| Summary: | 碩士 === 國立臺灣大學 === 地質科學研究所 === 106 === 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.
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