In the case of canals there is no difficulty in measuring the cross-sectional area.
Canals are always of some regular shape whose area can be found out by any geometrical formula.
But in case of natural streams and rivers, it is not very easy to determine the area of cross-section as their sections are not regular.
In this case area of cross-section of the river can be determined as follows.
A river is flowing from left to right.
Select AB, CD and EF sections of the river and pull cables at all these sections across the river.
The distance between sections AB, CD, and CD, EF may be anything, say 100 m.
Wooden battens are attached to all the cables and sections of river at the cable points are
divided into equal parts.
In our case we have divided each cable into seven equal parts as shown in Fig.10.2.
Thus the river width gets divided into seven equal parts longitudinally.
Now take a staff or any other graduated rod and measure the depth of the river at the centre points of the each of the seven parts.
Compute the areas of all seven compartments by multiplying the width of the water of each compartment by the depth of the water
The centre of each corresponding compartment. The sum of all these areas is the cross-sectional area (A) of the flowing water of the river.
If it is not possible to measure the depth of water with staff or graduated rod, it can be measured by using measures like Kelvin’s tube, Echo sounder etc.
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