A piezometric surface is surface formed by line joining water level in piezometers. The piezometric head is equal to height of water rise in piezometric tube.

The hydraulic head is also designated as hydraulic potential. The hydraulic head is sum of pressure head h_w and position head Z.

The seepage pressure p_s is given by,
p_s= h?_w,
where, h=hydraulic head or head lost due to frictional drag of water flowing through soil mass
?_w=uniy weight of water.

According to flow net derived by Kozney, flow lines and equipotential consist of shape of parabola.

Given,
Voids ratio e=0.78
critical gradient i_c=0.94
\(i_c=\frac{G-1}{1+e}\)
\(0.94=\frac{G-1}{1+0.78}\)
? G=2.67.

The phreatic line can be located by analytical method, graphical method-Casagrande method and experimental method.

A combination of ? and ? is called as complex potential (w) and is defined by equation: w=? +i ?.

In 1902, Col. Clibborn and Beresford developed hydraulic gradient ory of weir design as a result of ir experiment.

Given,
Discharge q=0.04 m³/s
z=12m
Area A=50 m²
coefficient of permeability k=1.6*10^-3m/s
hydraulic gradient i is given by,
i=q/(kA)
i=0.04/(50*1.6*10^-3)
i=0.5
? The effective pressure is given by
??=z??+iz?_w
??=12*9.59+0.5*12*9.81
??=173.94 kN/m².

Given,
k=6*10^-7cm/s
if viscosity is reduced to half, coefficient of permeability doubles.
?k=12*10^-7cm/s.