If the flow in an earth dam is plane and steady, the shape of the phreatic surface can be approximated by Dupuit's parabola. Thin sheet piling in the dam, deforms the phreatic surface in the vicinity of the sheet piling and at its tip the hydraulic gradient is singular. If the length of the sheet pile is much smaller than the thickness of the dam, the flux through the dam per sheet pile length is equal to the undisturbed flux, whereas if the length of the sheet pile is much larger than the thickness of the dam the corresponding flux is negligible.