The snow-thaw runoff of diurnal and seasonal permafrost is the main geomorphic agent in cold, high-latitude and high-altitude regions [1
]. Seasonal fluctuations in circulation and temperature lead to daily fluctuations in soil, resulting in the thawing of permafrost and ice, which can significantly alter soil erosion [5
]. Meltwater is more conducive to soil erosion than rainfall in both mid- and high-latitude upland watersheds [6
] and threatens soil and slope stability [7
Previous research has focused on the development of gully erosion on natural landforms [9
], while man-made landforms (e.g., open-pit mining dumps) have been neglected. In northern China, hundreds of labile, gully filled open-pit coal mine dump fields are surrounded by natural steppe. Here, open-pit mining has caused severe disturbances to the mine field; moreover, the dumping sites, containing a large amount of peeling material, are barren and characterized by high slopes (usually up to 100 m), loose slopes and soil compaction platforms, complex material composition, and uneven subsidence [11
]. This leads to the destruction of vast amounts of vegetation communities [12
] and to an extremely serious risk of soil erosion [13
]. Gullies caused by freeze-thaw erosion and meltwater erosion have severely eroded the soil in the dump, inducing slope stability of the dump and sedimentation (one of the most harmful [14
]) of surrounding grassland [9
]. To understand the dynamics of gully erosion, it is necessary to perform an accurate measurement of soil erosion during a freeze-thaw cycle; in this way, it is possible to provide a basis for both land management, and soil and water conservation management.
Land managers should be able to choose effective erosion control measures. For this purpose, gully erosion should be analyzed quantitatively. The method of gully extraction relies on the collection of geographic data. The most traditionally used methods are simulation experiment [19
], field surveys [20
], 137Cs approach [22
], and numerical simulations [23
], which is extremely complicated, time- and resource-consuming work. Existing erosion modeling studies include applications and adaptations of simple empirical models, such as the Universal Soil Loss Equation (USLE) [24
] and its revised version, the RUSLE model [25
]. The ‘vertical column’ representation, used in the geomorphological continuity equation, is generally applicable to the vertical variation of the surface height, but not to the horizontal movement of steep surfaces such as steep slopes and riverbanks [26
]. Over the past decade, the advances in remote sensing technologies have greatly facilitated the mapping and quantification of soil erosion processes [10
With the development of unmanned aerial vehicles (UAVs) and sensors, the technology of low-altitude (<1000 m) photogrammetry based on UAV platforms has shown its unique superiority. UAV Photogrammetry has the feature of automation; moreover, it has the advantages of a flexible system, convenient take-offs and landings, low cost, and high imaging quality. In addition, it can rapidly cover small areas, while at the same time cover difficult flight areas and provide high-resolution images of complex terrain areas. As such, it has rapidly become a complement of both field surveys and satellite and aircraft remote sensing [31
At present, the UAV Photogrammetry technology has been widely used in topographic surveys [32
], environmental monitoring [33
], vegetation information extraction, coastal zone extraction [35
], precision agriculture [36
], glacier dynamics [37
], landslides [38
], cultural relics protection [39
], and building risk assessment [40
]. In addition, thanks to the use of multiple flights and of single tilt-shift lenses to improve data accuracy, the spatial information density and accuracy of UAV Photogrammetry can be compared with that of airborne LiDAR and TLS [41
]. Several authors have adopted centimeter-level precision, high-resolution Digital Surface Model (DSMs) volume estimation, erosion rate measurement, morphological analysis, and trench monitoring from UAV image calculations [31
]. In this study, we employed a low-cost quadrotor to perform both a multi-temporal DoD calculation and a micro-topography change detection at a hillslope in an open-pit mine dump. Furthermore, we estimated the erosion of the study area during a winter freeze-thaw cycle and the relationship between the degree of erosion and the micro-topography of the area.
The majority of existing gully erosion studies have focused on runoff erosion during the rainy season [45
], while neglecting meltwater erosion. Therefore, the purpose of this paper is to investigate the effects of meltwater erosion on gullies during the winter season. We acquired data by using oblique photography and developed a comprehensive 3D model of the gully to quantify the amount of erosion produced in the gullies. The study was performed in the Baorixile open-pit coal mine dump site in Hulunbeier, China.
A spatial analysis (including the geomorphometry analysis and the hydrological analysis) of the erosion gully of the dump was performed using the centimeter-resolution DEMs obtained by the UAV. The measurement of the amount of erosion in the gully by using conventional measurement methods may be an extremely complicated, time- and resource-consuming work. The use of UAVs and SfM technologies may significantly improve productivity and measurement accuracy. Using 3D and GIS spatial analysis technologies, it may be easy to obtain the characteristic parameters of the typical erosion gullies of open-pit mine dumps. By performing a statistical analysis of the slope, slope length, LS factor, aspect, and average daily solar radiation of the erosion gully, as well as a regression analysis with the erosion degree, a high causality was found in this study between the erosion degree on the one side, and slope and average daily solar radiation on the other side, during a freeze-thaw cycle.
This study showed that significant improvement to measurement efficiency and accuracy can be achieved by augmenting traditional soil loss and runoff measurement with 3D surface change information. High-resolution DEM can show the fragile areas of erosion and development of the drainage field during a freeze-thaw cycle. Both UAV and SfM technologies provide a solid basis for the establishment of a freeze-thaw erosion model of soil gully erosion, which is of great significance to monitor the development of surface erosion.