In the design of a canal one has to find out bed width (B) depth (D), longitudinal slope (S), and velocity of flow (V).
If discharge (Q) and silt factor (f) are given, the method of finding.
This aspect limits only finding the sectional dimensions of the canal.
But how to compute the discharge at a particular reach is an important aspect of canal design.
Discharge in the off-taking canal does not remain constant throughout the length.
Outlets fixed on the canal at regular intervals draw discharge from the canal and supply it to the fields for irrigation.
Evaporation and percolation losses also go on increasing with length of the canal.
Hence because of discharge withdrawn by outlets, and also continuous evaporation and seepage losses.
The remaining discharge in the canal goes on decreasing as canal flows towards the tail.
As discharge at various points on the canal is not constant, the section of the canal will also be changing.
While fixing the discharge for any canal to be withdrawn from a head regulator, one must know the following data.
(i) Gross command area of which the proposed canal is going to be in charge.
(ii) Percentage by which G.C.A. is to be multiplied, to determine the cultural command area.(iii) Various crops that will grow after the commissioning of the proposed canal.
(iv) Duties of water for various crops.(v) Intensities of irrigation during Rabi and Kharif crops.(vi) Losses due to seepage and evaporation.
The discharge required at a particular point on the canal depends upon the area to be irrigated lying D/S of that point.
Also upon the seepage and evaporation losses occurring in the canal itself, lying D/S of that point.
Since area under irrigation, and losses due to evaporation and seepage, go on increasing as we proceed towards.
The U/S side, we have to design the canal sections at different points.
Design of the canal is always start from the tail end of the canal and proceeded step by step towards.
The U/S side till the head regulator of the canal is reached.
An example of how discharge is determined at various points on the canal is given here.
Let a 5 km long minor distributory is to be design.
Let discharge required at the tail, on an irrigated area basis, is 2.02 cumec.
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