For good quality of irrigation products and to avoid crop failures due to natural famines and scarcity of water a huge amount of capital should be invested in irrigation projects. For example in our country for five year plan (1997 ? 2002) capital invested in irrigation projects is 115000 lakh crores. This says that irrigation water is a costly commodity.

If lining is not provided in canal n irrigation water is lost in form of percolation and absorption as seepage losses. This loss is very significant as it reduces potential of irrigation water, which is a costly commodity. So, in order to reduce seepage losses lining of canal is needed.

With help of lining canals can be designed for both in smaller cross section and shorter in length. Steep gradients can be provided as higher velocities are allowed, and flat slopes can also be provided without silting action on lined canals. Therefore, due to se reasons command area can be increased.

Let us assume side slope as 1.5 : 1 (1.5H : 1V), rugosity constant as 0.015.
Now let us design a triangular section.
A = y² (? + cot?), P = 2y(? + cot?), and R = A/P = y/2
Tan? = (1/1.5) and cot? = 1.5 and ? = 0.59
A = y² (0.59 + 1.5) = y² (2.09)
P = 2 y (0.59 + 1.5) = y (4.18)
R = 0.5y
Now Manning?s equation gives
Q = 1/n x A x R^2/3 x S^1/2
20 = (1/0.015) x y² (2.09) x (0.5y) ^2/3 x (1/7000)^1/2
y= 3.02 m
Freeboard = 0.75 m
Total depth = 3.02 + 0.75 = 3.77 m and side slope = 1.5 : 1

V = 1/n x R^2/3 x S^1/2
3 = (1/0.012) x R^2/3 x (1/6000)^1/2
R = 4.7 m
Design a trapezoidal section canal.
A = y (B + y? + y cot?), P = B + 2y? + 2y cot?
Now Tan? = 1/1, ? = 0.785 radians, and cot? = 1
Now A = y (B + y x 0.785 + y x 1) = y(B + 1.785y), P = B + 2y(0.785) + 2y(1) = B + 2.57y
A = Q/V = 400/3 = 133.33 sq.m
100 = y (B +1.785y), B = 100/y ? 1.785y
R = A/P = 133.33/P
4.7 = 133.33/B + 2.57Y
y= 5.5 m
B = 14.4 m
Total Depth = 5.5 + 0.75 = 6.25 m
Bed Width = 14.4 m.

Due to uncontrolled seepage losses water in canal gets percolated and water table level in underground rises. This level rises up to or near ground level. This renders land unfit for agriculture as crop roots bring up alkali salts to ground level. This phenomenon is known as waterlogging and land is called thur.

The reason is very simple. The lined surface provides a smooth surface for flow of water with minimum resistance, whereas unlined canal provides more resistance due to vegetation, undulations, rocks etc. Moreover velocity of flow in lined canal is high when compared with velocity in unlined canal, as capacity of canal is dependent on velocity directly if velocity increases so does capacity of canal.

Coefficient of roughness is indirectly proportional to velocity in any mamatical formula, or condition, or equation. So, refore value of this coefficient is more for unlined canals than lined canals. Hence velocity is more for lined canals than unlined canals and refore capacity for lined canals is more.

Floods are catastrophic disasters in nature. If a canal is not lined or it constructed on weak foundations n re is chance of regular floods, sometimes even flash floods. In unlined canals re is constant eroding of embankments which ultimately leads to floods. So, refore lining of canal helps to keep a check or stop floods, which is a major danger.

The expenditure on Annual repair (A.R) and Maintenance (M.O) costs are reduced with lining of a canal. The expenditure maybe due to removal of weeds and plants, minor repairs like covering cracks, leakages, and undulations in bed, removal silt deposited on bed. So, refore if lining is done expenditure (costs) can be reduced.