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.