## 5 Discussion on Wind-Sand Hazards and Sand Control along the Railway
### Sand Control System  
#### 5.2.1 Mechanical Sand Control System  
Studies have shown that single sand control measures fail and become ineffective after a short while in aeolian sand environments [29], causing railways to be exposed to wind-sand hazards again. On that account, a sand control mode combining mechanical and biological measures has been adopted along the railway in the study area according to the \"blocking-fixing\" principle to achieve a satisfactory sand control effect. Mechanical sand control measures mainly include upright sand fences and reed checkerboards. Upright sand fences, erected on the outermost side of the mechanical sand control system, realize wind prevention and sand resistance by reducing incoming wind velocity and depositing sand particles. The wind-sand flows intercepted by upright sand fences enter reed checkerboards, which play a sand-fixing role by increasing surface roughness (Figure 11a). Aeolian sand environments have spatiotemporal variation characteristics, so the setting of the sand control system should fully consider the variations in the angle between railway and sand drift direction and in sand-driving wind direction. The railway trend in the study area is almost parallel to the sand drift direction, so sand-guiding measures should be stressed. The upright sand fences on the outermost side are arranged in a folded pattern (Figure 11b), as these can achieve a more significant sand accumulation effect than straight sand fences can under this pattern. Wind directions in the summer are disorderly in the study area, especially the Ruoqiang area. The angle between the sand-drift direction and railway is large in the summer, so straight upright sand fences were adopted in some sections (Figure 11c).  
Sand control measures should be adapted to local conditions. Sand control width should be set reasonably to reduce capital and human resource inputs. Spatial variations  
Figure 11: Mechanical sand prevention system and its specific patterns: (**a**) overall layout; (**b**) folded sand barriers; (**c**) straight sand barriers.  
Figure 10: Railway trend and annual RDD: (**a**) Qiemo; (**b**) Ruoqiang.  
in aeolian sand environments require that sand control measures be adjusted accordingly. Concerning areas facing serious wind-sand hazards, the number of rows of upright sand fences and the width of sand-fixing checkerboards on both sides of the railway should be adjusted for wind-sand hazards. Figure 12a shows the passage of the railway through a mobile dune area. The upwind side is erected with three rows of upright sand fences, which are 240 m away from the railway at the closest. The downwind side is erected with two rows of upright sand fences, which are 170 m away from the railway at the closest. Affected by the aeolian sand environment, the status of upright sand fences on the upwind side is inferior to that on the downwind side. Figure 12a,b shows that the distances between the upright sand fences on the upwind and downwind sides and railway are reduced by 30 m, and one row of upright sand fences is reduced on each side of the railway. As shown in Figure 12c,d, each side of the railway is erected with one row of upright sand fences. Differing from Figure 12b, both Figure 12c,d show reduced distances between the upwind sand fences and railways and increased distances between the downwind sand fences and railways. Additionally, in Figure 12d, the downwind side is arranged with lateral guidance devices to lead wind-sand flows through railway culverts, thus weakening the direct erosion of railway subgrade by the wind-sand flows.