## 1 Introduction
(**a**) Schematic diagram of the tailings pond; (**b**) laser measurement; (**c**) slope estimation method. instrument due to tailings accumulation. Based on the slope estimation method, the fixed section estimation method was proposed. This method assumes that the dry beach profile is approximately a fixed surface under the condition of regular ore drawing and will only rise in the vertical direction with ore drawing. A coordinate system of the reservoir length and dam height is established, and monitoring points are set to fit the initial dry beach profile to obtain the dry beach profile function. Then, the length of the dry beach and the elevation of the beach top are calculated by combining the external slope function of the dam body and the reservoir water level function. In recent years, Unmanned Aerial Vehicles (UAVs) have revolutionized remote sensing and photogrammetry, providing fast, versatile, and high-resolution solutions for environmental monitoring. UAV technology is characterized by high-efficiency data acquisition and high-precision measurements, making it a fundamental tool for modern terrain and environmental analysis. With continuous technological advancements, UAVs have been widely adopted in various fields, including structural geology [14; 15], geomorphology [16; 17; 18], monitoring hazards in coastal and cliff areas [19; 20], disaster monitoring [21; 22], construction [23; 24], and agriculture [25; 26]. Many commercial and open-source software programs have been developed [27]. Additionally, some scholars have explored their application in tailings pond monitoring [28; 29]; however, their specific use for dry beach monitoring remains relatively limited. With these developments, photogrammetry and image processing techniques have also been increasingly applied to dry beach monitoring. [PERSON] et al. [30] trained a neural network model to automatically divide the water and dry beach using the improved Mask R-CNN algorithm. On this basis, pixel coordinate values were extracted and combined with the monocular distance measurement model to measure the length of the dry beach. [PERSON] et al. [31] used the YOLOv5-seg model to segment satellite images to extract the dry beach area and used the edge-detection algorithm to extract the contour of the dry beach and determine the length of the dry beach. [PERSON] et al. [32] proposed an online measurement method of the dry beach length based on photogrammetric monitoring. [PERSON] et al. [33] used a monocular camera to obtain images of the water boundary on the tailings dam, extract the pixels of the water line, and measure the length of the dry beach in real-time using the geometric conversion formula between the pixel length and the actual length of the monocular camera. These methods calculate the distance based on the relationship between image pixels and the actual length, which have the advantages of high automation and low cost in dry beach monitoring. However, due to the lack of three-dimensional spatial information, the description of the surface morphology and slope of the dry beach is relatively simplified. The slope measured using this method is the average slope, based on the dry beach length and the elevation difference between the beach top and the water surface. However, it is challenging to determine the slope at any point of the dry beach. In this paper, a new semi-automatic method for measuring the length and slope of the tailings pond dry beach based on UAV 3D model reconstruction is proposed. UAV-based 3D reconstruction and point cloud models can accurately capture the surface changes and terrain details of dry beaches, providing detailed measurements of their length and slope at any location.