台灣地區之河流輸砂量與岩性、逕流量及地震之相關性

Abstract

本論文利用近20至50年來台灣島上13條主要河川之水文及山崩資料，來進行各項研究。從輸砂量估算的結果顯示，台灣各流域之平均年輸砂量介於0.12 Mt至93.81 Mt之間。其中，以北部頭前溪上游流域為最低，中部濁水溪流域為最高，單位面積的輸砂量則以二仁溪流域的88,667.33 ton/km2/yr為最高。平均岩石強度以北部大漢溪流域內之56.32 MPa為最高，節理密度則以林邊溪流域內平均53.08 條/立方公尺最多。各流域的平均崩塌率則以大甲溪流域內的9.26 %最高，頭前溪流域的0.89 %最低。 就輸砂量及山崩的分析結果，與地質材料、地形、降雨及地震間之相關探討，可以得出幾點結論：(1)岩石強度越低或不連續面越發達，將提高山崩發生之機率；(2)流域內的地形因子，如高程、坡度及坡向等，與山崩的分布間存在著相關性；(3)年輸砂量與年逕流量之間呈現良好的正相關，當單一颱風之累積降雨量超過400 mm，其輸砂量會佔流域年輸砂量之20 %以上；(4)地震會導致崩塌地的分布往山頂發展，也會延長高輸砂濃度回降到地震前平均值之所需時間。 從溪水化學性質的探討可以得知，超過60 %的溶解物質來自矽酸岩，表示矽酸岩類的風化是台灣各主要河川溪水中溶解物質的最主要來源，與台灣地層中主要岩性組成即以矽酸岩類為主有關。整體而言，影響溪水主要離子濃度的因素包括：(1)濕季高流量的稀釋作用、(2)各流域的母岩組成，以及(3)海洋鹽沫。 由分析各河流的輸砂濃度之回歸週期可以發現，流域內崩塌率越高，發生異重流所需之再現週期越短，顯示颱風期間的崩塌事件是異重流中沉積物的重要來源。而在岩石強度較高之流域內，需要較大的降雨量才可以誘發異重流，呼應了岩石強度是控制山崩發生的重要因素之一。

The study gathers the 20-50 yrs hydrometric and landsliding data of 13 main rivers. Analysis results display that the annual sediment discharges from these river catchments range from 0.12 Mt for the Touchien River to 93.81 Mt for the Choshui River. The maximum sediment yield is 88,668 ton/km2/yr for the Erhjen River. The investigations of rock properties display that the Tahan catchment has the highest rock strength of 56.32 MPa and the Linpien catchment has the highest joint density of 53.08 m-3; and further, the landslide interpretations show that the Tachia catchment has the highest landslide ratio of 9.26 %. By analyzing the influences on sediment discharge and landslides, some inferences are obtained: (1) rock mass with higher rock strength and lower joint density could resist landsliding and sediment yielding; (2) the sediment discharge induced by a typhoon having rainfall > 400 mm would occupy more than 20 % of annual sediment discharge; (3) earthquake cause that landslides are distributed away from streams and prolong the duration of consumption of landsliding debris. More than 60 % of dissolved material is attributed from silicates. Because the major lithologies in Taiwan are belong to silicates. In conclusion, the chemical properties of river water are influenced by dilution, lithologies, and sea spray. The recurrence intervals of hyperpycnal flow will be shorter while more landslides occur in the catchment. The study also finds that higher rainfall threshold is requested to form a hyperpycnal flow in a catchment with higher rock strength.