The energy transfer is affected between water and soil by virtue of viscous friction exerted on water flowing through soil pores. The force corresponding to this energy transfer is called seepage force.

When water flows through a saturated soil mass, total head is sum of piezometric head, velocity head and position head.

The position or elevation head is elevation of a point with respect to any arbitrary datum. The elevation head can be taken at any point with respect to a datum.

In an isotropic soil k_x=k_y=k. But for anisotropic flow medium is k_x?k_y. The laplace equation is \(k_x\frac{?^2 h}{?x^2}+k_y\frac{?^2 h}{?y^2}=0.\)

In quick condition,
p_s=z ??
iz ?_w=z ??
? i= i_c= ??/ ?_w.

When water flows through a saturated soil mass, total head at any point in soil mass consist of i)piezometric head or pressure head ii) velocity y head iii) position head.

According to Leliavsky (1965), failure of Norora weir in India was due to excessive water pressure (uplift pressure) causing floor to be blown upwards.

Flow occurs between two points only when re is a difference in potential energies or simply potential.

The location and shape of phreatic line or top flow line separating saturated and unsaturated zones is first essential step to draw flow net.

When water flows through a soil mass, due to its viscous friction, an energy transfer is effected between water and soil. This energy transfer is seepage pressure.