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4 joints. Also the pipe should be designed with a wall thickness sufficient to prevent collapse due to negative pressures. Thin-walled plastic pipe and aluminum irrigation pipes are not recommended, because they are typically not strong enough to accommodate negative pressures in siphons. Although there are no theoretical upper limits to the diameter of a siphon, economics and practicality usually dictate hose/pipe diameters in the range of ¾- inch to 30-inches, but commonly in the range of 2 to 8 inches. Additional siphoning capacity, if needed, is usually provided by more siphon pipes rather than by larger diameter pipes. Also, rather than having a single siphon and valves for conveying and controlling all of the water, it is often best to have several smaller- diameter siphons, with or without throttling valves – so that each can operate independently of the others. Priming: The siphon must be primed (filled with water) or pumped in order to start the flow of water. Both ends of the siphon tube need to be closed to prime the siphon. A gate or butterfly valve can be used to close the ends of the pipe. This can then be used to control the flow during and after priming. There are two common methods for priming the siphon: • Fill both the outlet leg and inlet leg of the siphon pipe with water through a fitting at the apex. The fitting must be large enough to allow air to escape as water fills the pipe (i.e. greater than the pump diameter). Place an airtight cap on the fitting when the pipe is full of water and open the discharge valve to begin the flow of water. • Fill the longer outlet leg with water through a port at the apex. A vacuum pump can be used to draw air out and water in through the inlet side of the siphon. The apex port is then plugged and the discharge valve at the outlet opened to begin the flow of water. Sometimes an intake valve is used to assist with priming, and, if needed, this valve needs to be opened before the discharge valve to start the flow of water. Other Siphon Applications Although this article focuses on the use of siphons for reservoir drawdown, siphons can also be used for dewatering applications during construction, as a means to pass additional inflows under major storm events, or a permanent means for withdrawing water on an on-demand need for reservoirs and canals. The theory and design considerations are the same as described above. Conclusion Siphons can, in the correct circumstances, provide a low(er)-cost alternative to drawdown a reservoir. The key operational parameters are: (1) the required hydraulic lift cannot exceed the effective local atmospheric pressure adjusted for vapor pressure and frictional losses; (2) the discharge point of the siphon must be lower in elevation than the body of water to be siphoned; and (3) the pipe or hose used for the siphon must be designed to withstand negative pressures. Common Pitfalls in Siphoning: • Ensure required lift height is feasible for site location and elevation [(Hmax<(DCE-RWS)] • Check pipe strength against collapse. Lowest pressure does not always occur at apex. • Ensure outlet velocity does not exceed inlet leg velocity (use a longer outlet pipe length and/or smaller diameter pipe) • Review need for vacuum break valves, throttling valves, and air chambers as described. References • Morrison-Maierle, Inc. (2012), “Guidelines for Use of Pumps and Siphons for Emergency Reservoir Drawdown.” Technical Note #9, Montana Dept. of Natural Resources and Conservation, December. • Lombardi, Tina (1996), “Technical Review: Siphon – An Economic Lake Drain Option.” ASDSO. Other References and Resources • Analysis of Siphon Lake Drain Performance for a Small Earthen Dam (1996) by Bryant, Scott D; Jewell, C. Douglas • Installation and Operation of a Siphon System to Provide Water Supply Releases (2003) by Thomas, Craig; Horvath, Richard • No Drain to Pull? Well That Sucks! Pennsylvania’s Emergency Response Teams May Be Able to Help! (2006) by Reisinger, Richard A. • A Quick Guide to Siphons (2004) Watershed News. 10/2004 • To Siphon or Not to Siphon: That is the Question (Among Others) A Repair History of the Crossgate Dam, Raleigh, North Carolina (1998) by Leumas, James K. • Belle Fourche Irrigation District Siphon Project • Reclamation - Central Arizona Project • Lake Burnt Mills Dam Rehab