Technical Manual: Conduits through Embankment Dams (FEMA 484)

Chapter 8—Potential Defects Associated with ConduitsFigure 107.—Concrete deterioration from alkali-aggregate reaction.risk of cavitation can be evaluated by computing the cavitation index for flow, whichis a function of velocity and pressure. Normally, for flow velocities less than 40 ft/s,cavitation will be minimal. Discontinuities or irregularities on flow surfaces and/ormisalignments in conduits carrying high velocity flow can induce cavitation. Thesediscontinuities, irregularities, or misalignments cause the flowing water to separatefrom the conduit surface, resulting in negative pressure zones and bubbles of watervapor. When these bubbles travel downstream and collapse next to the conduitsurface, the high pressure impact removes small particles of the conduit surface(pitting). As the pitting continues, a progressively deepening cavity develops, whichcauses additional irregularities that leads to even larger cavities farther downstream(also known as a Christmas tree pattern). Cavitation is common just downstream ofmechanical control equipment, such as gates or valves (figure 108) where pressureflow changes to free flow. Damage from cavitation and abrasion can appear to besimilar. Cavitation damage appears as a plucking out of the surface material with nofine scale evidence of flow direction. Abrasion damage is normally flow directional.The use of aeration devices (e.g., ramps and/or slots) installed along flow surfaces inmodern structures has been found to be an effective method for preventingcavitation damage. All new structures should include aeration devices, and existingstructures that have experienced cavitation damage can be retrofitted to includethese. However, the most effective solution is to eliminate the source of thecavitation, rather than attempting to minimize the resulting damage. For furtherguidance on cavitation, see Reclamation’s Cavitation in Chutes and Spillways (1990a).181