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Western Dam Engineering Technical Note August 2016 21 erosion occurs when the permeability and porosity of the foundation or abutment is significantly higher than adjacent embankment material, resulting in greater flow velocity through the foundation or abutment. The difference in flow velocity at the interface of the materials allows for embankment material to be transported into the foundation or abutment. Sinkholes or depressions in the embankment and sediment deposition downstream in channels or ditches are signs that any of the three internal erosion mechanisms may be occurring through the embankment and into the foundation or abutment. Figure 14. Internal Erosion of the Embankment into the Foundation by BEP [2] Figure 15. Partial Failure of Fontenelle Dam Due to Internal Erosion into Untreated Fractures in the Foundation [5] Internal Erosion along/Out of/into Conduits or Drains Concentrated leak erosion can occur along conduits due to cracks or low stress zones around conduits from poor compaction during construction or differential settlement. BEP may be the initiating mechanism when the material along the full pipe length comprises low- plasticity erodible material. The seepage flow can be from reservoir head or from a hole in a conduit that is flowing under pressure. The material usually exits downstream around the daylight of the conduit. If a crack or hole occurs in a conduit not flowing under pressure, concentrated leak erosion can cause material to be transported into the conduit. Sediment in the conduit discharge or clogging of the conduit can be a sign of concentrated leak erosion into the conduit. Sinkholes and depressions on the embankment surface can also be signs that erosion is occurring along or into the conduit below. Figure 16. Internal Erosion along a Conduit [1] Figure 17. Internal Erosion along Conduits is a Common Failure Mode for Dams prior to the Use of Filter Diaphragms [1] Figure 18. Internal Erosion into and out of a Conduit [1]