地形对包兰铁路沙坡头段防护体系的影响

The Influence of Terrain on the Protective System of Shapotou Section of Baotou-Lanzhou Railway

选择沙坡头铁路防护体系典型断面,利用野外风场观测与风洞模拟,讨论地形在道路防沙体系中的作用。结果表明,防护体系前沿栅栏沙丘下风向10倍沙丘高度的水平范围内,地表摩阻风速通常小于临界值;风速递减系数随防护距离的增大而增大,地形倾斜和起伏对贴地层风力减弱的贡献占防护体系风速削弱程度的43%。自流沙区至铁路路基,潜在输沙率迅速波动降低,其中地形变化使得防护体系内潜在输沙率降低50%以上。这表明,地形是沙坡头铁路防护体系发挥卓越功能的重要因素,栅栏沙丘已成为防护体系的重要构成部分。

To explore the role of terrain on preventing the railway from blown sand disaster, a typical transect within the protective system of Shapotou section was selected and its airflow measured in field and over ascaled down model in wind tunnel. The airflow over this transect shows high speed wind centers on the top of the fence dune and on the edges of river terraces, while low speed wind centers appear at the leeward of the fence dune and downwind of the river terraces. The high wind speed center gets stronger as the wind speed in- creases. The reduction coefficient （Rc（x,z）） is used to describe the airflow variation along this transect. A sharp in- crease of （Rc（x,z）） is found behind the fence for a horizontal distance 8 times the fence height. Rc（x,z）increases with the increasing protection distance. Under the reference wind speed of 8-9 m/s, the decline and fluctuating terrain contribute about 43% the reduction of the near surface airflow to the total airflow reduction by the shel- terbelt. As wind speed increases, airflow diffusion is weakened and （Rc（x,z）） decreases to below zero. Dune to- pography and the protection measure play a role on the dune surface airflow. Though the fence dune has a windward slope gradient three times the gradient of the semi-fixed dune and the fixed dune, the speed-up ratio （s） is almost the same. It is the drag force of fence exerting on the winds that effectively reduces the windward speed- up ratio, and leads to the large amount of sands accumulating around the fence to form a distinct artifi- cial dune form. Friction wind speed decreases rapidly from the moving dune area towards the railway. Under reference wind speed of 6 m/s, 9 m/s and 12 m/s, the friction wind speed over dune surface decreases by 49%, 45% and 39% from the moving dune area to the railway, respectively. The friction wind speed in the fixed dune section （model） is always lower than 0.5 m/s due to the decline terrain. The fence dune has a protection distance ten times the dune height in which the friction velocity is always less than the threshold friction veloci- ty. It is supposed that, when there are lacking of checkerboard and vegetation cover, the fence dune and the de- cline terrain would reduce 88% the latent sand flux from the moving dune area under wind speed of 5-9 m/s and 50% under wind speed of 12 m/s. All the results indicate that terrain is one of the most important factors to prevent the railway from blown sand disaster. The fence dune has become an indispensible part of the railway protective system.