基岩河道(bedrock channel)形貌(例如:河寬與坡度)的變化,能夠反應出局部下切速率的改變,在河流縱剖面處於均衡狀態(steady state)之下,岩石抬升速率(U)與下切速率(E)相等,則可藉由分析基岩河道形貌的變化,瞭解構造間的相對活動性。位於台灣中部的大安溪貫穿一系列約南北走向的構造,例如: 天狗斷層、東勢背斜、枕頭山斷層等,提供我們一個適當的機會去檢視河道形貌在空間上的變化與構造活動性的關係。 本研究利用民國75年的40公尺數值地形模型(DEM)、五千分之一和一萬分之一像片基本圖,分析河道坡度、河道寬度、集水區面積,以及野外量測岩石強度等資料。期望1.了解河道坡度、寬度與岩石強度在空間上的變化與構造位置及活動性之間的關係;2.建立大安溪水力經驗公式(hydraulic empirical functions) (如: 坡度、河寬、流量分別與集水區面積的乘冪關係)與前人研究比較;3.結合水利經驗公式與剪應力下切模型(shear stress incision model),在假設均衡河流縱剖面和氣候、岩性一致的條件下,計算陡峭指標(ks)並討論與構造抬升速率的關係。 分析結果顯示河道形貌異常的變化與岩石強度較無直接相對應關係,因此河道形貌異常變化主要受構造影響。而由河道坡度與寬度在經過構造時的異常變陡或變窄趨勢,推測天狗斷層、雙崎斷層、東勢背斜、枕頭山斷層在大安溪流域內具有相對較高的活動性。所建立的大安溪流域的水力經驗公式中,各乘冪關係中的指數,θ、b、c分別為.0.53、0.51、0.90,與前人研究經驗值非常相近,顯示出水力經驗公式同樣可適用在大安溪流域;另外θ與模擬河道在均衡狀態下的θ(0.5)十分相近,推測目前大安溪可能處於均衡狀態之下。相對於坡度的反應,本研究的河寬變化與構造間有較明顯的關係,其原因有: 1.河道形貌在調節下切速率的改變有不同的反應機制;2.坡度的變化相對於寬度較不容易被保存下來;3. 40公尺DEM的解析度造成的高程誤差對坡度影響較大。ks在空間上的變化雖然仍表現出天狗斷層的高活動性,卻無法指示出其他本研究推測具有較高活動性的構造,其原因可能為ks主要是由坡度推導而來,並無考慮到寬度的變化,因此若能將ks與kw結合,則更能代表相對的岩石抬升速率。
The variations of bedrock channel morphology, such as channel width and slope, have been proposed to reflect the different incision rate. On the other hand the incision rate has been suggested to be equal to the rock-uplift rate when the river longitudinal profile is in steady state. This study aims at understanding and deriving the relative local rock-uplift rate along a river by observing the changes of its bedrock channel morphology. In central Taiwan Daan River transverses a series of N-striking structures such as Tienkou fault, Tungshih anticline, and Chentoushan fault and gives a good opportunity to examine the channel dynamics across these structures. Using a 40m digital elevation model and 1/5000 and 1/10000 Orthophoto Base Maps and measuring rock strength, this study obtained the channel width, slope, drainage area, and rock strength along the Daan River and figured out the relationship to the relative activities of structures. This study also built the hydraulic empirical functions of Daan River, and further combined them with shear stress model in steady state to calculate the steepness index (ks) and discuss the correlation between ks and rock-uplift rate. According to the results, the channel morphology dose not relate to lithology along Daan River, but the anomaly variations of channel morphology are influenced by structures. The abrupt changes in channel width and slope indicate the Tienkou fault, Shuangchi fault, Tungshih anticline, and Chentoushan fault have relatively higher activities. The exponents of hydraulic empirical functions are similar to the previous studies, and further the concavity index (θ) of Daan River is nearly the same as the θ modeled in the steady state, indicating Daan river is in steady state. From the results, the channel slope is less relative to structures, and it might result from three factors. First, the timing of the changes of channel slope and width to adjust the incision rate is not the same. Second, to preserve the variations of channel slope is more difficult than channel width. Third, the elevation accuracy of DEM has deeper influence on the channel slope than on the channel width. Furthermore the ks dose not respond to these structures implied as active strucutres, except Tienkou fault. Because the ks is derived from the channel slope without involving the width, further studies should combine the steepness index (ks) with width coefficient (kw), which could better represent the rock -uplift rate.