臺灣海岸山脈東翼小流域之河階分布與類型的探討
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Date
2013-06-01
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中國地理學會
Abstract
海岸山脈東翼的流域普遍狹小，河階地形卻非常發達。以往的研究以本區較大河流為主，對控因的探討則多籠統說明抬升快速和軟岩出露等之重要性，較缺乏對河階分布、類型的整體性描述，也少見以量化方式呈現河階生成和底岩特性、流域面積的關係。因此，本研究以海岸山脈東翼中小型流域為對象，判釋本區河階的形態與分布，再配合野外露頭調查結果區分本區河階類型，更細緻地檢視河階的類型及其分布趨勢，並討論可能的影響因素。經航照判釋，海岸山脈東翼207條中小型溪流中有河階發育者共104條，約佔半數，且有近九成的河階面積分布於砂頁岩互層或泥岩等軟岩區。整體而言，河階最發達的區域在靜浦至新蘭間，結合河階形態與125個露頭資料，可歸納出：該區域溪流中下游多為面積和比高較小的侵蝕型河階；面積和比高較大且位於河流中上游的階地，則多屬堆積型河階，且上覆沈積物多為土石流相。統計分析也發現流域面積愈大，有河階發育之溪流的比例愈高，且河階總面積愈大。流域面積超過200 ha時，有河階發育的流域比例已超過九成。總結以上，地體構造的背景大致決定了海岸山脈東翼的原初陡峭地形和軟岩出露於中下游的位置，在本區高抬升速率的背景下，一條溪流的集水範圍不用很大，就有能力在軟岩區下切和拓寬河道，侵蝕型階地得以發展，中上游則還有因土石流埋積體被下切而成的堆積型階地，使本區河階地形特別發達。
Fluvial terraces are well-developed in the eastern flank of Coastal Range, even in the very small drainage basins. Rapid uplift rate and widespread exposure of less resistant bedrocks are believed the main causes, however, investigation and quantitative analysis of terrace forming materials are needed to clarify the major causes. By re-interpreting the terraces from the aerial photographs and investigating more outcrops, this research aims to identify the spatial pattern of different terrace types and further clarify the factors which contribute to the well-developed terraces.Fluvial terraces are interpreted from 104 drainage basins out of 207 medium to small basins in the study area. In term of area of terraces, up to 90% are located in the less resistant bedrocks. Statistically, there is a strong positive correlation between the area of drainage basins and of the total area of terraces. The spatial pattern of two types of terraces is classified that the depositional terraces (larger terrace area, higher relative height and overlain with relatively thick debris-flow facies) tend to occur in the upper to middle segments and the erosional terraces (smaller terrace area, lower relative height and overlain with thin sediments) tend to occur in the middle to lower reach along the streams. It is inferred that erosional terraces reflect the continuous but episodic downcuttings into bedrocks while the depositional terraces here are the products of incision into poorly consolidated sediments which may be driven by debris flow.
Fluvial terraces are well-developed in the eastern flank of Coastal Range, even in the very small drainage basins. Rapid uplift rate and widespread exposure of less resistant bedrocks are believed the main causes, however, investigation and quantitative analysis of terrace forming materials are needed to clarify the major causes. By re-interpreting the terraces from the aerial photographs and investigating more outcrops, this research aims to identify the spatial pattern of different terrace types and further clarify the factors which contribute to the well-developed terraces.Fluvial terraces are interpreted from 104 drainage basins out of 207 medium to small basins in the study area. In term of area of terraces, up to 90% are located in the less resistant bedrocks. Statistically, there is a strong positive correlation between the area of drainage basins and of the total area of terraces. The spatial pattern of two types of terraces is classified that the depositional terraces (larger terrace area, higher relative height and overlain with relatively thick debris-flow facies) tend to occur in the upper to middle segments and the erosional terraces (smaller terrace area, lower relative height and overlain with thin sediments) tend to occur in the middle to lower reach along the streams. It is inferred that erosional terraces reflect the continuous but episodic downcuttings into bedrocks while the depositional terraces here are the products of incision into poorly consolidated sediments which may be driven by debris flow.