Contributions of composite erosion forces on undisturbed Pisha sandstone slope in different seasons

Abstract: Soil erosion is very severe in Pisha sandstone area, located in the middle reaches of the Yellow River in China, and the erosion modulus can reach 30 000-40 000 t/(km2?a). Although the Pisha sandstone area only accounts for 2% of the Yellow River basin, the coarse sediment (particle size > 0.05 mm) derived from this area accounts for 25% of the total sediment deposition in the lower reaches of the Yellow River. The alternation and coupling effects of water erosion, wind erosion and freeze-thaw erosion have become important reasons of fragile ecological environment and serious soil erosion in this area. To reveal the seasonal variation characteristics of multi-dynamic composite erosion in Pisha sandstone area, this study analyzed the seasonal interaction characteristics of water erosion, wind erosion and freeze-thaw erosion on Pisha sandstone slope, and quantitatively determined the contribution of each erosion force to the original Pisha sandstone slope by integrating field observation in-situ, 3D laser scanner and GIS technologies. The data included the four-phase topographic point cloud data of three bare experimental plots from March 2018 to April 2019, the measured erosion amounts of each plot and meterological data of the study area. The three bare experimental plots included undisturbed plots with hydraulic plus freeze-thaw and wind composite erosion, a control plot with hydraulic plus freeze-thaw composite erosion and a control plot with freeze-thaw erosion only, and the size of each plot was 12.5 m by 2.5 m. The results showed that, from March to June and from November to April of the next year, the main erosion forces on Pisha sandstone slope were freeze-thaw plus wind forces, and from July to October the main erosion force was hydraulic force. From March 2018 to April 2019, the total erosion amount of the undisturbed plot, the control plot with hydraulic plus freeze-thaw composite erosion and the control plot with freeze-thaw erosion was 111.63, 84.90 and 19.65 kg, respectively. Namely, the composite erosion amount was the highest for the plot with hydraulic plus freeze-thaw and wind composite erosion, followed by the plot with hydraulic plus freeze-thaw composite erosion, and that with the freeze-thaw erosion. The superimposition effect of multi-dynamic composite erosion on Pisha sandstone slope was obvious. In the three periods from March to June, July to October and November to April of the next year, the erosion proportions of the undisturbed plot were 10.58%, 76.61% and 12.81%, respectively; The erosion proportions of the control plot with hydraulic plus freeze-thaw composite erosion were 10.43%, 78.57% and 11.00%, respectively; and the erosion proportions of the control plot with freeze-thaw erosion were 31.90%, 28.65% and 39.45%, respectively. In the three periods, for the Pisha sandstone undisturbed slope, the contribution rates of hydraulic erosion were 21.85%, 71.42% and 11.12%, the contribution rates of freeze-thaw erosion were 53.09%, 6.58% and 54.20%, and the contribution rates of wind erosion were 25.06%, 21.99% and 34.69%, respectively. Namely, the seasonal erosion amount of each erosion force were the highest for the freeze-thaw erosion, followed by wind erosion and water erosion from March to June and from November to April of the next year. Water erosion followed of the total erosion amount. In the whole study period, water erosion accounted for 58.45%, and freeze-thaw and wind erosions both accounted for 41.55%. In 2018, the influence degree of each erosion force on Pisha sandstone slope was the highest for water erosion, followed by freeze-thaw erosion and wind erosion. In addition, 3D laser scanning technology provided a convenient and efficient mean for monitoring topographic fluctuation and calculating soil erosion in complex terrain areas. The research results can provide valuable information for comprehensive control of composite erosion in Pisha sandstone area.