# Study on Siphon Drainage Capacity of Slopes with Long-Horizontal Pipe Sections
## 4 Discussion
### Critical Lift Variation at Siphon Restart  
The limiting lift for each pipe diameter before and after the stop flow period is shown in Figure 10. The restarting lift loss rate of the siphon visually reflects the siphon restarting capacity and rain and dry season sustainability for that pipe diameter and length. The siphon restarting capacity of the short pipe siphon is much larger than that of the long pipe siphon: the loss rate along the length of the short pipe siphon is about 1% to 3.5% for the 30-50 m pipe siphon, about 12% to 22% for the 100 m pipe siphon, while the distribution of the 200 m pipe siphon loss rate shows a significant variance of about 15% to 31%. For smaller pipe diameters of 3 to 5 mm, the maximum loss rate of siphon restart lift does not exceed 20%, while for larger diameters, especially for 8 mm siphons, the siphon restart critical lift fluctuation is more significant. While the lift reduction rate for short 8 mm diameter pipes does not exceed 4%, the lift reduction rate for long pipes increases to 19-30%, with a risk of failure. Many experimental studies have shown that the static contact angle is a dynamic value with a range of variation, known as contact angle hysteresis [17; 18; 19]. The upper limit of the static contact angle is called the static forward contact angle, while the lower limit is called the static backward contact angle. As the name implies, the static forward contact angle is the contact angle between the liquid and the solid when the liquid phase advances (the front end of the forward direction of the liquid slug), while the static backward contact angle is the contact angle between the liquid and the solid when the liquid phase retreats (the back end of the forward direction of the liquid slug). The dynamic change in the static contact angle caused by the contact angle hysteresis means there may be pressure loss in the fluid in the tube even before the movement starts. When the liquid slug is in the critical state of static-moving, the dynamic advancing and retreating contact angles take the values of static advancing and retreating contact angles, respectively. At this point, the pressure drop caused by the contact angle hysteresis is the maximum of the pressure drop caused  
Figure 10: Average initial limit lift and restart limit lift change. by the static contact line. Taking the concept of contact angle hysteresis into consideration explains well the drop in critical lift during siphon restart. The test results show that the average length of slug bubbles in the horizontal section of the long-distance siphon drain increases with pipe diameter, as shown in Figure 11. In the horizontal section of D-3-5 pipes, there is no direct rule of slug bubble length with pipe diameter, but the statistical results of D-5 and D-6.5 have evident values of jump variation difference. Combining the statistical values of bubble lengths in the five pipe diameter horizontal sections shows that the average bubble length increases with the increase of pipe diameter when the pipe diameter exceeds 5 mm. In addition, the dispersion of the bubble length distribution is also correlated with the tube diameter, i.e., the dispersion increases positively with the tube diameter. The average bubble length in the horizontal section of the siphon was more considerable for the 6.5 mm and 8 mm diameters compared to the bubble length in the tube with diameters below 5 mm, and no bubbles were observed in the vertical section of the siphon during the standstill period. This is because the bubbles in the vertical section of the siphon move to the horizontal section under buoyancy, which pushes the bubbles in front of them to move further and merge with them to form longer bubbles. The fusion tendency occurs when the distance between bubbles in the same phase is small [20], and the comparison results of the average length of bubbles in the tube before and after the stopping period (change value curve) confirm this idea.