Key Features

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Velocity Area Streamflow Method

In the velocity-area method, stream velocity and water depth measurements are taken along a transect perpendicular to the stream. Total discharge (Q) is calculated by integrating the stream velocities with the cross sectional area of the stream profile defined by the transect.

Different types of current meters are available to measure stream velocity. Propeller-type meters have a horizontally aligned vane that rotates in proportion to the stream velocity. The number of rotations can be recorded visually, audibly or digitally. Cup-type meters work on the same principle, but the vane is oriented vertically. Electromagnetic meters measure the voltage induced when a conducting fluid flows through a magnetic field. These tend to be used in coastal and marine studies due to the high conductivity of saline water. Ultrasonic meters use sound wave propagation in water to measure water velocity. Some versions use the impedance on the time for an ultrasonic wave emitted transmitted from one side of the river to reach a receiver on the other side. Other versions use the Doppler effect by measuring the wavelength of ultrasonic waves reflected off suspended particles in the stream flow.

The procedure is to:

  1. Choose a suitable site along the stream with a straight reach, uniform laminar flow conditions and relatively constant depth and width. Sites with extreme turbulence, protruding obstructions, eddies, stagnant zones or divided channels should be avoided.
  2. Set up tagline consisting of a tape measure perpendicular across the stream to be used for locating the velocity/depth measurements. Measurements are taken along 10-20 verticals across the stream transect. Each vertical should partition stream flow equally, so that verticals should be closer together where water is faster or deeper.
  3. At each site, use a current meter to measure stream velocity and a graduated pole to measure stream depth. Typically, flow is measured at a depth considered to reflect average velocity conditions (0.6 of the stream depth measured downward from the surface). Other approaches such as two measurements taken and averaged for each vertical, at 0.2 and 0.8 of the water depth are used (refer Table 1)

  4. The stream flow can be calculated using the mid-section method:

    (Equation 1)

    Mathematical equation

    where the Xi are the distances to successive measurement points along the transect, where stream velocity (Ui) and water depth (Yi) are measured, starting with X1 being the initial point on one bank and Xn being the final measuring point on the opposite bank.

Table 1: Different procedures for determining mean velocity at a vertical (after Gordon et al, 2004)
Number of points in Vertical Depth of measurement
(from water surface)		 Application Mean velocity Equation
1 0.6D D < 0.5m or quick measurement required V = V0.6
2 0.2D and 0.8D Preferable where size of meter allows (D>0.5m) V = 0.5(V0.2 + V0.8)
3 0.2D, 0.6D and 0.8D Irregularites distort the velocity profile and depth is sufficient V = 0.25(V0.2 + V0.8 +2V0.6)
1 Just below surface (~0.6m) High or fast-flowing conditions where lowering meter is difficult V = kVsurf where k typically = 0.85
Many Range of depths including 0.2D, 0.6D and 0.8D When high precision required or velocity profile is of interest Integration of the area bounded by the profile divided by D

D = vertical distance between water surface and streambed, measured downwards

A simplified version of this method that estimates the water velocity by timing the movement of a float over a set distance can be useful in obtaining quick approximations of stream flow, particularly during floods (Hauer & Lamberti, 1996). This method is to:

  1. Estimate the cross sectional area (A) of the stream by using a steel tape to measure both the stream width and a few depth measurements.
  2. Measure a stream reach of adequate length (L) to allow a travel time of over 20 seconds for the float. Mark the starting and finishing points of this reach with a stake or a string across the stream.
  3. Choose a float that is only slightly buoyant to reduce wind effects. An orange, chunk of ice, half-filled fishing float or a waterlogged stick are good options.
  4. Place the float upstream of the defined starting point of the reach, so that the float is travelling at the velocity of the stream by the time it reaches the starting marker. Measure the time that the float takes to travel between the upstream and downstream markers using a stopwatch. An average time (t) is obtained by taking multiple readings.

  5. Use a correction factor (k) to account for surface velocity being faster than the average stream velocity so that:

    Q = AkL/t (Equation 2)

    The correction factor k generally varies between 0.80 for rough stream beds to 0.90 for smooth, with 0.85 most commonly used.

    Instead of a float, a slug of coloured dye can be introduced into the stream and its movement down the stream reach measured (Gordon et al, 2004). This would tend to reflect an average stream velocity rather than the surface velocity that a float is influenced by, so a correction factor (k) is not required. However, due to dispersal during flow it can be difficult to define the centre of the dye slug.

References