## 4 Results and Discussion
### Mean Wind Characteristics
#### 4.1.1 Wind Profiles and Wind Speed Amplification Factor  
The distribution of wind speed along the vertical direction is referred to as the wind profile. Influenced by the terrain, the mean wind speed may exhibit different patterns in the vertical direction. The definition of mean wind speed is given by Equation (1). The bridge damage due to excessive wind speeds can be prevented by understanding the distribution of the wind profiles. The wind speed amplification factor is defined as the ratio of the wind speed at a certain height above the ground to the mean wind speed at a reference height [25], and the calculation expression is shown in Equation (2). \[U=\sqrt{U_{x}^{2}+U_{y}^{2}+U_{z}^{2}} \tag{1}\]  
where \(U\) is the total mean wind speed; \(U_{x}\), \(U_{y}\), and \(U_{z}\) are the mean wind speeds in the \(x\), \(y\), and \(z\) directions, respectively. \[C_{u}=\frac{U_{b}}{U_{ref}} \tag{2}\]  
where \(C_{u}\) represents the wind speed amplification factor; \(U_{b}\) is the mean wind speed at the main beam; and \(U_{ref}\) is the mean wind speed at the height of the main beam when the oncoming wind reaches the terrain. The wind profile and wind speed amplification factor are both normalized by \(U_{ref}\) (the mean wind speed at the height of the main beam when the oncoming wind reaches the terrain). The distribution of wind profiles on the southwest bridge tower is shown in Figures 6 and 7. It can be observed that the wind speed generally increases with height. The mean wind speed on the southwest bridge tower is highest when the oncoming wind direction is 180\({}^{\circ}\), followed by the 0\({}^{\circ}\) direction, and the 30\({}^{\circ}\) direction is lowest. The reason for this phenomenon is that when the oncoming wind direction is 180\({}^{\circ}\), although there are high mountains around, there is a narrow gully between the mountains, as shown in Figure 2. The lateral movement of the wind is limited by the two mountains, which cause the wind to accelerate as it passes through this narrow gully. The highest wind speeds  
Figure 5: Location of the bridge towers and main beam monitoring points. appear in the wind direction of 180\({}^{\circ}\) owing to the southwest bridge tower being located on this gully. When the wind direction is 0\({}^{\circ}\), the wind flows along the river channel, and the terrain is more open without the canyon wind acceleration effect. Thus, the wind speed is lower than that of the 180\({}^{\circ}\) wind direction. When the wind direction is 30\({}^{\circ}\), the obstruction effect of the high mountains on the oncoming wind is more significant, resulting in the lowest wind speed. Additionally, when the oncoming wind is uniform, although there is some difference in the wind profile with the oncoming wind speeds, the degree of the difference is small. For example, at the midpoint of the bridge tower under 180\({}^{\circ}\) wind direction, the dimensional wind speed difference between an oncoming wind speed of 6 m/s and 14 m/s is only 0.04, as shown in Figure 6c. When the oncoming wind is ABL inflow, due to the varying wind speeds at different heights under ABL inflow, significant differences with uniform inflow in the wind profile at the bridge site are caused. The wind profile decreases with increasing oncoming wind speed. Additionally, the higher the wind speed, the smaller the decrease extent in mean wind speed, and when the oncoming wind speed reaches above 12 m/s, the wind profile remains largely unchanged. The distribution of wind profiles on the northeast bridge tower is shown in Figures 8 and 9. When the oncoming wind direction is 180\({}^{\circ}\), the wind speed on the northeast bridge tower is the lowest, which is due to the tower on the leeward side of the high mountains, and the wind is significantly reduced by the obstruction of the mountains. When the oncoming wind direction is 0\({}^{\circ}\), the wind speed is the highest.