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2 Simple Steps to Siphoning Introduction This article discusses a practical approach to the design and implementation of siphons – specifically applicable to small-dam owners and operators. Many older dams were not constructed with an outlet or other means of draining the reservoir. Lowering the reservoir may be needed for temporary construction or for emergency response. Siphons can be a low-cost means of providing a reservoir outlet if one does not exist. Operational Theory Siphons used in reservoir drawdown operate by atmospheric pressure pushing water over an obstacle (i.e., reservoir water over an embankment dam) and discharging on the other side at a lower elevation than the reservoir. In the same way a barometer works, atmospheric pressure pushes liquid up a siphon into the region of reduced pressure at the top/apex of the siphon. The region of reduced pressure at the top of the siphon is caused by liquid (water) falling on the exit side, creating a pressure differential. The maximum height, or lift, of a siphon is limited by the atmospheric pressure at the site. The height a siphon can lift water will, therefore, be lower for dams at higher elevations (for instance the western United States). There are several parameters that must be evaluated when establishing the feasibility and design of a siphon. Bernoulli's equation can be applied to estimate a siphon’s maximum lift, discharge capacity, diameter, and pressure. Maximum Siphon Lift The most critical parameter for a siphon at a given site is to determine whether it is hydraulically possible to ‘push’ the water the desired height over the dam or spillway crest. The required lift height can be determined by comparing the dam crest elevation (DCE) to the lowest desired reservoir water surface elevation (RWS); see Figure 1. At sea level, atmospheric pressure is generally 14.7 psi which is equivalent to a column of water about 34-ft high. Thus, 34 ft is the maximum theoretical height for a siphon. However, the maximum achievable lift is reduced by friction and other minor losses in the system due to velocity head. Therefore, it is good practice to assume that the maximum lift achievable by atmospheric pressure at sea level is equal to only about 20 to 25 ft of water. Atmospheric pressure can be assumed to decrease by about 4 percent (or 1 ft) for every 1,000-ft increase in elevation. Therefore, the maximum lift height of a siphon can be conservatively taken as: 𝐻𝑚𝑎𝑥=20′−𝑅𝑊𝑆1,000 Where 𝐻𝑚𝑎𝑥 = Maximum achievable siphon lift. RWS = Lowest desired reservoir water surface in feet of elevation DCE = Dam crest in feet of elevation. The maximum achievable siphon lift, Hmax, must be less than the value of (DCE – RWS). If the dam crest is too high compared to the desired reservoir- drawdown elevation, consider routing the siphon through a spillway or a temporary notch in the dam crest to reduce the required lift. Predicted Siphon Discharge Estimating the discharge capacity will help the designer Figure 1: Typical Syphon Schematic