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.