HIGH-RESOLUTION REMOTE SENSING IMAGERY CHARACTERS OF ZHOUQU EXTRA-LARGE-SCALE DEBRIS FLOW AND GEOLOGICAL SIGNIFICANCE OF IDENTIFYING ANCIENT DEBRIS FLOWS DIRECTLY BY USING REMOTE SENSING METHODS IN GANSU PROVINCE
Zhouqu extra-large-scale debris flow in 2010 was the newest action since Quaternary in Zhouqu area and caused the severest casualties and property losses in the history of the world's debris flow hazards. It could provide scientific bases for debris flow disaster relief and prevention to analyze the spatial distribution of debris flow deposits and forecast time of occurrence of debris flows in the future accurately. The primary task was to identify the spatial distribution of debris flows in different periods quickly and accurately by using an advanced technology. Remote sensing technology has the unique advantage in a large amount of data acquisition, high spatial resolution and quickness and can play an important role in identifying Zhouqu extra-large-scale debris flow and ancient debris flow deposits in different periods. On the high-resolution aerial remote sensing image, Zhouqu extra -large-scale debris flow had obvious spectral characteristics which could be used to accurately delineate the scope of debris flow disaster. Accumulation forms including lateral moraines on both sides of the valley formed by the debris flow and the fan at the terminal of the accumulation area were other identification signs. On the optical aerial images (0. 2m), the fans in the outlet of the tributaries of Bailongjiang River which lie to the south of Wachang Village, and the south-west of Malianping Village and lateral moraine embankments in these tributaries had the same typical shape features as those of Zhouqu extra-large-scale debris flow deposits. The fans correspond to 1~(st) terraces completely which were formed in the mid-Holocene in Zhouqu area. So these fans were formed by the ancient debris flows. While through comparing radar remote sensing backscattering coefficient with modern debris flows and analyzing the sedimentary deposit sections, the ancient debris flows still exist in the south-east of Zhouqu County. The remote sensing direct identification of debris flows has the important geological significance through comparative analyses of modern and ancient debris flows in different periods and spatial positions that it can not only reveal the development history of the debris flow since Quaternary in Zhouqu area and reflect indirectly the process of climate change at the geological period scale, but also provide data support for susceptibility analysis of the debris flow in the future.