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WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 1, ISSUE 1, MARCH 2013
SIPHONING, LOW LEVEL CONDUITS, FILTER DESIGN, CONSTRUCTION CONSIDERATIONS
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<br /> <br /> <br /> <br />2 <br />Simple Steps to Siphoning <br />Introduction <br />This article discusses a practical approach to the design <br />and implementation of siphons – specifically applicable <br />to small-dam owners and operators. Many older dams <br />were not constructed with an outlet or other means of <br />draining the reservoir. Lowering the reservoir may be <br />needed for temporary construction or for emergency <br />response. Siphons can be a low-cost means of <br />providing a reservoir outlet if one does not exist. <br />Operational Theory <br />Siphons used in reservoir drawdown operate by <br />atmospheric pressure pushing water over an obstacle <br />(i.e., reservoir water over an embankment dam) and <br />discharging on the other side at a lower elevation than <br />the reservoir. In the same way a barometer works, <br />atmospheric pressure pushes liquid up a siphon into <br />the region of reduced pressure at the top/apex of the <br />siphon. The region of reduced pressure at the top of <br />the siphon is caused by liquid (water) falling on the exit <br />side, creating a pressure differential. The maximum <br />height, or lift, of a siphon is limited by the atmospheric <br />pressure at the site. The height a siphon can lift water <br />will, therefore, be lower for dams at higher elevations <br />(for instance the western United States). <br />There are several parameters that must be evaluated <br />when establishing the feasibility and design of a <br />siphon. Bernoulli's equation can be applied to <br />estimate a siphon’s maximum lift, discharge capacity, <br />diameter, and pressure. <br />Maximum Siphon Lift <br />The most critical parameter for a siphon at a given site <br />is to determine whether it is hydraulically possible to <br />‘push’ the water the desired height over the dam or <br />spillway crest. The required lift height can be <br />determined by comparing the dam crest elevation <br />(DCE) to the lowest desired reservoir water surface <br />elevation (RWS); see Figure 1. At sea level, <br />atmospheric pressure is generally 14.7 psi which is <br />equivalent to a column of water about 34-ft high. Thus, <br />34 ft is the maximum theoretical height for a siphon. <br />However, the maximum achievable lift is reduced by <br />friction and other minor losses in the system due to <br />velocity head. Therefore, it is good practice to assume <br />that the maximum lift achievable by atmospheric <br />pressure at sea level is equal to only about 20 to 25 ft <br />of water. Atmospheric pressure can be assumed to <br />decrease by about 4 percent (or 1 ft) for every 1,000-ft <br />increase in elevation. Therefore, the maximum lift <br />height of a siphon can be conservatively taken as: 𝐻𝑚𝑎𝑥=20′−𝑅𝑊𝑆1,000 <br />Where 𝐻𝑚𝑎𝑥 = Maximum achievable siphon lift. <br />RWS = Lowest desired reservoir water <br />surface in feet of elevation <br />DCE = Dam crest in feet of elevation. <br />The maximum achievable siphon lift, Hmax, must be <br />less than the value of (DCE – RWS). If the dam crest <br />is too high compared to the desired reservoir- <br />drawdown elevation, consider routing the siphon <br />through a spillway or a temporary notch in the dam <br />crest to reduce the required lift. <br /> Predicted Siphon Discharge <br />Estimating the discharge capacity will help the designer <br />Figure 1: Typical Syphon Schematic
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