Technical Manual: Conduits through Embankment Dams (FEMA 484)

GlossaryResistivity: A measure of the resistance to current flow in a material.Resolution (ASCE, 2000): The smallest increment in measurement that can bedistinguished.Riprap (FEMA, 2004): A layer of large, uncoursed stone, precast blocks, bags ofcement, or other suitable material, generally placed on the slope of an embankmentor along a watercourse as protection against wave action, erosion, or scour. Riprap isusually placed by dumping or other mechanical methods and in some cases, is handplaced. It consists of pieces of relatively large size, as distinguished from a gravelblanket.Riser pipe: A vertical pipe section at the upstream end of a spillway that allowswater to drop into the conduit and be discharged downstream.Risk (FEMA, 2004): A measure of the likelihood and severity of adverseconsequences (National Research Council, 1983). Risk is estimated by themathematical expectation of the consequences of an adverse event occurring, that is,the product of the probability of occurrence and the consequence, or alternatively,by the triplet of scenario, probability of occurrence, and the consequence.Risk reduction analysis: An analysis that examines alternatives for their impact onthe baseline risk. This type of analysis is begun once the baseline risk indicates risksare considered too high and that some steps are necessary to reduce risk.Rockfill dam: See Dam, rockfill.Rutting: The tire or equipment impressions in the surface of a compacted fill thatresult from repeated passes of the equipment over the compacted fill when the soil isat a moisture and density condition that allows the rutting to occur. Rutting usuallyoccurs when soils are not well compacted and/or are at a water content too high foreffective compaction.Sand (ASTM D 653, 2002): Particles of rock that will pass the No. 4 (4.75–µm)sieve and be retained on the No. 200 (0.075-mm) U.S. standard sieve.Sand boil: Sand or silt grains deposited by seepage discharging at the groundsurface without a filter to block the soil movement. The sand boil may have theshape of a volcano cone with flat to steeper slopes, depending on the size andgradation of particles being piped. Sand boils are evidence of piping occurring in thefoundation of embankments or levees from excessive hydraulic gradient at the pointof discharge.413