4.3. Discussion
Using different remote sensing methods allows quantitative assessments of several exogenous processes at different scales and qualitative levels. Landslide processes may have different appearances, and their assessment requires different approaches (Broeckx et al. , 2019, 2020). Each tool solves the problem differently. Volumetric landslide changes evaluation is possible by calculating the difference of 3D models obtained by terrestrial scanning systems and unmanned aerial vehicles. Assessment of planar displacements is possible by visual interpretation of remote sensing data and field monitoring of the boundaries of the study object using a total station survey. Satellite images, as well as simple aerial photos, do not allow to produce a volumetric picture. However, TLS does not allow to establish the exact position of the landslide body boundaries. Scanning total stations, allow to solve classical tachymetric tasks - that give the most exact position of object borders under research; and to make scanning of landslide object for the subsequent three-dimensional reconstruction. TLS helps to estimate volumetric changes with millimeter accuracy. However, when estimating landslide structures located on the banks of reservoirs, access from all sides is difficult. It may give so-called shadows – areas with no information on three-dimensional data. The UAV method has not such a disadvantage; however, even though the method allows creating three-dimensional models comparable to TLS detail and accuracy, multi-time measurements alignment is still tricky because of the UAV navigation system inaccuracies.
Nevertheless, the error of georeferencing of multitemporal models is considered to be acceptable when assessing landslide process changes because of their high intensity. Returning to the most optimal method, weighing all the advantages and disadvantages of the applied methods, it can be noted that the use of UAV to assess planar and volumetric changes on slopes allows obtaining both a three-dimensional model and a ultrahigh-resolution orthophoto, with which the retreat of the slope scarp can be monitored. However, as noted earlier, the construction of accurate difference-time maps requires high-precision positioning of the aircraft itself, which is achieved using high-precision GNSS receivers that allow making corrections in real-time kinematics or post-processing. As the established practice of using such airborne systems has shown, the best results in comparing multi-temporal data can be achieved by simultaneous use of UAV image data with high-precision positioning, taking into account corrections from the base station and ground reference points.
The use of modern unmanned systems allows taking images with multispectral cameras that provide additional possibilities for analyzing the relationship between the spectral characteristics of rocks composing the landslide body and the intensity of the processes. However, comprehensive mathematical and statistical modeling is possible only with spatial data on fundamental factors of landslide formation – the steepness of slopes, soil texture and type of rocks, air temperature and precipitation data, especially for the winter period, groundwater depth data, and reservoir level fluctuations. However, for verification of modeling results, one needs data on currently developing landslides along the Kuibyshev reservoir banks, which is to be collected in the near future.
Active shoreline erosion currently occurs in almost all water reservoirs in Russia. In total, 36% of the shoreline of all water reservoirs in Russia is affected by these processes (Burova, 2020). Shoreline erosion is most active within large reservoirs with a capacity of more than 10 million m3. For such water bodies in the European part of Russia about 40% of shores is transformed, in Siberia - 36%, in the Far East - 35%, while in small reservoirs, located in the same regions, only 13-15% of the total shoreline is affected by shoreline erosion (Ovchinnikov & Maksimishina, 2002).
Among the large water bodies of the European part of Russia, the most affected by bank destruction are the Kuibyshev (75%), Volgograd (72%), Saratov (70%), and Gorky (65%) reservoirs, located in forest-steppe and steppe zones. More than 50% of the banks are actively retreating at the Kama and Novosibirsk reservoirs. For other water bodies in Russia, shoreline degradation usually occurs on less than 40% of the shoreline.
Analysis of actual data on linear shoreline retreat rates showed that the maximum average annual retreat rates during the first ten years of development of water reservoirs, i.e. during the first, active stage of the process development, amount to 10-20 m/year. In some years, the indicated rates of bank erosion at the first stage can be significantly higher, up to 100 m/year. Thus, in the eastern part of Kamskoye Ustie (Sites 1-2), a 70-90 m wide strip of coastline was destroyed by landslides during the first 30 years of the reservoir’s existence. The average annual intensity of shoreline processes of the largest reservoirs of Russia at the second stage of development in the regime of steady-state or slowing-down deformations is, as a rule, significantly smaller and usually does not exceed 1-2.5 m/year. The data obtained at the studied site are comparable with the data of previous studies (Dedkov, 1991), conducted mainly by standard surveying methods.
Nevertheless, at some sites, recent studies show much larger values of shoreline displacement, mainly in the areas with loams. This, for example, occurs near the village of Izmeri in Spassky District, Republic of Tatarstan (55.130452, 49.469740), where maximum retreat values reach 9 m/yr, with average values of 1- 6 m/yr (Usmanov et al. , 2018).
Observations of coastal retreat and landslide processes have been made at the Kuibyshev reservoir (Kotlyakov et al. , 2007; Gaynullinet al. , 2014a, 2014b; Bespalova et al. , 2020), mainly focusing on the assessment of coastal retreat and land losses. Long-term observations of landslide processes are conducted mainly by state agencies by installing ground control points at random sections. Unfortunately, these data are not available for scientific studies and are not published.
The use of modern methodological approaches allows obtaining information on landslide processes independently, quickly, and efficiently. These allow estimating changes on the whole front of the landslide slope and dangerous areas, inaccessible to observations by traditional methods.