15:30 〜 17:00
[HGM02-P12] Different evolution mechanisms of fill terraces in the upper reaches of the Laonong River, southwestern Taiwan
キーワード：fill terrace, Laonong River, Taiwan, fluvial landscape
The Meihsiu Creek (MC), Putangpunas Creek (PC) and Chingshui Creek (CC) catchments are adjacent tributaries in the upper reaches of the Laonong River with similar geological and climatic backgrounds, and large-scale fill terraces are developed at the confluence with the trunk river. However, the aggradation ages, scales and times of these terraces are all different. Especially during Typhoon Morakot in 2009, a large-scale landslide occurred in PC, forming a new alluvial fan to bury the older terrace and a barrier lake on the Laonong River, but the MC and CC only experienced smaller-scale landslides. Therefore, this study focused on investigating the landslide characteristics of PC, as well as re-examining and exploring the fluvial landscape differences among these three tributaries.
Based on the preliminary results, we propose four comments, as follows: (1) The large-scale landslide in 2009 of PC may be related to the simultaneous occurrence of the paleocolluvium rotational slide on the right flank and the translational slide on the left flank. This viewpoint is different from those of previous studies, which attributed the movement mainly to the deep-seated gravitational slope deformation on the right flank. (2) The average channel-gradient ratio of PC is larger, and a series of canyons and knickpoints are present. However, the channels of MC and CC are wider and gentler, which can preserve more clastic materials from the upstream, making it difficult for the riverbed to be incised and protect the stability of slopes. Furthermore, according to the age of the terraces of MC and CC noted by previous studies, it can be inferred that the frequency of landslides in the upstream is relatively low. (3) A large amount of loose colluvium is still preserved in the upstream of the PC and is continuously transported downstream. The repeated erosion and aggradation of fan terraces respond to high-frequency and small-scale deposit supply, can be expected to continue in the future. (4) In summary, the landscape of these tributaries represents differential sensitivity to natural hazard events, among which PC has the highest sensitivity. The landscape of each tributary at the same time not necessarily respond to the natural hazard events of different time scales.
Based on the preliminary results, we propose four comments, as follows: (1) The large-scale landslide in 2009 of PC may be related to the simultaneous occurrence of the paleocolluvium rotational slide on the right flank and the translational slide on the left flank. This viewpoint is different from those of previous studies, which attributed the movement mainly to the deep-seated gravitational slope deformation on the right flank. (2) The average channel-gradient ratio of PC is larger, and a series of canyons and knickpoints are present. However, the channels of MC and CC are wider and gentler, which can preserve more clastic materials from the upstream, making it difficult for the riverbed to be incised and protect the stability of slopes. Furthermore, according to the age of the terraces of MC and CC noted by previous studies, it can be inferred that the frequency of landslides in the upstream is relatively low. (3) A large amount of loose colluvium is still preserved in the upstream of the PC and is continuously transported downstream. The repeated erosion and aggradation of fan terraces respond to high-frequency and small-scale deposit supply, can be expected to continue in the future. (4) In summary, the landscape of these tributaries represents differential sensitivity to natural hazard events, among which PC has the highest sensitivity. The landscape of each tributary at the same time not necessarily respond to the natural hazard events of different time scales.