Before we actually discuss various methods to measure the velocity of flow, it is worthwhile to explain the variation in the velocity along the cross-section of the river.
The velocity of flow is not uniform throughout the cross-section of the river.
It is almost zero at the bed and increases parabolically as we proceed towards the surface of water.
But the velocity is maximum slightly below the surface.
At surface velocity is somewhat less than the maximum.
Now if we proceed along the width of the river, the velocity is almost zero along the banks.
The velocity goes on increasing as we proceed towards the centre of the river and it reaches maximum at the centre.
Now if we consider, both along depth and along the width of the river, we can easily pin point that velocity of flow is maximum at centre width of the river but slightly below the surface of water.
The velocity is zero along banks and bed of the river.
If we draw contours of equal velocity they will be as shown in Fig. 10.3 (a).
Seeing variation in velocity along depth as well as along width of the river, it can be easily understood that finding the average velocity of flow is not very easy.
One has to take average of the velocities along depth as well as width for all the seven compartments.
Launch your Graphy