ИСТИНА

Twentieth century was notable for the high activity of natural hazards. In the Caucasus, debris flows were among the most destructive hazards. It is important to study extreme-magnitude cases of debris flows in the current conditions, as well as historical cases. The largest volume of debris flow deposits is associated with the degradation of glacial moraine complexes. The magnitude and parameters of disastrous debris flow events can be used as benchmark values to assess debris flow hazard for specific areas and to model maximum parameters of debris flow activity. We examine results of monitoring the debris flows in the Central Caucasus mountains, where catastrophic debris flows occurred in the 20th and early 21st centuries. Observations of debris flows and research of their formation factors has been conducted under direct supervision and with participation of authors from 1960 to 2012. Recently, in addition to the traditional methods of research, modern monitoring technologies (such as temperature and water-level data logging, and satellite remote sensing) have been introduced. The monitoring covers the upper reaches of the Baksan River (a major tributary of the Terek River) in an area of 2,100 km2, for 112 debris-flow catchments, including 75 catchments with modern glaciation. A continuous series of observations contains information on 420 debris flow events. Dates and locations of debris flow passage have been identified for a period starting in the late 19th century. Lichenometric method for determining the age of deposits allowed to record manifestations of catastrophic debris flows during the 2nd millennium AD for comparison of their magnitudes with current events. The comparison of areas of the modern debris flow deposits with ancient debris flow deposits on alluvial fans and in river valleys, demonstrated that the debris flow disasters of 20th -21st centuries are on a par with the most powerful events that occurred in the past millennium. This suggests that modern debris flow processes are at the maximum of the regional scale, and compare with 0.001% event probability level. This can be used to adjust the debris flow assessment methods. One-time removal of debris material from 1 km2 of the catchment area (runoff module) in different basins of the high-mountain tributaries of the Baksan River ranged from 150,000 to 600,000 m3, equivalent to the layer of debris flow deposits of 40-70 mm per catchment area. A promising task for future research could be a comparative analysis of endogenous and exogenous factors of debris flow formation, using data obtained for the Caucasus and information about extra large debris flow events after the Wenchuan earthquake in 2008 in China.