## 5 Discussion on Wind-Sand Hazards and Sand Control along the Railway
### Sand Control System
#### 5.2.2 Biological Sand Control Measures  
Studies have revealed that the morphology of dunes varies greatly from one side of the river to the other. The shrubs on both sides of the river intercept some sand particles. Their growth also modifies the near-surface wind regime, reduces wind velocity, and increases  
Figure 12: Distribution of sand control system along railway: (**a–d**) distances between upright sand fences on both sides and railway and number of upright sand fences. 1–3 represents the upright sand fences.  
the threshold of sand-driving wind velocity, playing an important role in wind prevention and sand fixation. The Qarqan River section (DK149-DK161) parallel to the railway has a high vegetation coverage rate. When wind-sand flows are slowed and intercepted by vegetation and the river, the sand drift rates are significantly reduced, indicating that vegetation provides effective protection for the railway. Therefore, vegetation coverage along the railway can be gradually improved by fully utilizing the water systems flowing through the study area. Biological and mechanical sand control measures can be combined to reduce wind-sand hazards. The perennial rivers flowing through the study area include the Ruoqiang River, the Washixia River, and the Qarqan River. Originated mainly in ice and snow melting water, these rivers show great differences in flow during wet and dry seasons. Their flood peaks usually occur from June to August, i.e., the hottest period. The winter and the spring are dry seasons. In the flood peak period, the downstream inclined plain in the Gobi area is threatened by floods. The underground water level is highly variable due to the topography or landforms. The study area has a harsh natural environment, so the species selected for biological sand control must be able to withstand drought, cold, high temperature, and wind erosion. Based on the natural environment and wild plant resource distribution along the railway and the experience from built-up projects, this paper proposes three ideal sand-fixing shrub species, i.e., _Calligonium mongolicum_, _Tamarix chinensis_, and _Haloxylon ammodendron_.  
It takes a relatively long period for biological sand control measures to exert their effect fully. In an environment facing wind erosion and sand burial, sand fences are erected by rows and columns along the outer edges of forest belts to protect plant seedlings and guarantee their survival rate. Meanwhile, it is also necessary to ensure a sufficient water supply. In deserts, drip irrigation is often adopted to reduce evaporation and supply water directly to plant roots (Figure 13). The factors considered in setting the width of biological sand control layout are the degree of wind-sand hazards along the railway and the angle between the railway trend and sand drift direction. In the principle of \"adapting sand control measures to the severity of wind-sand hazards\", the width of the biological sand control layout on the upwind side is greater than that on the downwind side. In case of a larger angle between railway trend and sand drift direction, the shelter forest belt will also be wider.