Abstract
The behavior of the free water surface at discontinuities in permeability for steady flow has been known for more than 40 years and has been used in numerical solutions and flow net sketching. The present paper extends that analysis to unsteady flow, primarily for use in numerical solutions. For a rising free surface the diffraction between zones is given in graphical form. For a falling free surface (almost) no diffraction solution exists and the free surface is either horizontal or tangent to the line of discontinuity. However, sharp curvatures may occur in the vicinity of a permeability change so care must be exercised in extrapolating free surface slopes to neighboring regions.
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