Hydro-meteorological preconditions and geomorphological consequences of extreme flood in the small river basin in the wet subtropical zone (the Tsanyk River case study, Sochi region)

Abstract

The overall increase in precipitation and the frequency of extreme rainfall is confirmed by several meteorological observations both on the Black Sea coast of the Caucasus, and in other regions with similar environmental conditions. In this context, research on the drivers of an extreme meteorological event becoming a severe flood and an extreme geomorphological (erosion) event becomes especially important. The article analyses the factors contributing to the development of a flash flood on the watershed of the river Tsanyk (Greater Sochi area) and estimates its geomorphological impact. Analysis of the hydrologic-meteorological situation during the Tsanyk flood on 7–9 September 2018 and in the previous period showed that the event is unique for the investigated watershed from the hydrologic perspective. After a prolonged period without precipitation, the rainfall on 7–8 September resulted in a flash flood with a rapid water level rise (117 cm in 40 min.). We determined the rates and volumes of channel erosion based on a repeated survey of the channel section, estimation of the speed and length of the banks being flushed out — composed of loose sediments and bedrock — using the pin method. The amount of erosion on the banks composed of loosened sediments was on average 2–3 times higher than on the flysch banks. At the same time, the material export for one erosion event in September 2018 is comparable to (or even exceeds) the total amount of bank erosion for a year, previously identified based on stationary observations.