Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damsensure that the reservoir releases do not cause loss of life or significant propertydamage downstream.11.4.1 SiphoningA siphon is a closed conduit system formed in the shape of an inverted U. A siphon(figure 150) can be used to partially drain a small reservoir. A single or a series ofsiphons can be constructed. Typically, siphons are placed up and over theembankment dam and extended to the toe of the dam. The downstream portion ofthe siphon can be charged with water and then released to create the siphonic actionto start the siphoning of water. The downstream end of the siphon should beequipped with a gate or valve to facilitate creating the siphonic action. Multiplemethods should be considered for priming siphons, such as a vacuum pump, waterpump, or hand pump. Provisions for breaking the siphon (siphon breaker vent)should be provided at the crest of the embankment dam, should the need arise. Atthe discharge end of the siphon, the area should be properly protected to ensure thatthe discharging water does not cause erosional damage. A siphon over anembankment dam is illustrated in figure 151. The siphon in figure 151 is shownextending over the dam crest to avoid excavation into the embankment dam. If thesiphon must be excavated through the embankment dam crest, the guidanceprovided in chapter 5 should be utilized.Siphons are typically constructed of either PVC, HDPE, steel pipe and typically donot exceed 12 inches in diameter; however, in some instances, siphons as large as15 to 18 inches in diameter have been successfully utilized for small embankmentdams. Because of the negative pressures prevalent in the siphon, the pipe should besufficiently rigid to withstand the collapsing forces. Pipe joints must be watertight,and the designer must take measures to avoid cracking of the pipe caused bymovement or settlement of the embankment dam. In order to prevent absolutepressures within the pipe from approaching cavitation or collapsing presssures, thetotal drop of the siphon should be limited to a maximum of 20 feet. Duringemergencies, some cavitation damage may be an acceptable tradeoff.Embankment dam owners and surrounding property owners should be aware thatthe use of siphons results in more frequent fluctuation in reservoir level whencompared to more traditional pipe-and-riser spillway systems. This is a result of theinherent inefficiency of the siphon prior to priming of the system. Siphons primewith a head between 1 and 1¼ times the diameter of the siphon above the siphoninvert. For example, the water surface in the reservoir will need to rise about 12 to14 inches before a 12-inch diameter siphon becomes most efficient. Once thesiphon primes, outflow increases very little with increases in head (reservoir level)(Monroe, Wilson, and Bendel 2002, p. 20). If a series of siphons is used at a site,they must be properly spaced to avoid close proximity. To close of proximity to278