About hydraulic structures protection against seepage erosion

Many hydraulic structures — river levees embankments, overflow weirs, locks, spillways, etc. — are often built on pervious soil, or a more pervious stratum, sometimes neighboring the river bed, underlies the soil on which the foundation of the structure rests. As a consequence of the hydraulic gradient along discharge boundary, an upward seepage force takes place at the downstream toe of the structure. If high hydraulic gradients are present, seepage force exerted by water flowing can be sufficient to cause the removal of foundation material.