MASS FORMATION OF DEBRIS-FLOWS IN THE LOW MOUNTAINS OF SAKHALIN ISLAND: CONDITIONS AND REPEATABILITY

Abstract

In the debris-flows basins of the Southern and Middle part of Sakhalin island are formed debris-flows, mudflows, slushflow and suspended stream. Average volumes of debris-flows: 500 - 10 000 m3, maximum-more than 500 000 m3. In July-October, periods of mass formation of debris-flows and mudflows are repeated from 1 time in 3-5 years to 1 time in 10-12 years. These periods last from 1 to 3-5 days. These periods occur during deep cyclones (when more than 50 mm of precipitation falls per day). During these periods some hundreds of debris-flows and mudflows form in most mud basins. For example, during the passage of typhoons Ojin and Phyllis on August 3-5, 1981 on the South-Western coast of Sakhalin Island, 154 debris-flows and mudflows with a volume of up to 12 thousand m3 descended on a section 55 km long. At the same time, 55 debris-flows and mudflows with a volume of up to 35 thousand m3 descended on the South-Eastern coast of Sakhalin Island (on a section 30 km long). Periods of mass formation of debris-flows and mudflows on Southern Sakhalin were noted in 1947, 1951, 1964, 1965, 1970, 1972, 1978, 1981, 1992, 1993, 2000, 2002, 2003, 2009, 2010, 2011, 2013, 2016, 2018. The high intensity of the manifestation of debris-flows and mudflows processes on Sakhalin Island is due to the following factors: a large depth of relief (500 – 1000 m), large slopes of debris-flows and mudflows channels (35°-50°), strong precipitation (daily maximum precipitation – 230 mm, hourly – more than 50 mm), as well as the geological structure of debris-flows and mudflows basins: young rocks, weakly cemented, easily eroded and soaked (siltstones, mudstones, sandstones) saturating the debris-flows with clay fractions; strong Intrusive, volcanogenic and metamorphic rocks (diorite porphyrites, andesites, diorites, dacites, Greenstone shales, serpentinites), form a boulder-block fraction of mudflows.