Sunday, September 1, 2013

Flow Lines and Flow net of soil


Flow Lines 

-are the paths which water particle follow in the course of seepage. Water flows from the point of higher head to low head.

Equipotential lines 

-are the lines formed by joining the points of same head or potential on the flow lines.


If we draw the flow lines and the equipotential lines for a given flow of water through a given soil we get a net like sketch which is known as flow net. Flow lines are always at 90 degrees to  the equal potential lines.

Properties of Flow net:

  1. Flow lines and equal potential lines intersect each other at 90 degrees.
  2. The areas bounded by the flow lines and equal potential lines form approximate squares.
  3.  Flow nets must satisfy the boundary conditions of flow field.
  4.  Quantity of water flowing through each flow channel is the same.
  5.  The potential drop in any two consecutive equal potential lines is same/constant.
  6. Flow lines and equal potential lines are smooth curves.
  7. Flow lines do show refraction at the interface between two soils having different coefficient of permeability.

Application of Flow Net:

(i) Determination of seepage: For the seepage through a flow net for isotropic condition                   Total Discharge, q = k.h.Nf/Nd
       where,  k = co-efficient of permeability
                    h= head causing flow
                  Nf= Total numbers of flow channels
                  Nd= Total numbers of potential drops.

(ii) Determination of seepage pressure: The change in the effective pressure due to flow of water is known as seepage pressure. The seepage pressure at any point can be determined by
                                  Ps= H.Yw
 where, H= hydraulic potential at any point
                = h - n.h/Nd = (Nd-n).h/Nd
                n = numbers of potential drops.