TRANSIT AND ACCUMULATION OF THE GERKHOZHAN-SU RIVER DEBRIS FLOWS AT THE SECTION OF DEBRIS FLOW CHECK CHANNEL IN THE TYRNYAUZ CITY
Debris flows of the Gerkhozhan-Su River are distinguished by high activity and danger and pose a threat Tyrnyauz city (Central Caucasus) and its inhabitants. To protect the Tyrnyauz city from debris flows of the Gerkhozhan-Su River a debris flow check channel was built in the early 1960s. It was partially destroyed after the disastrous debris flows of 1999–2000 and then restored. Subsequent debris flows (2011 and 2017), passing through the channel, left traces on its walls and also deposited debris material at the bottom of the channel, gradually filling it. Specific characteristics of the transit and accumulation of debris flows at the section of debris flow check channel were determined using route surveys and video-footage analysis of the Gerkhozhan-Su River debris flows. The dynamics of debris flows on August 14–15, 2017 at the section of debris flow check channel was recorded on numerous videos. The photo and video information was processed in the ArcGIS software – scaled multi-temporal photos taken before and after debris flows were linked and combined with video frames. The debris flow waves passage velocity was determined by means of video materials with the measurement of distances on a high-resolution satellite image. Debris flows in 2017 were sweeping through the channel at a velocity of 9–12 m/s. Sections of temporal debris flow mass deposit with the preserved structure of debris flow were revealed. Subsequent debris flow waves passed over and carried the deposited debris flow mass down from the channel. It is shown that the determination of the debris flow cross-section from debris flow traces on the walls of the channel can lead to overstatement in the values of the calculated debris flow discharges. It is revealed that after each debris flow at the bottom of the channel, the thickness of the debris flow deposits increased by 1.5 m on the area of the former pedestrian bridge. Protection role was noted of debris flow check channel and the need for its timely cleaning from deposits to ensure the debris flow safety of the Tyrnyauz City
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