# Study on Siphon Drainage Capacity of Slopes with Long-Horizontal Pipe Sections
## 2 Siphon Drainage Test
### Siphon Resting Period and Restart  
In the application cycle of the project, the resting and restarting process of the fluid in the pipe in a short siphon drainage system can be simplified to a transient process. In contrast, in a siphon with a horizontal section length of more than 10 m, the rising water level causes a gradual gas-liquid two-phase step-by-step push in the horizontal section, and the duration of this process cannot be ignored. In this pushing process, the rising water level causes an increase in potential energy, converted into kinetic energy, causing fluid movement in the tube. When the end of the horizontal section of the fluid segment passes the horizontal section, gravitational potential energy converts into kinetic energy. The siphoning is completed when the fluid in the horizontal section also moves under the negative pressure generated by the descending fluid motion. According to the calculation formula of pipe flow movement, the pipe diameter and length are the key factors affecting the reduction rate of restarting lift under the premise that the pipe, fluid composition type, temperature, atmospheric pressure, and other conditions are determined. The siphon lift loss caused by the pipe diameter size is mainly determined by the along-stream loss and air accumulation. The flow rate and pipe diameter are negatively related to the along-stream loss coefficient. Therefore, the smaller the pipe diameter, the larger the along-travel loss coefficient for a stationary fluid in the siphon stopping period. The air accumulation problem is determined by the main flow pattern of the fluid in the siphon. During the standstill period, many segmental bubbles are observed in the horizontal section of the siphon. The main shape of the bubbles varies with the size of the tube diameter: the segmental bubbles in the D-3-5 pipes are smaller in size, and the gas-liquid  
Figure 5: Bubble pattern in the siphon tubes: (a) D-3-4; (b) D-5; (c) D-6.5; (d) D-8. interface at both ends is less affected by gravity, so the upper and lower sides of the bubbles can be considered symmetrical along the central axis of the siphon and form two independent gas-liquid interfaces, as in Figure 6a,b. In the horizontal section of the D-6.5-8 pipes, the presence of segmental bubbles occupying the entire cross-section of the siphon can still be observed, but more floating bubbles replaced the segmental bubbles against the pipe wall. The interface morphology on both sides of the slug bubbles changed significantly: the longitudinal profile of the gas-liquid interface was symmetrical at a point on the central axis of the siphon or was a single curved surface with the wall surface forming upward-facing wall bubbles, as shown in Figure 6c,d. When the water supply level rises, the fluid in the rising section of the siphon will then push the fluid in the horizontal section towards the drain. In the D-3-5 pipes, the horizontal section of the fluid undergoes a discontinuous motion under the action of the thrust. In this form of flow, bubbles and liquid velocity are the same, alternating through the pipe and a liquid film on the pipe wall is not formed. According to the simplified model, it is known that the pressure drop of the slug flow can be divided into three parts, namely, the frictional pressure drop of the bubble and fluid movement and the frictional pressure drop generated by the movement of the gas-liquid-solid three-phase contact line. In the D-6.5-8 pipes, the tremendous frictional resistance between the floating bubbles and the tube wall seriously hinders the advance of the fluid. Marine engineering of microbubble resistance reduction technology has also confirmed the existence of \"a three-phase gas-liquid-solid wall with attached bubbles and the liquid to make a relative motion\" mode [12]. The liquid along the resistance declines when the gas-liquid motion is not synchronized.