Drainage of Agricultural Lands - Practices of Irrigation and On-farm Water Management - page 350

Office building

Weather
station

Main drain

Tertiary drain

Farm implement
shed To natural

channel

Fig. 9.12 Sample layout of surface drainage system in an agricultural farm

“drainage area” of the drain. “Drainage area” of a drain is the area from which the
runoff falls to that drain.
For concrete channel, both rectangular and trapezoidal type can easily be con-
structed. For earthen channel (or earthen vegetative waterway), only trapezoidal type
with smoothing of bottom corners is recommended.

9.4.4 Sample Work Out Problem

Example 9.4

Surface drainage should be planned for a new agricultural farm to drain out irri-
gation tail-water and seasonal rainfall runoff. Maximum rainfall intensity at the
site in 20 years record is 35 mm/h. The tertiary drain would have to carry runoff
from 4 ha land. The secondary drain would have to carry thrice of tertiary, and the
main drain to carry discharge of four secondary drain (of similar flow). Determine
the design discharge capacity of the (a) tertiary, (b) secondary, and (c) main
drain.

Solution

We know, Q = CIA

Here,

10, 000m 2 = 40,000 m 2
Design rainfall intensity, I = 35mm/h = 9.72222E

Drainage area, A = 4ha = 4 ×

− 06 m/s

Runoff coefficient, C = 0.6 (as of agricultural land)