MEASUREMENT OF DISCHARGE BY AREA VELOCITY METHOD FOR BIG RIVERS
We had said earlier that cables may be stretched across the river and river width divided into small compartments by fixing tags to the cable.
But in case of very large and wide rivers it is not possible to stretch cables across the river.
In that case pivot method is used.
It consists of a long high pole installed at one bank of the river.
A long wooden plank is fixed to this pole at some height.
The plank consists of a number of equidistant holes.
P is the point of observation.
The last hole B, on the plank is made in such a way that the observer from point P when views through hole B7 strikes point B7 at the opposite bank of the river.
B7 is the last point where river water at H.F.L. is touching the opposite bank. See Fig. 10.10.
Now by observing from P through points B1, B2, B3……B7, corresponding points A1, A2,A3, in the river can be fixed.
The velocity of flow for each compartment is then found out with the help of a current meter.
Depths of water at the centre of each compartment are found out by going in boat.
The discharge of the river is finally found out by adding together the discharges flowing in all the compartments separately.
