Explain velocity area method of runoff measurement.

The velocity area method is a hydrological technique used to estimate the flow rate or discharge of water in rivers, streams, and open channels. It involves measuring the velocity of water flow at various points across a cross-section of the channel and combining it with the corresponding cross-sectional area to calculate the total flow rate.

The velocity area method follows these steps:

1. Cross-Sectional Measurement: The first step is to measure the cross-sectional area of the channel at a specific location where flow velocity will be measured. This typically involves taking measurements of the channel width and depth at regular intervals along a transect perpendicular to the flow direction. The cross-sectional area (A) is then calculated by multiplying the width (W) by the depth (D).

[ A = W \times D ]

1. Velocity Measurement: Next, the flow velocity (V) is measured at multiple points within the channel cross-section using a flow meter, current meter, or other velocity measurement devices. The velocity is typically measured at discrete depths or velocity points across the width of the channel to capture variations in flow velocity.

2. Velocity-Area Calculation: The flow velocity data obtained from the measurements are then multiplied by the corresponding areas to calculate the discharge (Q) for each velocity point. The discharge at each point is given by:

[ Q = A \times V ]

1. Total Discharge Calculation: Finally, the individual discharge values calculated for each velocity point are summed to obtain the total discharge for the entire channel cross-section. This provides an estimate of the flow rate or discharge (Q) of water passing through the channel at the measurement location.

The velocity area method is widely used in hydrology and hydraulic engineering for monitoring and managing water resources, assessing flood risk, designing hydraulic structures, and conducting streamflow measurements. It provides a practical and accurate means of quantifying flow rates in rivers and streams, especially in situations where flow conditions are variable or channels have irregular shapes.