The Relationship between Landslide and Vegetation during Typhoon events in the upstream of Tao-Cheng River, Hsinchu
Date Issued
2009
Date
2009
Author(s)
Chen, Chi-Wen
Abstract
In this study, we discussed the relationship between landslide and vegetation distribution in 6 different typhoon events from 1996 to 2008. Through analyzing the particulate carbon from rivers, we found that the average percentage of the particulate carbon is between 5.57% and 6.39% and the average concentration of the particulate carbon is between 0.22mg/l and 2.39mg/l in the dry season. In the wet season, the average percentage of the particulate carbon is between 1.43% and 2.29% and the average concentration of the particulate carbon is between 0.41mg/l and 5.4mg/l. This result indicated that in the wet season, some geological material from landslide or surface erosion entered the river, causing the increase of the concentration of particulate carbon. When the scenario of high flow and high sediment discharge happened, more suspended load joined in, bringing about the decrease of the percentage of the particulate carbon in the suspended load. The dry season had opposite result. Among several typhoon events, the concentration of the particulate carbon can reach 0.7mg/l to 23.67mg/l because the huge landslides caused a large number of vegetation and soil entering the river. With analyzing the different bands of the SPOT satellite in 10 years, the Normalized Difference Vegetation Index (NDVI) in this study area is between 0.47 and 0.63 before typhoon events and between 0.38 and 0.46 after typhoon events. The highest gap of NDVI value (0.22) occurred in Typhoon Herb and the lowest (0.06) occurred in Typhoon Sinlaku. Extreme disparity of NDVI value would caused by the numbers of landslide areas increasing or a huge scale of vegetation collapse. From mapping the landslide areas, we found that there are higher new generation ratio of landslide, between 69.98% to 83.30%, and lower reactivated ratio of landslide between 10.93% and 40.37%. The landslide ratio is between 0.60% and 1.29%. For the correlation between rock strength and landslide ratio of the three formations in the study area, we can found the highest rock strength of Datonshan formation has the lowest landslide ratio and highest value of NDVI. This result revealed that the formation with higher rock strength, the vegetation will be better and landslides are not easy to occur. From the statistics of the sediment discharge and rainfall in each typhoon events, there are the highest sediment discharge (3.52Mt) and average daily rainfall (276.92 mm) in Typhoon Aere and the lowest sediment discharge (1.8kt) and average daily rainfall (19.88mm) in Typhoon Taraji. For the correlation between landslide ratio, rainfall and sediment discharge we can found that higher rainfall caused the higher sediment discharge. But the landslide ratio would not increase relatively. This result revealed that when high rainfall occurred, the high sediment discharge is not affected by the landslide instantly but the erosion of the colluvial deposit previously or the high flow scraped the river itself did.
Subjects
particulate carbon
NDVI
landslide
sediment discharge
typhoon
Type
thesis
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