Lining of a canal is economically justified only in case the annual benefits accruing therefrom exceed the initial cost plus interest on the capital investment and maintenance charges, i.e. when its cost inclusive of interest on capital and maintenance charges can be repaid in monetary benefits accrued during the life of the lining.

The maximum rate of expenditure on lining that is economically justifiable as per IS : I0430 – 1982 is as under:

Annual value of water lost by seepage from unlined canal = pLsdW Rs.

Annual saving in value of water otherwise lost by seepage = { (pLsdW – PLSdW) Rs.

= [LDw(ps – PS)] Rs.

Total annual benefits resulting from lining of canal, ‘a’ = {LdW(ps – PS) + B + M] Rs…[1]

Additional capital expenditure on construction of lined canal = TLC Rs.

Net present worth (NPW) of the total annual benefits ‘a’

Over the life of the canal, y

$NPW = a \frac{(1 + x)^y – 1}{x(1 + x)^y}$ [2]

For lining to be economically feasible, the additional initial cost o the lined canal (TLC) should be equal to or less than net present worth of savings, i.e. TLC $\leq$ NPW, i.e. value of ‘a’ worked out in

2] Is equal to or less than.

1] Where, C = cost of lining (Rs $m^2$) including the additional cost of dressing the banks for lining and accounting for the saving, if any, resulting from the smaller cross sections and hence smaller area of land, quantity of earth work and structures required for the lined sections. This saving will be viable on new canal excavated to lined cross sections from the beginning, but not on lining of existing unlined canals, s and S = seepage losses in unlined and lined canals respectively ($m^3/m^2$ of wetted perimeter/day of 24 hr), p and P = wetted perimeter (m) of unlined and lined sections respectively, T = total perimeter of lining (m), d = number of running days of the canal per year, W = value of water saved ($RS/M^3$), L = length of canal (m), Y = Life of canal (yr) assumed as 60 years for concrete, brick tile and boulder lining (IS : 10430 – 1982), M = annual saving (Rs) in operation and maintenance due to lining, B = annual estimated value (Rs) and of other benefits for the length of canal which include prevention of water logging, reduced cost of drainage for adjoining lands, reduced risk of breaching, etc, x = percent rate of interest per year. Transmission loss of 0.60 $m^3/s \ per \ 10^6 \ m^2$ of wetted perimeter of lined canal is generally assumed.