Technical Manual: Conduits through Embankment Dams (FEMA 484)

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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
Chapter 11—Appropriate Emergency ActionsFigure 150.—Siphon used to lower the reservoir water surface through theupper entrance of an outlet works intake structure.Figure 151.—A simple siphon constructed over the crest of an embankment dam.each other may cause problems at the intakes of the siphons. Air may be suckedinto the siphon pipe, causing the vacuum to be lost.The advantages of a siphon include:• The installation of a siphon can be performed in a relatively short amount oftime.• A siphon can be constructed with automatic operation to eliminate the need forfrequent manual manipulation.• The reservoir does not have to be completely drained. Maintaining a partialpool allows for the maintenance of some of the aquatic habitat.• A siphon allows for the removal of cool water from relatively deep areas withinthe reservoir to promote cold water fish habitat downstream. In areas wheretrout populations are threatened by high water temperatures, a siphon can beused to combat the rise in stream temperature.279
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Technical Manual:Conduits throughEm
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PrefaceTens of thousands of conduit
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Prefacecondition. The hazard classi
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PrefaceLakeLine MagazineMaryland Da
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Conduits through Embankment Dams11.
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Contents23 Precast concrete pipe be
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Contents81 On first filling, a high
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Contents117 This conduit was severe
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Contents158 An example of a histori
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ContentsFigures in the AppendicesNo
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ContentsB-33 Aerial view of Como Da
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ContentsB-81 Cross section of Wiste
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ContentsMASW, multichannel analysis
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Symbolsc, cohesionE', modulus of so
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ContentsD 3350D 4221D 4253D 4254D 4
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IntroductionConduits convey water f
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IntroductionFigure 3.—A 15-inch d
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Introductionunstable. When seepage
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IntroductionAn embankment dam with
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A vortex may form at the location w
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IntroductionWater from the reservoi
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IntroductionFlowing water from the
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IntroductionThe process of dislodgi
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Introductionbe observable at the su
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IntroductionFigure 11.—Example of
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Introductionwhere conduits were ass
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Introduction• Conduits within lev
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IntroductionFigure 12.—An example
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Introductionmore recent practiced h
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IntroductionThe “best practices
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Chapter 1GeneralConduits have been
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Chapter 1—GeneralFigure 14.—Ant
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Chapter 1—GeneralFigure 17.—Fai
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Chapter 1—Generalmeans the condui
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Chapter 1—Generalfacilitate futur
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Chapter 2Conduit MaterialsVarious m
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Chapter 2—Conduit MaterialsFigure
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Chapter 2—Conduit Materials• Co
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Chapter 2—Conduit MaterialsSteel
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Chapter 2—Conduit Materials2.3.2
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Chapter 3Hydraulic Design of Condui
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Chapter 3—Hydraulic Design of Con
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Chapter 4Structural Design of Condu
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Chapter 4—Structural Design of Co
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Chapter 5Foundation and Embankment
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Chapter 5—Foundation and Embankme
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Chapter 6Filter ZonesZones of desig
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Chapter 6—Filter Zonesseepage or
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Chapter 6—Filter ZonesFigure 87.
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Chapter 6—Filter Zones3. Upstream
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Chapter 6—Filter Zonesbedrock sur
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Chapter 6—Filter Zoneszones, and
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Chapter 6—Filter Zonesfiltering t
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Chapter 6—Filter Zonessoil. This
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Chapter 6—Filter Zones6.8 Specifi
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Chapter 6—Filter ZonesVarious deg
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Chapter 6—Filter Zones• For pre
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Chapter 7Potential Failure Modes As
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Chapter 7—Potential Failure Modes
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Chapter 8Potential Defects Associat
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Chapter 8—Potential Defects Assoc
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Chapter 9Inspection and Assessment
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Chapter 9—Inspection and Assessme
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Chapter 12—Renovation of Conduits
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Chapter 13Replacement of ConduitsGe
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Chapter 13—Replacement of Conduit
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Chapter 14Repair and Abandonment of
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Chapter 14—Repair and Abandonment
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ReferencesThe following references
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ReferencesBureau of Reclamation, De
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ReferencesCooper, Chuck, E. Wesley
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ReferencesJacobsen, S., “Buckling
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ReferencesNatural Resources Conserv
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ReferencesU.S. Army Corps of Engine
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ReferencesWooten, R. Lee, Peter S.
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Additional ReadingThe following ref
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Additional ReadingCarter, Brent H.,
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Additional ReadingJohnson, Brian, J
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Additional ReadingNatural Resources
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Additional ReadingU.S. Army Corps o
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Additional ReadingVan Aller, Hal, R
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GlossaryThe terms defined in this g
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Glossary• Backward erosion piping
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GlossaryCathodic protection system
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GlossaryConsequences (FEMA, 2004):
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Glossarymalfunction or abnormality
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GlossaryDrawdown (FEMA, 2004): The
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GlossaryFailure mode (FEMA, 2004):
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GlossaryGate chamber: An outlet wor
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GlossaryImpervious: Not permeable;
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GlossaryLeakage (FEMA, 2004): Uncon
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GlossaryOpen cut: An excavation thr
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GlossaryPolyvinyl chloride (PVC): A
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GlossaryResistivity: A measure of t
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GlossarySettlement (FEMA, 2004): Th
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GlossaryStandard Proctor compaction
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GlossaryThermoset (ASTM F 412, 2001
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GlossaryReferencesAmerican Concrete
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IndexAAbandonment of conduits, 225,
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IndexCompaction of backfill, 6, 10,
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IndexDDamembankment, 113-130, 333-3
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IndexExisting conduit, 23, 31, 45-5
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IndexHydraulic fracture, 4-11, 17,
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IndexMechanical caliper, 246Metal c
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IndexPressure flow, see Conduit, pr
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IndexSliplining, 39, 45-47, 51, 164
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IndexUUltrasonic pulse-echo, 245, 2
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Appendix AHistory of Antiseep Colla
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Appendix A—History of Antiseep Co
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Appendix BCase HistoriesIndexDam Lo
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Appendix B—Case HistoriesDam Loca
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Appendix B—Case HistoriesProject
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Appendix B—Case HistoriesFigure B
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Appendix B—Case Historiesproject,
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Appendix B—Case Historieswatersto
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Appendix B—Case HistoriesLessons
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Appendix B—Case HistoriesThe cond
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Appendix B—Case Historiesroadway.
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Appendix B—Case Historiesentire c
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Appendix B—Case Historieshauled i
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Appendix B—Case Historiescoal tar
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Appendix B—Case Historiespipe. Du
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Appendix B—Case Historiesof sever
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Appendix B—Case HistoriesThe down
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Appendix B—Case Historieshose and
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Appendix B—Case HistoriesADownstr
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Technical Manual: Conduits through Embankment Dams (FEMA 484)

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