Discharge measurement by area- velocity method is quite time consuming and tedious.

The moving boat method is one of the advanced techniques in the measurement of discharge. 

On large rivers, conventional methods of discharge measurement may be difficult during floods. 

The moving boat method can be used to overcome problems in such a situation. 

Also this method can be used at remote locations, at sites where unsteady flow conditions require rapid completion of measurement or at locations where no fixed facilities are available.

This method was developed by U.S Geological Survey.

In this method, a special propeller type current meter which is free to move about a vertical axis is towed in a boat at a velocity Vb at right angles to the stream flow. 

If the flow velocity is Vf  the meter will align itself in the direction of the resultant velocity Vr making an angle α with the direction of the boat. Further, the meter will register the velocity Vr.

Vb = Vr cosα     Vf = Vr sinα    

The distance travelled by the boat along its path between two consecutive observation points is given by :

 Lb = ʃ Vb dt = ʃVr cosα dt

The discharge is calculated by :

     △Q = (Lb * davg) V 

where davg is the average depth of flow and V is stream velocity 

This method gives the discharge within 4% of that measured by conventional area-velocity method.

For better accuracy, the number of observation points should be increased.

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