利用高精度地形測量觀察底岩型河川在洪水事件時的搬運作用

Abstract

在河流地形演育中，河流如何搬運沉積物是一個重要的課題，特別是在洪水事件時對於大量或大型沉積物的搬運，經常主導著河流地形的變化。過去研究受限於現地觀察記錄地形的變化較為困難，河流作用研究中多以電腦數值模擬或縮尺模型在實驗室內進行，此二者皆有無法完全忠實反映現地河流狀況的問題。然而隨著科技的進步，高精度的地形空間資訊已可有效率的取得，並得以廣泛的使用，本研究利用地面雷射掃描法(terrestrial laser scanning, TLS)針對立霧溪中游段燕子口一帶現地測量河流地形，並利用不同時期的測量結果觀察河流沉積物的搬運作用。 燕子口河段中分布了許多從支流帶來的大型片麻岩礫石，其大小可達十數公尺，其岩性與周圍大理岩底岩差異甚大，易於辨識，且該河段的上下游皆無相似片麻岩礫石分布，而具有其獨特性。故本研究以這些礫石為目標，在雨季及乾季前後利用TLS觀測河道礫石的分布，比較不同時期的礫石變化，並與水文資料比對，討論乾季及雨季中不同水流情況下的搬運能力，以及不同尺寸礫石與搬運距離的關係及搬運模式。本研究將搬運作用分為滑動、傾倒、轉動、滾動、跳動等五類，根據礫石型態不同可對應到不同的搬運模式與搬運閾值，其結果發現巨型礫石可搬運的主因並非單純受水力大小影響，而是和礫石的三軸位態及其樣貌有關。 此外本研究對比自動攝影系統每半小時所拍攝之河道照片，分析颱風事件對河流搬運作用所造成之影響。除了觀察河流沉積物的搬運作用外，TLS的高精度測量結果亦可觀察河流侵蝕作用中的磨蝕與拔蝕現象。綜合以上，本研究以TLS獲得了高精度的地形模型，並更進一步了解了河流沉積物的搬運作用過程。

How sediments are transported in rivers has long been a critical issue in fluvial geomorphology, especially the transportation process of large amount of sediments during flood events. In previous studies, this question has been generally tackled using numerical simulations and flume experiments. Results from direct observations of sediment transport in the field are scarce. We therefore intend to observe in situ fluvial sediment transportation processes during flood events based on terrestrial laser scanning (TLS) technique in a mountain bedrock channel in eastern Taiwan. Along the Swallow Grotto (Yanzikou) section of the Liwu River, there are many huge boulders with different sizes, up to tens of meters. Most of these boulders, which came from rock falls of a nearby tributary, are composed of gneiss, distinctively different from the marble bedrock of the river channel. Thus, we chose these boulders as our survey targets in this study. We surveyed the boulders by TLS annually between the rainy and dry seasons to observe their geometry. By comparing the scanning results of the river segment in different time, we are able to analyze the movement mechanism and transport distance of each boulder and the relationship between the transportation and fluvial hydraulic conditions. We divided the transportation modes into 5 types: sliding, toppling, rotation, rolling and saltation, and analyzed the relationship between transportation modes, boulder conditions, and the size threshold of transported boulders. We found that the hydraulic condition is not the only reason to cause the movement of the boulders, but the condition of the boulders is also an important factor to affect their movements. We used an automatic camera to take photographs at the same site every 30 minutes to identify the flood events that caused the boulder movements. Furthermore, we also observed processes of fluvial erosion in this study based on the TLS data, including abrasion and plucking processes of the boulders. These results would allow us to further understand the sediment transportation processes in various geomorphic systems.