**2. The distribution pattern and cause mechanism of sand disasters along the Qinghai-Tibet Railway**

In terms of space, the sand disasters along the Qinghai-Tibet Railway are mainly distributed from Golmud (K815 + 380) to Tibet's Cuona Lake (K1531 + 280). The sand damage in this section is not evenly distributed, mainly concentrated in the Hongliang River (K1104 + 690), Tuotuo River (K1224 + 810), Za'gya Zangbo (K1445 + 560) river valleys, and the areas on both sides of the river and Cuona Lake section (K1528 + 710 to K1531 + 280) and other areas [6]. The railway section with severe sand damage is 78.8 km (**Figure 1**).

In terms of time, the Qinghai-Tibet Railway has many windy days in winter and spring. Especially the main plateau, including Tuotuo River, Wudaoliang, Amdo,

*Desertification and Its Control along the Qinghai-Tibet Railway DOI: http://dx.doi.org/10.5772/intechopen.101701*

#### **Figure 1.**

*Sand disasters along the Qinghai-Tibet Railway. (a) Hongliang River section, (b) Tuotuo River section, (c) Za'gya Zangbo section, (d) Cuona Lake section.*

and other areas, the number of strong wind days is more than 100 days. Moreover, the climate is relatively dry in winter and spring. For example, in the Tuotuo River area, according to the observation data of the weather station for many years, the average annual wind speed is greater than 4 m s−1. The number of windy days is more than 140 days. The number of sandstorm days in the year is 15–22 days. The annual precipitation is about 200 mm, mainly from June to September (about 85%). There is no snow cover in winter and spring, and the surface is extremely dry. At the same time, this period is also a period of high winds, with the highest wind speed reaching 32 m s−1. Therefore, the Qinghai-Tibet Railway sand disaster occurred in these seasons.

The main body of the Qinghai-Tibet Railway is located in the rapids area in the middle of the westerly belt. Observation of sandstorms on the Hongliang River, Tuotuo River, Cuona Lake, and other road sections with severe sand damage revealed that the wind along the railway is strong and the amount of sand transported is relatively large. The sanding wind has a single wind direction and a long duration. It is dominated by westerly winds. These conditions have provided sufficient impetus for wind-sand activities [6]. The sand materials along the Qinghai-Tibet Railway mainly come from river sediments, desertified meadows and grasslands, and lake sediments. River facies sediments are mainly concentrated in river valley areas. Affected by topography and wind, sand materials develop from the river valley area to the two banks. The wind-sand disasters are particularly serious on the downwind bank of the river valley. This type of sand material is mainly distributed in the valleys and banks of the Hongliang River, the Xiushui River, the Beilu River, the Tongtian River, the Tuotuo River, the Za'gya Zangbo, and the Basuoqu River. Sandy meadows and grassland sand sources are typical non-point source sand sources, which are widely distributed. The main damage area is the Tuotuo River section. The impact of sandy meadows and grassland sand sources on railways mainly has two aspects. On the one hand, under favorable wind

conditions, sand particles accumulate on the roads and directly harm the railway. On the other hand, sand particles enter the river valley with water flow, and deposit in the river valley area, becoming an important source of river facies sediments. Lake sediments are also an important source of sand material along the Qinghai-Tibet Railway, especially the Cuona Lake section, which has also become the most severe sandstorm area along the Qinghai-Tibet Railway. In addition, the sources of sand along the Qinghai-Tibet Railway also include rock weathered debris, sandy Gobi, wind erosion of ancient dunes, and activation of fixed dunes. The sources of these wind-sand disasters either directly harm the railways, or compound with each other, and superimposed on the railways.

After the completion of the Qinghai-Tibet Railway, due to the appearance of the roadbed, the original relatively stable dynamic balance of the plateau sand movement was disturbed spatially, and the flow field structure and transportation and accumulation conditions of the near-surface sand flow were changed, resulting in the deposition of sand materials near the railway. The hazards of sandstorms were highlighted. Through the wind tunnel simulation experiment on the characteristics of the flow field of the Tuotuo River section of the Qinghai-Tibet Railway, the formation mechanism of the sand damage of the roadbed was studied in combination with the flow field structure on both sides of the roadbed and the horizontal gradient distribution of the wind speed profile. It was found that when the airflow passed through the railway subgrade, there were obvious obstructed uplift areas, current gathering acceleration areas, deceleration and subsidence areas, and dissipation and recovery areas. The railway subgrade affected the characteristics of wind-sand flow by changing the movement state of the airflow, the separation of the boundary layer, and the size of the return zone. The formation mechanism of railway sand hazards was mainly determined by the functional zones where the air currents were located on both sides of the railway. When the wind-sand flow run near the railway subgrade, as the airflow encountered obstacles and rises, energy consumption was large, the wind speed of the bottom airflow decreased, and sand particles accumulated at the foot of the windward slope of the railway in the way of falling and depositing, causing sand burial on the railway subgrade. The airflow on the windward side of the subgrade mainly caused wind erosion to the middle of the subgrade or the shoulder of the subgrade due to the uplift and the acceleration of the current collection due to obstacles. When the airflow crossed the railroad track on the leeward side due to decelerating settlement and vortex movement, the sand carrying capacity was drastically reduced, and the sand flow was in a state of supersaturation, which will inevitably accumulate a large amount of sand particles carried on the leeward slope [7, 8].
