Technical Manual: Conduits through Embankment Dams (FEMA 484)

Conduits through Embankment Damserosion severely damaged the partially constructed siphon. Temporary repairs wereunder way a few days later, but not yet completed, when another storm caused theembankment dam to again be overtopped. Erosion of the embankment dam, at thelocation of the siphon, by the second storm event caused the embankment dam tobreach.A decision was made to abandon the siphon spillway and instead construct aconcrete riser and barrel structure. However, the siphon was later repaired and wassuccessfully used to drain the reservoir (although several large pumps were alsorequired).Lessons learned (after Leumas, 1998)• A siphon may be an attractive option for providing an existing embankmentdam with a permanent means to drain a reservoir in lieu of excavation of theembankment to install a traditional bottom drain. However, many designelements should be considered in deciding whether to install a siphon, a lowlevel outlet works, or sliplining the existing deteriorated conduit. Each methodhas its own advantages and disadvantages. The long term performance of thefinal selection and public safety considerations, rather than cost, should be thebasis for the selected design.• Diversion during construction is a vital element to be considered in the designprocess. An acceptable level of risk for diversion requirements, which balanceseconomics for the project and an owner’s liability, must not compromise thesafety of the downstream public.• Bad weather can occur during any project. Contingency planning should bemade during the design process, which addresses what to do in the event thatthe capacity of diversion measures is exceeded. Such planning should have areadily available notification list of State dam safety program staff, emergencymanagement officials, and other State and local representatives, who can assistin the event of an emergency.ReferenceLeumas, James, To Siphon or Not To Siphon: That is the Question (Among Others)—ARepair History of the Crossgate Dam, 1998 ASDSO Annual Conference, Las Vegas,1998.B-38