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RULES FOR FLOW NET CONSTRUCTION
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1. WHEN MATERIALS ARE ISOTROPIC WITH RESPECT TO PERMEABILITY, THE
PATTERN OF FLOW LINES AND EQUIPOTENTIALS INTERSECT AT RIGHT ANGLES. DRAW A
PATTERN IN WHICH SQUARE FIGURES ARE FORMED BETWEEN FLOW LINES AND
EQUIPOTENTIALS
2. USUALLY IT IS EXPEDIENT TO START WITH AN INTEGER NUMBER OF
EQUIPOTENTIAL DROPS, DIVIDING TOTAL HEAD BY A WHOLE NUMBER, AND DRAWING FLOW
LINES TO CONFORM TO THESE EQUIPOTENTIALS. IN THE GENERAL CASE, THE OUTER FLOW
PATH WILL FORM RECTANGULAR RATHER THEN SQUARE FIGURES. THE SHAPE OF THESE
RECTANGLES (RATIO B/L) MUST BE CONSTANT.
3. THE UPPER BOUNDARY OF A FLOW NET THAT IS AT ATMOSPHERIC PRESSURE IS A
"FREE WATER SURFACE". INTEGER EQUIPOTENTIALS INTERSECT THE FREE WATER SURFACE
AT POINTS SPACED AT EQUAL VERTICAL INTERVALS.
4. A DISCHARGE FACE THROUGH WHICH SEEPAGE PASSES IS AN EQUIPOTENTIAL
LINE IF THE DISCHARGE IS SUBMERGED, OR A FREE WATER SURFACE IF THE DISCHARGE IS
NOT SUBMERGED. IF IT IS A FREE WATER SURFACE, THE FLOW NET FIGURES ADJOINING
THE DISCHARGE FACE WILL NOT BE SQUARES.
5. IN A STRATIFIED SOIL PROFILE WHERE RATIO OF PERMEABILITY OF LAYERS
EXCEEDS 10, THE FLOW IN THE MORE PERMEABLE LAYER CONTROLS. THAT IS, THE FLOW
NET MAY BE DRAWN FOR MORE PERMEABLE LAYER ASSUMING THE LESS PERMEABLE LAYER TO
BE IMPERVIOUS. THE HEAD ON THE INTERFACE THUS OBTAINED IS IMPOSED ON THE LESS
PERVIOUS LAYER FOR CONSTRUCTION OF THE FLOW NET WITHIN IT.
6. IN A STRATIFIED SOIL PROFILE WHERE RATIO OF PERMEABILITY OF LAYERS IS
LESS THAN 10, FLOW IS DEFLECTED AT THE INTERFACE IN ACCORDANCE WITH THE DIAGRAM
SHOWN ABOVE.
7. WHEN MATERIALS ARE ANISOTROPIC WITH RESPECT TO PERMEABILITY, THE
CROSS SECTION MAY BE TRANSFORMED BY CHANGING SCALE AS SHOWN ABOVE AND FLOW NET
DRAWN AS FOR ISOTROPIC MATERIALS. IN COMPUTING QUANTITY OF SEEPAGE, THE
DIFFERENTIAL HEAD IS NOT ALTERED FOR THE TRANSFORMATION.
8. WHERE ONLY THE QUANTITY OF SEEPAGE IS TO BE DETERMINED, AN
APPROXIMATE FLOW NET SUFFICES. IF PORE PRESSURES ARE TO BE DETERMINED, THE
FLOW NET MUST BE ACCURATE.
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FIGURE 1 (continued)
Flow Net Construction and Seepage Analysis
7.1-261
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