Аннотация:The Big Sochi is a lovely resort on the Black Sea coast of the Greater Caucasus. It has always been a region of high landslide risk due to widely spread clays and marls saturated by water from abundant precipitations. Besides, it is a region of rather high seismicity. In recent years landslide monitoring and risk assessment have become vital due to strongly increasing anthropogenic impact.
The Big Sochi area amounting to 3500 sq.km stretches along the Black Sea coast for about 150 km. Near the coast it is a highly populated area while in a few km from the beach there are only isolated villages in the mountains. A plenty of existing landslides are shallow, very slow (rates of movements up to several sm/Y) with maximum dimensions up to tens or a few hundred meters at the most. InSAR methods seem to be very promising for landslide remote sensing in the Caucasus region due to application of data of frequent regular inspection of large hard to reach areas, acquisitions during day and night almost independent from the meteorological conditions. Besides, existence of archived data makes it possible to follow historical changes in landslide activity. We discussed some aspects of InSAR application for landslides studying on the Black Sea coast of the Northern Caucasus using satellite radar imagery with different wavelengths in [1-3]. Nowadays usage of S-1 images is of particular interest because of short time-interval between acquisitions (6-12 days) and free data access in almost real time of sensing via Internet.
The Big Sochi area has been fully covered by Sentinel-1 acquisitions from ascending and descending tracks since 2015. SBAS ENVI SARscape processing of more than 200 acquisitions from 43A, 145A and 123D tracks for the time period 2015-2018 enabled us to develop detailed surface displacement maps. Based on the calculated surface displacement fields the alert maps were plotted showing places where average displacement rate in the line of site (LOS) projection exceeds the 10 mm/Y threshold. Joint analysis of the S-1 InSAR alert maps with existing GIS maps of the study area (topographic, maps of landslide phenomena and risk assessment [4], interactive map of current landslide events) enabled us to update for several regions the existing landslide inventories based on ground survey. In the Big Sochi area ground survey have problems with mapping of a plenty of small in dimensions and very slow moving landslides in highly populated areas where the majority of buildings are private houses with restricted entry. So, field works provide comprehensive data for undeveloped areas and incomplete picture for densely built-up territories. On the contrary, compact districts are very favorable for S-1 InSAR while rural slopes have lack of reflectors of radar signal. So, in the Big Sochi these methods well complement each other. About 100 new active landslide phenomena were revealed with S-1 InSAR images in the Big Sochi. The time series analyses permitted us to conclude that for the majority of landslides the period of activation is usually between November and June, although many landslides move permanently.
Thus, S-1 InSAR in the coastal part of the Big Sochi area appeared to be a powerful and low cost tool improving the existing systems of landslide monitoring based on ground survey.