# Wind Tunnel Measurements of Surface Shear Stress on an Isolated Dune Downwind a Bridge
## 1 Introduction  
Railways in arid and semiarid regions in Western China suffer from sand hazards [1, 2, 3]. On one hand, wind-blown sand movement could bury rail tracks, attack railway power systems and roll over the trains. On the other hand, railway structures such as high subgrades, bridges and windbreak walls are huge enough to perturb airflow over the nearby sand dunes, which are stable and moving regularly under initial airflow conditions [4, 5, 6]. Therefore, it is important to study how the railway structures change the shape and moving patterns of sand dunes in the neighborhood. In previous studies, particular attention has been paid to the effects of wind-blown sands on the railways and subgrades [5, 6, 7, 8], however, the effects of railways on the wind-blown sand, especially on the sand dunes movement, are not well explored. Based on the bridge engineering of the Dun-Gel railway in a sand valley, we observed and characterized the surface shear stress on the sand dunes downwind of a bridge in the wind tunnel.  
Sand particle deposition and erosion processes are closely related to the threshold friction velocity of sand entrainment [9; 10; 11; 12; 13; 14]. Many studies have shown the relationships between surface shear stress acting on the ground and sand entrainment [15; 16; 17]. To understand the formation and migration of dunes, one first needs to know the stationary wind stress exerted on a given sand topography [18]. Using the visualization techniques to investigate the flow structures around the roughness elements, researchers have shown that vortex strongly governs the pattern and magnitude of surface shear stress at the presence of roughness elements [13]. Hence, it is important to study the fluctuation of surface shear stress.  
This study is a part of a comprehensive environmental assessment of the Dun-Gel railway project located in Dunhuang city, Gansu Province, China (Figure 1). Located in the northwest of China, Gansu lies between latitude 32\({}^{\circ}\)11'-42\({}^{\circ}\)57\" north and longitude 92\({}^{\circ}\)13'-108\({}^{\circ}\)46' east, with a total area of 425,800 km\({}^{2}\). The terrain of Gansu province is long and narrow, and the landform is complex and diverse, including mountains, plateaus, flat rivers, river valleys, deserts and gobi. The climate type of Gansu is mainly temperate continental arid climate. Our purpose is to study how the railway structures affect the shape and moving patterns of sand dunes in the neighborhood by investigating the shear stress on the sand dune surface, which determines the deposition or erosion of the sand dune surface. In this study, spatial variation in the surface shear stress was measured directly by mounting Irwin sensors at numerous discrete points on the sand dune model [19]. The relationship between the surface shear stress on the sand dune and the position of the bridge was analyzed. By analyzing spatial variation changes of surface shear stress on and downstream of the sand dune model, the erosion and deposition zones were identified. Moreover, future changes in the shape and moving patterns of the sand dunes can be predicted.