The three most common types of flow meters in use are cup, propeller, and magnetic meters. Cup and propeller meters determine flow velocity according to the number of revolutions of the cups or propeller over a given time interval. Magnetic meters measure the difference in water pressure as water flows around a sensor.

Whichever meter you use, it needs to be mounted on a rod or strung at the end of a cable to allow the propeller or other mechanism to be held in one place while the measurement is taken. Usually the meters are mounted on what is called a top-setting rod. A top-setting rod actually consists of two rods: a support rod and a smaller rod that can slide up and down the support rod. This second rod holds the business end of the meter (let’s call it the propeller) and allows it to be raised or lowered to the desired depth.

You may be surprised to learn that the velocity of water changes with depth. Hydrologists have determined that average velocity in a stream occurs just below mid-depth – at 0.6 times the total depth to be exact. That is where you want to place your propeller. Top-setting rods are designed so that you can slide the propeller up or down the support rod – which rests on the stream bottom – and measure velocity at the desired depth.

The USGS Method

1. String a measuring tape across the stream at right angles to the flow. Tie the tape off at both sides of the stream. Make it taut enough so that it doesn’t sage near the middle. Measure the stream width. Leave the tape in place.

2. First, determine the width intervals you will measure. The official method requires that at least 20 points of measurement be made across the width of the stream. To do this, divide the total stream width by 20 to calculate the distance between points. If you have been lucky enough to find a station that has a relatively uniform depth and velocity, or if it is a narrow stream, 20 points may be more than you need. In many cases, especially in very small streams (and depending upon the accuracy you desire), it is adequate to measure velocity at 1-foot, or one-half-foot intervals even if that means you may only have five or ten measurements. Measuring points should be closer together or more frequent wherever there is a lot of variation in the depth or velocity of the cross section.

3. Start at the very edge of one bank and work your way across the stream, measuring velocity with the meter at each of the 20 points and noting your distance from the bank edge where you started. For example, if your stream was 20 feet wide, you would make measurements at one-foot intervals. The first measurement would be taken at zero feet from the edge (the velocity will likely be zero), the second at 1 foot and so on to 20.

NOTE: Stand at least 1 foot away on the downstream side of the tape and hold the meter and rod next to the tape. Be sure you are standing far enough from the meter to ensure that the eddies around your boots are not interfering with the flow measurement.

4. At each measuring point, read and record the total depth, multiply the total depth by 0.6 to determine the depth of average velocity, set the propeller at the new depth, read and record the velocity. Also, remember to record your distance from the bank for each measurement.

5. The total amount of water moving through your section is a function of the size of the stream (cross-sectional area) and the velocity. Used the velocity measurements and the depth and distance measurements you recorded to calculate the total volume of water flowing through the section (total discharge).

6. Total discharge is calculated as the summation of the discharge from each of the intervals measured, as described and illustrated above and on page 61.

**Finding the Average Velocity**

Stream velocity varies vertically (from surface to bottom) at each point in a stream. Stream hydrologists have developed a standard technique to ensure consistency in determining the "average" velocity at a given point. The USGS method assumes that at points where the depth is less than 2.5 feet, the average velocity occurs at six-tenths of the total depth. Where the stream is deeper than 2.5 feet, the velocity is measured at two-tenths and eight-tenths of the total depth, and the average of the two readings is used as the average velocity at that point.

The next section discusses measuring stream flow with a simple float.

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