ИСТИНА

Big Sochi has always been a famous Russian rresort and since the Olympics Sochi-2014 its popularity has been growing constantly. Monitoring of surface deformations which develop due to increasing anthropogenic loading is vital. Our lab in the Schmidt Institute of Physics of the Earth of the Russian Academy of Sciences has been studying different aspects of InSAR application in the Big Sochi region since 2012 (DmitrievP.N. et.al.,2012, Kiseleva et al., 2014; Mikhailov et al., 2014, Smolianinova, 2018,2019,2020). Adler is one of the most favorable parts of the Big Sochi for InSAR monitoring because of high density of low rise residential development being perfect reflectors of radar signal, moderate and smooth topography (up to 300-400m) and snow free winters. For this region extremely slow surface movements (several sm/Y) are typical. We present surface deformation maps based on processing radar images of ALOS-1 (18 images 2007-2010), ENVISAT (12 images 2010-2012) and Sentinel-1 (more than 300 images 2015-2020 from 2 ascending and 2 descending orbits) SBAS ENVI SARscape software was used for processing. Surface deformations in the Adler district are mainly caused by two processes: subsidence and landsliding. On the InSAR maps of surface movements landslides and subsidence areas here can be easily distinguished. Zero topography is a reason to consider a "hot spot" (where surface deformation exceeds a given threshold value) as a subsidence area and alternatively "hot spots" on hill slopes are most likely landslides. The main subsidence areas in Adler are located in the Imereti lowland which is situated on the southern slope of the Greater Caucasus Mountain Range between the Mzymta and Psou rivers. It stretches along the Black Sea coast for about 8 km and 0.8-2.0 km inland. Before construction of the Olympic-2014 facilities started here in 2007, the territory of the lowland was almost undeveloped, occupied by swamps, marshes and regularly floodable agricultural fields. The specific organic, so called “weak soils” are widely spread here, particularly in the central part of the lowland. Being wet these soils may lose their bearing capability and can be squeezed out from under constructions, thus, leading to their subsidence. Special technologies were involved to consolidate soils of basements. We applied InSAR to monitor efficiency of all the stabilizing measures. For the Imereti lowland we present maps of pseudo-vertical displacements for all the datasets of images. Pseudo-vertical movements were estimated as vertical projection of the LOS displacements assuming horizontal displacements in this flat area being negligibly small. We revealed seven main areas of subsidence in the Imereti lowland derived from S-1A datasets for 2015-2020. Maximum values of total subsidence since 2007 were fixed in the Central part of the lowland where thick lagoonal deposits with considerable content of “weak soils” are located according to geotechnical mapping. In this area subsidence for the 5 years period 2015-2020 is more than twice as big as for 2007-2012. Total subsidence here for the whole period 2007-2020 is more than 40 cm. Analyzing time-series we pointed out continuous subsidence for all the seven subsiding areas although rates of subsidence vary. There is no noticeable correlation between subsidence and precipitation. We revealed four areas of progressive subsidence and three stabilizing areas. These results help to estimate efficiency of drainage systems and stabilizing actions for the particular areas. Based on S1A 2015-2020 Vlos maps we revealed about 20 active landslides from tens to few hundred meters in size. About half of them were not previously registered by field works because it is difficult to register small displacements on territories occupied by private housing. It was found out that incorporating acquisitions from different orbits (43A and 145A as well as 123D and 21D) reduces layover and shadow areas and considerably improves detection of active landslides. We selected five landslides clearly visible on Vlos maps to demonstrate detailed investigation of landslides movements. Rate of movements of all these landslides was not uniform during period 2007-2020. To compare activity of the landslides we computed down-the-slope rates (Vsd) where it was possible. We also averaged Vlos values in six month running window. These averaged time-series demonstrate periodic behavior. Extreme values of averaged values for different landslides sometimes are shifted within no more than 1-2 months. On the contrary to subsidence in the Imereti lowland, landslides activity depends upon precipitation. Extreme values of the mean Vlos for six month intervals match peaks of the graph of the total precipitation for six months intervals. Maximum displacement rates correspond to February-April, and minimum - August-October. Usually heavy but short rains do not trigger landslide activity while long lasting moderate rainfalls may be crucial. Our results demonstrate high efficiency of InSAR to detect landslides and subsiding areas and analyze dynamics of these movements in the highly populated Adler settlement of the Big Sochi, where extremely slow movements can hardly be fixed by field methods. InSAR demonstrated its efficiency to detect many new landslides and subsidence areas as well estimate activity of areas during the last 13 years, i.e. from starting construction of Olympic-2014 facilities up to now.