Simulating rill development in frozen black soil using 3D laser scanning

Snowmelt runoff and soil freezing have been combined to seriously exert on the rill networks and soil erosion intensity in the black soil region of northeastern China. The dynamic development of rill networks can be used to quantitatively extract the parameters of erosion. Previous studies have focused primarily on the runoff erosion caused by rainfall. Only a few studies were to consider the combined effects of snowmelt runoff and freezing. Currently, available methods cannot fully meet the high precision of rill morphology. Therefore, it is very necessary to systematically investigate the rill erosion under combined effects using high-precision extraction. This study aims to simulate the snowmelt runoff scouring on the frozen (-20 °C freezing) and non-frozen slopes (10 °C), refilled with black soil from northeastern China. A 0 °C ice-water mixture was used to simulate the snowmelt runoff scouring. Meanwhile, three-dimensional laser scanning was employed to obtain the point cloud data of the slope surface after different times of scouring, until the eroded surface stabilized. The Digital Surface Model (DSM) of Difference (DoD) with point cloud reverse engineering was used to detect and parameterize the micro-topographic pattern (e.g., development of rill networks and erosion area, erosion volume, rill length, and rill density). The results show that there was a significant impact of freezing-induced temperature on the development of rill networks: (1) The frozen slope was more prone to develop the rill networks. The erosion rate after each scouring was significantly higher than that on the non-frozen slopes. Upon reaching stability, the erosion area, erosion volume, and rill length were 291%, 557%, and 437% of those on the non-frozen slopes, respectively; (2) The cumulative erosion volume of the frozen and non-frozen slopes showed similar patterns during the rill development. But there were noticeable differences along the cross-sectional direction of the rills. On the frozen slopes, the width-depth ratio (R_W/D) of the rill cross-sections decreased rapidly at the rill intersections with accelerated downcutting, whereas, the width and depth of rills increased linearly and stably. On the non-frozen slopes, the R_W/D at the mid-slope cross-sections increased with the number of scouring events, thus accelerating lateral erosion, while the R_W/D at other sections decreased with the number of scouring events with accelerated downcutting; (3) The relative error range was -12.70% to 4.42% for the Differential Digital Surface Model (DoD) with ArcGIS hydrological processing and GeoMagic point cloud reverse engineering models for the frozen slopes, with an absolute mean relative error of 5.48%, and an extraction accuracy of over 95%. The combined extraction with GeoMagic and ArcGIS showed a great contribution to obtain the rill parameters on the freeze-treated slopes. The finding can serve as a promising potential to the rill development under complex eroding settings.