Technical Manual: Conduits through Embankment Dams (FEMA 484)

GlossaryFailure mode (FEMA, 2004): A physically plausible process for an embankmentdam failure, resulting from an existing inadequacy or defect related to a naturalfoundation condition, the dam or appurtenant structure’s design, the construction,the materials incorporated, the operation and maintenance, or aging process, whichcan lead to an uncontrolled release of the reservoir.Filter cake: A thin layer of soil particles that accumulate at the face of a filter whenwater flowing through a crack carries eroding particles to the face. The filter cakeforms when eroded particles embed themselves into the surface voids of the filter.The filter cake is effective in reducing further water flow to that which would occurthrough a layer of soil with the permeability of the eroded soil particles.Filter collar: See Filter, collar.Filter diaphragm: See Filter, diaphragm.Filter: A zone of material designed and installed to provide drainage, yet prevent themovement of soil particles due to flowing water.Chimney: A chimney filter is a vertical or near vertical element in anembankment dam that is placed immediately downstream of the dam’s core. Inthe case of a homogenous embankment dam, the chimney filter is typicallyplaced in the central portion of the dam.Collar: A limited placement of filter material that completely surrounds aconduit for a specified length within the embankment dam. The filter collar islocated near the conduit’s downstream end. The filter collar is usually includedin embankment dam rehabilitation only when a filter diaphragm cannot beconstructed. A filter collar is different from a filter diaphragm, in that a filterdiaphragm is usually located within the interior of the embankment dam.Diaphragm: A filter diaphragm is a zone of filter material constructed as adiaphragm surrounding a conduit through an embankment. The filterdiaphragm protects the embankment near the conduit from internal erosion byintercepting potential cracks in the earthfill near and surrounding the conduit.A filter diaphragm is intermediate in size between a chimney filter and a filtercollar. The filter diaphragm is placed on all sides of the conduit and extends aspecified distance into the embankment.First filling: Usually refers to the initial filling of a reservoir or conduit.Flood (FEMA, 2004): A temporary rise in water surface elevation resulting ininundation of areas not normally covered by water. Hypothetical floods may beexpressed in terms of average probability of exceedance per year, such as401