The application of drone technology in investigating the morphometric changes of the river (Case study: Vaz river, Chamestan)

Introduction
The newly emerging and rapidly evolving technology of Unmanned Aerial Systems (UASs), covering a wide range of devices and enabling photogrammetric applications from user-driven platform, presents new possibilities for research in geomorphology by obtaining spatially accurate geographic data. The low-level altitude of flight allows researchers to obtain imagery at high levels of detail, which is necessary for a detailed analysis of even fine fluvial forms of the dynamic river channels or landslides. After the early 2010s, the use of UAVs equipped with an RGB camera has become widely applied in geomorphology. It has expanded the capability of low-altitude aerial photo shootings and subsequent SfM photogrammetry. The basic principle of SfM photogrammetry is the same as the conventional photogrammetry, but SfM enables automatic alignment of many (hundreds to thousands) images thanks to automated detection of numerous tie-points, and then produces dense point cloud and rasterized DEMs, as well as orthorectified mosaic images. One of the advantages of SfM photogrammetry is its flexibility across camera platforms. conventional aerial images, and ground-based images are commonly used platforms to 3D models by SfM.
 
Materials and Methods
The Vaz watershed is located in the northern slope of the Alborz Mountain range and in the south of Chamestan city, which in terms of political divisions, this basin is located in Noor city of Mazandaran province. First, using the library method, international and domaestic scientific literature was studied in order to examine the background and different methods of using UAVs in river studies and to prepare a study framework. After imaging, it was processed in Metashape software (Metashape 1.5.5) and orthophoto image and digital elevation model (DEM) were extracted in centimeters in order to prepare cross-sections and check the morphometric and hydrological characteristics of the river.
Results and Discussion
The lateral changes of the Vaz River were studied across 2 different sections of the river. The first level of analysis included building an elevation model based on imagery provided by UAVs and processed in Metashape software, with a pixel resolution size of 4.6 centimeters. Through measuring cross sections of the channel at different flows at the edges and bankfull discharge of the channel, data was acquired. This data included active depth and width of the channel, cross sectional areas, average depth, hydraulic radius and width to depth ratio. These data could be combined with hydraulic data to calculate flow velocity, discharge, stream power, shear stress and other parameters which enable the quantification of river processes. In the statistical comparison of 2 reach at channel edge discharge, some parameters such as cross-sectional area, hydraulic radius, flow velocity, maximum and average channel depth, width-to-depth ratio, channel width susceptible to flooding, channel depth ratio, discharge and Froude number, decrease at the downstream section comparing to the upstream section. However, shear stress and specific stream power of the river increase in the downstream. At the bankfull discharge of the channel, all parameters except specific stream power of the river decrease comparing to the 1st section. The specific stream power increases at the 2nd section. Afterwards the research investigated the change in cross sections across the 2 sections. At the upstream, sand mining, road construction across the channel and construction of a secondary channel had changed the composition and material of the right banks of the river. At the downstream, dam construction, had changed the flow and sedimentation regime of the river.
 
Conclusion
The data acquired from UAVs enables extraction of relevant morphometric and hydrological data regarding the channel and its transverse sections, due to geometric precision and high resolution. The high precision of this data provides an appropriate alternative for field works such as mapping and surveying with different cameras. Orthophoto images prepared using UAVs have enabled the more accurate identification of geomorphic features and landforms. According to data acquired from the 2 different sections of River Vaz, it was demonstrated that upstream and downstream sections are not similar and variation in morphometric characteristics causes difference in hydraulic characteristics such as flow velocity, shear stress and specific stream power. These varieties have led to different geomorphic forms at the studied sections.