INTERPRETATION OF FLOWNET
Flow Rate:
Let the total head loss across the fl ow domain be DH, that is, the difference between upstream and downstream water level elevation. Then the head loss (Dh) between each consecutive pair of equipotential lines is
where Nd is the number of equipotential drops, that is, the number of equipotential lines minus one. In Figure , DH 5 H 5 8 m and Nd 5 18. From Darcy’s law, the fl ow through each fl ow channel for an isotropic soil is
where b and L are defi ned as shown in Figure By construction, b/L < 1, and therefore the total flow is
where Nf is the number of flow channels (number of fl ow lines minus one). In Figure , Nf =9. Theratio Nf /Nd is called the shape factor. Finer discretization of the flownet by drawing more flow lines and equipotential lines does not signifi cantly change the shape factor. Both Nf and Nd can be fractional. In the case of anisotropic soils, the quantity of flow is
Hydraulic Gradient:
You can fi nd the hydraulic gradient over each square by dividing the head loss by the length, L; that is,
You should notice from Figure 14.3 that L is not constant. Therefore, the hydraulic gradient is not constant. The maximum hydraulic gradient occurs where L is a minimum; that is,
You should notice from Figure that L is not constant. Therefore, the hydraulic gradient is not constant. The maximum hydraulic gradient occurs where L is a minimum; that is,
where Lmin is the minimum length of the cells within the fl ow domain. Usually, Lmin occurs at exit points or around corners (e.g., point A in Figure ), and it is at these points that we usually get the maximum hydraulic gradient.