## Materials and Methods
### UAV-Based Optical Survey for Detection of Coastal Processes and Phenomena  
The optical survey techniques used within the study methodology can be divided into several tasks: mapping of the shoreline, constructing a digital surface model (DSM) for modeling the shoreline position, land use mapping, and pollution monitoring of water areas and coasts. Aerial photography was performed from the small UAV DJI Phantom PRO. According to the different study tasks, flight levels for the surveys were 100, 200, and 400 m above the surface (Table 1). Most extensive and long surveys were performed at 400 m elevation, while human-induced disturbances and shoreline fluctuations were detected from a height of 100 m.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Flight Altitude, m**} & \multirow{2}{*}{ \begin{tabular}{c} **Thematic** \\ **Tasks** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Overlap of** \\ **Acquired** \\ **Imagery, \%** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Mean Spatial** \\ **Resolution of** \\ **Orthomosaics,** \\ **cm** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Mean DEM's** \\ **and DSM’s** \\ **Spatial** \\ **Resolution, cm** \\ \end{tabular}  } & \multirow{2}{*}{ \begin{tabular}{c} **Mean Point** \\ **Density in** \\ **Dense Point** \\ **Clouds,** \\ **Points Per sq.** \\ **Meters** \\ **(Water/Ground)** \\ \end{tabular}  } \\ \hline \multirow{4}{*}{100} & \multirow{4}{*}{RGB} & \multirow{4}{*}{\ \begin{tabular}{c} detection of \\ shoreline \\ positions, \\ monitoring \\ and littering. \\ \end{tabular}  } & \multirow{2}{*}{80} & \multirow{2}{*}{3.1} & \multirow{2}{*}{6.2} & \multirow{2}{*}{97/311} \\  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & & \\ \hline \multirow{4}{*}{200} & \multirow{4}{*}{RGB} & \multirow{4}{*}{ \begin{tabular}{c} monitoring \\ of coastal \\ waters’ \\ pollution, \\ DSM, \\ \end{tabular}  } & \multirow{2}{*}{80} & \multirow{2}{*}{6.4} & \multirow{2}{*}{12.8} & \multirow{2}{*}{63/137} \\  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & & \\ \cline{1-1}  & & & & &  
Photogrammetric processing of optical survey data was performed in Agisoft MetaShape using a Structure-from-Motion (SfM) technique for imagery conversion into 3D models. The software includes a standard processing flowchart with aligning photos, dense point cloud generation, and building of models [43]. According to survey flight levels, the resulting DSM spatial resolution ranges from 6.2 to 25.4 cm. DSMs were then used to produce various parameters for the coastal zone, classification of coastal types, and determining shoreline position. Visual interpretation of UAV-based optical ortho-imagery (in three visible bands -Red, -Green, Blue, or RGB) with spatial resolution from 3.1 to 12.7 cm was performed on a basis of direct distinguishing features as detected objects (such as garbage, landfills, wastewater discharge points, and other sources of environmental problems) can be well identified only on images with a spatial resolution of the first decimeters or higher (Figure 3). At the stage of flight-planning, we also conducted a spatial analysis of water quality in terms of turbidity and algal blooms using satellite imagery from PlanetScope constellation. These multi-spectral data are acquired up to several times per day at a spatial resolution of 3 m, and free access to the data was granted under an individual research license [44]. We utilized surface reflectance products [45] to create short-term consistent time series of such a spectral index as a Surface Algal Bloom Index (SABI). SABI is a robust four-band index for detecting areas with high primary production of surface phytoplankton during the period of algal blooms [46]. SABI was calculated according to Equation (1). \[\text{SABI}=(\text{NIR}-\text{Red})/(\text{Green}+\text{Blue}), \tag{1}\]  
where NIR, Red, Green, Blue-spectral surface reflectance in appropriate channels of PlanetScope imagery.