月的濕季期間，輸砂量佔全年輸砂量97.47 %。另外，颱風事件的研究中發現，颱風事件的輸砂量平均占全年輸砂量的39.02 %，崩塌率平均為4.69 %。此意義顯示，在濕季或颱風暴雨帶來的高降水因素，主導了林邊溪的輸砂特性。 颱風事件造成之崩塌地分布的分析結果顯示，輸砂量較高之2009年莫拉克颱風、2008卡玫基颱風與2005年的海棠颱風的新生率較高，顯示新發生的山崩會供應河川豐富的地質材料而提高事件輸砂量。 2013年3月至2014年4月的河水化學分析結果顯示，林邊溪河水中陽離子的組成以Ca (52.1 %)為主、；陰離子與溶解態矽酸鹽的組成成分以HCO3(66.6 %)為主。中上游的河水化學性質陽離子來自碳酸鹽的比例最大(82.33 %)，其次為矽酸鹽(13.82 %)，最少的則是大氣輸入(3.85 %)。顯示變質岩內少量分布的碳酸鹽可提供河水大量來自碳酸鹽的化學成分。河水中的高SO4來源為變質岩當中的黃鐵礦風化，而非外部因素。下游新埤測站樣品的陽離子中，來自碳酸鹽的比例最大(62.12 %)；其次為雨水(14.90 %)；再者為矽酸鹽(14.74 %)；最少的為外部因素(8.23 %)。外部因素的比例雖少，但對河水化學的影響不可忽視。 2013年3月至2014年4月林邊溪的月化學風化通量為125 ton至14,844 ton，平均為2,944 ton，而濕季期間的化學風化通量佔全年化學風化通量的88.38 %。林邊溪的化學風化速率為110 t km-2 yr-1，矽酸鹽化學風化速率為29 t km-2 yr-1，約為世界平均的5倍。研究期間林邊溪的物理風化速率為4,062 t km-2 yr-1，約為化學風化速率的40倍，顯示林邊溪流域的風化以物理風化為主。對林邊溪而言，每年濕季颱風事件所造成的高輸砂量與大量山崩，會持續搬運風化作用的產物，使母岩的新鮮面不斷暴露至地表接受化學風化，造成高化學風化速率，顯示物理風化對化學風化有重大影響。

The weathering of geomaterial can be classified into physical and chemical weathering. The dissolved load and sediment discharge can be considered chemical and physical weathering product in a drainage basin. Therefore, we can understand the weathering in the drainage basin by studying the characteristics of dissolved load and sediment discharge in river water. The typhoon events would cause massive landslides in mountains, which are important sources of sediment discharge in Taiwan. This research utilizes sediment discharge during 1982 to 2013, 5 typhoon events during 2005 to 2012, and river chemistry during March 2013 to April 2014 of Linpien River to study the relationship between river chemistry, sediment discharge and landslide. The annual sediment discharges ranged from 0.06 Mt to 8.08 Mt, with an average of 1.40 Mt. The sediment discharge yielded during wet seasons (May to October) contributes 97.47 % to the annual sediment discharge. In addition, the average sediment discharge during typhoon events contributed 39.02 % to the annual sediment discharge and the average landslide ratio for five typhoon events during 2005 to 2012 was 4.69 %. It implies that the factor of high rainfall during wet seasons or typhoons controls the variations of sediment discharges in the Linpien River. The analyses of distributions of landslides caused by typhoon events revealed that the higher sediment discharge of Morakot in 2009, Kalmeigi in 2008 and Haitang in 2005 with higher newborn ratio, which implies that the newborn landslides would contribute abundant sediments to the river, and resulted in the increase of sediment discharge of the typhoon events. The analyses of major elements in water samples collected in the basin during March 2013 to April 2014 showed the major cation was Ca (52.1 %) and the major anion is HCO3 (66.6 %). The contribution of river chemistry from carbonate was highest (82.33 %), next was silicate (13.82 %), and the last was atmospheric input (3.85 %) in upstream samples. This implies that the trace carbonates in metamorphic rock could influence river chemistry significantly. The high SO4 in river water is contributed from chemical weathering of pyrite, not from anthropogenic pollution. The river chemistry of the downstream samples collected at Hsinpi Station was contributed most from carbonate (62.12 %), next was atmospheric input (14.90 %), then was silicate (14.74 %), and the last was external factor (8.23 %). Although the proportion of external factor is insignificant, the impact on river chemistry is innegligible. The monthly chemical weathering flux ranged from 125 ton to14,844 ton, with an average of 2,944 ton during March 2013 to April 2014. The chemical weathering flux during wet seasons contributed 88.38 % to annual chemical weathering flux. The chemical weathering rate of Linpien River was 110 t km-2 yr-1, and the silicate chemical weathering rate was 29 t km-2 yr-1. Both were 5 times higher than the world average. During the study period, the physical weathering rate was 4,062 t km-2 yr-1, which was about 40 times higher than the chemical weathering rate. It indicates that the weathering in the drainage of Linpien River is mainly physical weathering. For Linpien River, the high sediment discharge and the massive landslides caused by typhoon events during wet season would transport weathering products continuously. It makes the fresh surfaces of bedrock be exposed to earth surface, and results in the intense chemical weathering. In the conclusion, the physical weathering has significant influence on chemical weathering.