Subsections

• Using the Flowmeter

Measuring Velocity

Velocity can be measured directly, using a flowmeter (essentially a speedometer for water, Fig. 3.8 and Section 3.1.3) or inferred by timing the movement of a float in the water (Fig. 3.5). Velocity varies across a stream and with depth, depending primarily on the proximity of the streambed (Fig. 3.4). When using a flowmeter, a single measurement at approximately 60% of the depth of the stream will give a reliable vertical average.

Figure 3.4: Variation of stream velocity with depth (after Fig. 3.16, Sanders, 1998).

Figure 3.5: The float method for velocity determination (after Fig. 3.17, Sanders, 1998).

Figure 3.6: Students using the float method for velocity measurement (click on image for full-sized version). Fall '98 class, at Spring Creek, Richardson, Texas.

Using the Flowmeter

The flowmeter can be used to determine average velocity at a point, or across the entire stream (for small streams). The device is waterproof, but try to avoid submerging the LCD display. To use the flowmeter to measure stream velocity:

1. make sure the prop turns freely
2. note the measurement units, ``mi'' on the lower right side of the display denotes english (ft/sec) units, ``km'' denotes metric (m/sec)
3. point the prop directly along the flow, with the black arrow on the prop housing pointing downstream (with the flow). The prop should be fully submerged.
4. press the right button until ``V'' (velocity) appears The instantaneous velocity (in meters/sec) is displayed as the top number on the LCD screen.
5. press the left button to set the lower display to ``av'' for average velocity (initially this number is the maximum ``mx'' velocity)
6. press and hold both left and right buttons simultaneously for 2 seconds to zero the display, and start measurement
7. for point measurements, hold in the flow until the average velocity is constant, then remove the probe. Measurement (averaging) ceases when the prop stops turning, so the displayed value is the true average at the point.
8. for areal measurements (average velocity over a stream cross-section) move the probe in the flow in a steady back-and-forth motion, as if you were spray-painting. When the entire cross-section has been covered, remove the probe from the flow, and record the displayed value.

Figure 3.7: Impeller flowmeter. After http://www.globalw.com/graphics/flow.jpg.

Figure 3.8: Students using an impeller flowmeter (click on image for full-sized version). Fall '98 class, at Spring Creek, Richardson, Texas.