Technical Manual: Conduits through Embankment Dams (FEMA 484)

Appendix B—Case HistoriesProject name: Crossgate DamLocation: North CarolinaSummary: Problems encountered during the construction of a new siphon spillwayCrossgate Dam was constructed in 1955 in a rural area outside of Raleigh, NorthCarolina. The site was eventually incorporated into the Raleigh city limits, anddevelopment in the area downstream of the 25-foot high embankment dam resultedin a high hazard classification. Very little maintenance was performed on theembankment dam after it was constructed, and 40 years of neglect took its toll onthe dam and the CMP spillway conduits.In 1996, a developer agreed to upgrade the embankment dam in order to build newhomes around the reservoir. A new 12-inch diameter siphon spillway was designedto be installed in a trench excavated through the embankment dam crest (seefigure B-35). Leumas (1998, p. 710) discussed the design of the siphon:The design for the siphon was somewhat unique in that it served not only as a normalpool regulating device which discharged water from the surface of the reservoir, butalso as a “bottom drain” structure which could drain the reservoir by discharging flowfrom near the bottom of the reservoir. Also, the siphon was designed to be selfpriming,so that it would not need to be filled initially to start the siphon in order todrain the reservoir.Unfortunately, a hurricane arrived during construction of the siphon and proveddisastrous. The CMPs, which served as the only spillway, had already beenabandoned with grout, and the only outflow was by means of a small, temporarysiphon installed by the contractor. The flood overtopped the embankment dam, andFigure B-35.—A well designed siphon was to be installedthrough the dam crest (from Leumas, 1998).B-37