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The phreatic Line or seepage line is defined as the line within a dam section flow which they are positive hydrostatic pressure in the dam the hydrostag phreatic line graphically:

- AB is upstream face let it horizontal projection be L. On The Water surface measure a distance BC = 0.32. Then the point C is the starting point of the base parabola.
- The directrix of the parabola is located by utilising the principle that any point on the parabola is equidistant from the focus as well as from the directrix
- Hence with C as the centre and CF radius is drawn an arc to cut the horizontal line through CB in D. Draw a vertical tangent to the curve FD at D. Since CD = CF the vertical line BH is Directrix.
- Between F and H the last point G of the parabola will lie.
- To locate the intermediate point on the parabola the principal that its distance from the focus and directrix must be equal will be used. For example to locate any point P drawn vertical line QP at any distance x from F
- Measure QH with F as the centre and QH as the radius draw an Arc to cut vertical line through Q at point P
- Join all these points (C, P, G) to get the box parabola.
- Entry point correction the phreatic line is flow line it must start from B and not from C it should be perpendicular to the upstream face AB which is 100% equal potential line.
Therefore portion of the phreatic line at B is sketched freehand in such a way that it starts perpendicularly to AB. The base parabola should also meet the downstream.

dicularly to AB. The base parabola should also meet the downstream.

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