The Study on Fluvial Morphological Change of Daan Gorge
Date Issued
2012
Date
2012
Author(s)
Chen, Shiuan-An
Abstract
The position of Daan Gorge is between Zhuolan in Miaoli and Dongshi in Taichung, and it formed because of the tectonic uplifting of Chi-Chi Earthquake in 1999 and the erosion process of rejuvenation. This gorge formed before 2007, the length and width was about 1 km and 500 m respectively. Because the changes of river systems were very complex, and the understanding of tectonic uplifting and river erosion processes in short-term was limited, moreover, the environmental monitoring and managements of this area were necessary, so this research collected the meteorological and hydrological data first, then analyzed the characteristics of fluvial morphological changes of Daan Gorge by aerial photos, in-situ observations and channel cross-profile measurements. Finally, this research established the models of channel morphological changing of the channel slopes.
The slopes of channel sides were between 15.7 and 78.5 degree, and the depth of the main channel was about 10 to 15 m. As far as the long-term changes, this research figured out that the widths of channels were increased from about 40 m to more than 100 m first, because of the obstructing of tectonic uplifting; after 2003, it formed gorges quickly because of the process of deepening, widening and transportation, and the widths of channels were decreased to about 10 to 30 m. As far as the short-term changes, because there were no large rainfall and discharge events between the research period from July, 2011 to May, 2012, so the magnitudes of morphological changes were not large. On the top of the slopes, because they eroded directly by rainfall, and the rock types were mainly mudstone and shale, so the retreated ranges were larger, the largest average retreat range of the slopes were 0.74 m in the research period. At the bottom of the slopes, if they were in positions where the river eroded directly, the largest average retreat range would reached 0.39 m, but if they were not in positions where the river eroded directly, the changes were not large. In the middle parts of the slopes, the differences of the changes were large, and the positions of erosion and deposition processes were mainly controlled by slopes, but slopes may influenced by rock types and dip angles of rock formations indirectly. The mechanism of slope retreats was that after the river and rainfall eroding, the sandstone layers were often protrude the shale layers by differential erosion processes, therefore, the sandstone layers finally collapsed by gravity because of lacking support beneath, making the shale layers on the top and at the bottom of sandstone layers collapse easier.
This research finally established four types of channel slope changing models. However, the calculations of hydrological processes, the measurements of rock strengths and landforms under water level, and the weighed analyses of the impact factors of fluvial morphological changes all needed more analyses by further researches.
Subjects
Fluvial Morphological Change
Daan Gorge
Tectonic Uplifting
Total Station
River Channel Profile
Type
thesis
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