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20 Siphons require a priming method to initiate siphon action. Multiple methods can be considered such as vacuum pump, water pump, or hand pump. Siphons can also be designed to be self-priming so mechanical means are not necessary to fill the pipe to initially start siphon action to drawdown the reservoir. One way this can be accomplished is by designing the siphon system so when water level rises to emergency spillway level, air is expelled from the system starting full pipe flow. A self-priming siphon may require excavation into the embankment to locate it below the anticipated reservoir water level. Provisions for breaking the siphon (siphon breaker vent) should be provided at the crest of the embankment, should the need arise. Self-priming siphons will not stop until the siphon process is broken either by mechanical means (human intervention) or until the water level in the reservoir reaches the level of the siphon intake. The advantages of a siphon include: • Reservoir does not have to be completely drained. • Installation of siphons can be performed in a relatively short amount of time and are typically cost-effective. • Specialty contractors are not required if quality engineering oversight is available during construction. The disadvantages of this option include: • Some excavation of the dam may be required. If the dam crest is utilized as either a pedestrian path or vehicular path, some interruption of service should be anticipated. • A pump is required to initiate flows unless a self- priming siphon is installed. • Inefficient removal of water at heads below 1 to 1- 1/4 times the diameter of the pipe, which causes excessive fluctuations in the water surface when compared to pipe and riser spillways. • Not cost-effective for large reservoirs and watersheds. • Can be susceptible to vandalism, unless protective measures are taken. • Limited ability to drain reservoir deeper than 20 to 25 feet. In some situations, equipment for the siphon or pumps can be procured and stored at the dam with the intent of quickly installing the siphon temporarily for use during emergencies or as needed. This option must be carefully evaluated and may only be applicable for circumstances where a smaller-sized dam is located off-channel (i.e., not on a live stream) and thus would not invoke the need to release water frequently or at high rates. Contingency plans should be made during the design process, which outline actions to take in the event that the capacity of diversion measures is exceeded. Such plans should include a notification list of State dam safety program staff, emergency management officials, and other State and local representatives, who can assist in the event of an emergency. For readers needing more information about siphons, a detailed discussion of design, installation and operation of siphons was presented in Volume 1: Issue 1 of Western Dam Engineering: Technical Note – “Simple Steps to Siphoning." A case study of problems encountered and lessons learned with the installation of a new siphon at Crossgate Dam in Raleigh, North Carolina, is presented in the paper, “To Siphon or Not To Siphon: That is the Question (Among Others) A Repair History of Crossgate Dam Raleigh, NC.” Installation of a High-Level Outlet Conduit Installation of a high-level outlet conduit is similar to a traditional low-level outlet conduit but requires significantly less excavation or disturbance to the dam or reservoir operations. It typically involves installing a weir-box or stop-log structure into the dam embankment with the invert set at a higher elevation than the low level outlet. Common conduit materials used are reinforced concrete (cast-in-place or precast), metal pipe (steel, ductile iron, or cast iron)2, PVC, and high density polyethylene (HDPE). The appropriate material depends on loading (strength) requirements, pipe diameter, and other considerations including constructability, durability, and cost. 2 Note that corrugated metal pipe [CMP] is not recommended due to the potential of corrosion