Debris Flow Activity in Trans-Ili Alatau in the 20th — Early 21st Centuries

We examine the debris flow activity in Trans-Ili Alatau since 1900 till the present. Characteristics of the debris flow formation conditions and data on the largest debris flows are presented. A study is made of the interannual and intra-annual variability of debris flow activity. The debris flow activity was characterized by the number of debris flows in a year, and by the total volume of debris transported. For the period from 1900 to 2017 we recorded 481 debris flows. It was found that the genetic types of debris flows are dominated by rain-induced and glacial flows (87 % of the total number of debris flows). The other genetic types (snow-caused, seismogenic and anthropogenic) account for 13%. Most of the debris flows have a rain genesis (71% of the total number of rain-induced and glacial debris flows). Glacial debris flows account for 79% of the volume of debris transported. The volumes of the largest debris flows, both glacial and rain-induced, exceed one million cubic meters. They make up 3% of the total number of debris flows but they transported 70 % of debris. Small debris flows with a volume of less than 10 thousand cubic meters, in terms of their number, constitute 74%; however, they account for a mere 3% of the volume of debris flow deposits. Observations indicate that the earliest debris flows occurred in the second ten-day period of March, and the latest occurred in the first ten-day period of September. The distribution of the number of debris flows according to the dates of their occurrence clearly shows two maxima: one maximum in the second ten-day period of June, and the other maximum in the second ten-day period of July. The June maximum is due to rain precipitation, and the July maximum is associated with glacial debris flows. From 1950 to 2017 there were only 5 (7%) years without any debris flows. Four of them corresponded to the 2010s. The number of years with rain-induced and glacial debris flows was 55 (81%) and 39 (57%), respectively.