臺東鹿野地區河流地形作用與山崩事件之交互影響

Abstract

河流作用是地表侵蝕與堆積最主要的營力之一，因此河流地形之相關研究長期以來皆是地質學家所重視的議題；另一方面，臺灣地處活躍造山帶，山崩事件是影響地形演育的重要地表作用，主控了侵蝕作用與河流系統的沉積物供應，是臺灣中低海拔地形演化的一個主要因素。臺東鹿野地區的鹿野溪與卑南溪匯流處附近，有一系列發育良好的河階地形，地形上同時顯示過去曾有一個規模極大的古山崩事件（鸞山山崩），過去雖然曾經有針對兩者地形分別進行的研究，但並未詳細探討河流地形變化是否與山崩事件的發生具有因果關係，或是山崩事件崩落後對該處河流系統是否造成影響。有鑑於此，本研究的目的為藉由鸞山山崩與周圍河流階地的地形幾何、層位關係、以及其相對年代，來釐清鹿野地區河流系統的演育歷史，瞭解這些地表作用在河流演育的過程中其交互的影響。 應用即時動態全球定位系統（RTK-GPS）與雷射測距儀在野外進行山崩堆積物與其底岩間交界面的高程測量，再透過高精度數值地形模型進行地形分析，本研究得到過去山崩未發生前的古地形面。結果顯示兩千多年前山崩事件發生前，卑南溪主流古河道可能流於目前河道東側數百公尺處，此古河道於山崩發生時遭到山崩堆積物掩埋，使得河流改道至現今卑南溪河道所在位置，並下切形成外成峽谷。另一方面，本研究綜合前人的觀察與資料，推測兩千多年前此山壁內的古河道也並非是長久以來卑南溪所流經的位置，而可能是當鹿野溪大規模沖積扇（龍田階地）快速堆積發育時，大量的沉積物加積使得兩千多年前之卑南溪河道向東遷徙300至700公尺，流到位於縱谷斷層上盤的古河道處。其後又由於這個側向遷徙造成海岸山脈西翼邊坡過陡，引發了鸞山山崩事件。依據本研究發現的外成峽谷地形可得知，若利用現今卑南溪河岸的底岩階地比高計算長期底岩下切速率，可能會高估其數值。但經本研究觀察到卑南溪古河道與現今河道的高差計算得出此地長期河流下切速率為14－19.6 mm/yr，可對於鹿野鸞山地區估計河流下切速率時提供一個參考根據。整體來說，鹿野地區河流地形作用與山崩事件經過一系列的相互影響，才形成了目前所見的地貌。

Landsliding is an important surface process in landscape evolution, and controls the erosion and sediment supply in river systems in active orogenic belts. A series of river terraces is situated near the confluence of the Luyeh River and Peinan River in Taitung Luyeh area and is neighbored by a landform showing an enormous landslide event (Luanshan landslide). Although there have been separate studies on the two landforms in the past, there has not been a detailed analysis on whether the change in river topography is related to the occurrence of the landslide, or vice versa. The purpose of this study is thus to understand the evolutionary history of the river system in Luyeh area and delineate the effects of surface processes on river evolution with the analysis of topography, stratigraphic relationship, and the ages of the Luanshan landslide and its surrounding river terraces. Real-time kinematic GPS and laser rangefinder were used in the field to measure the elevation of the contact between the landslide deposits and the bedrock strath. Then, topographic analysis was conducted with a digital elevation model to obtain the ancient land surface before the occurrence of the landslide. The results indicated that the Peinan River, a paleo-channel, might have been flowing down several hundred meters to the east of the current channel where the landslide event occurred at two thousand years ago. This paleo-channel was buried by landslide deposits when the landslide took place. Therefore, the Peinan River moved back to the current position and incised to the formation of an epigenetic gorge. Based on the previous and the current studies, the paleo-channel of the Peinan River was not the original location of the river. Longtian terrace, an enormous alluvial fan with a large amount of sediments was developed by the Luyeh River, which might have pushed the Peinan River channel to migrate about 300 to 700 meters eastward and then flowed onto the hanging-wall block of the Longitudinal Valley fault. The western slope of the Coastal Range became rising at a sharp angle because of the eastward migration and lateral erosion, which might have triggered the Luanshan landslide. According to the epigenetic gorge found in this study, the long-term bedrock incision rate might be overestimated if the height of the strath terrace was used to calculate the incision rate. A long-term incision rate of the Peinan River at the Luanshan landslide could be estimated at 14-19.6 mm/yr by calculating the elevation difference of the paleo-channel observed in this study and the current river channel. This can provide a basis for estimating the incision rate of Peinan River in Luanshan area. In summary, fluvial processes and landslides have interacted with each other to form the current landscapes in the Luyeh area.