D.2 Discharge Measurement

This manual presents five basic techniques for making discharge measurements:

1. Measuring by wading (Section D.2.1)
2. Measuring from a bridge with a bridge rod (Section D.2.2.1)
3. Measuring from a bridge with a cable-suspended current meter (Section D.2.2.2)
4. Measuring from a bank-controlled cableway (Section D.2.3)
5. Measuring with small flumes and weirs (Section C.5.4)

Although other techniques exist, such as measuring from a boat, or using dye tracing and salt-dilution methods, they are rather complex for the non-specialist to apply, and are therefore beyond the scope of this manual. The non-specialist technician is advised to locate the gauging station at a spot along the watercourse suitable for conducting discharge measurements by one of the five basic techniques.

D.2.1 Measuring by Wading

D.2.1.1 General

If conditions permit, wading measurements are preferred to those obtained by other means.

Wading measurements are relatively easy to carry out, and computing the discharge can be simpler than for other techniques. However, in very small watercourses, e.g. ditches, the presence of the field technician in the water may significantly affect the flow. In this case, the technician should stand on a plank or log placed across the watercourse (Figure D-2).

D.2.1.2 Checking Safety Conditions

Check the overall conditions at the gauging station to determine if the watercourse can be waded safely.

Providing the channel bed is firm and provides good footing, a general rule of thumb for deciding whether conditions are safe is:

Depth (metres) multiplied by velocity (m/s) does not exceed a value of one.

This rule of thumb may be exceeded if a strong safety line can be installed across the metering section (Figure D-3).

Figure D-2. Ditch flow measurement using planks.

Figure D-3. Measuring by wading, with safety line angled upstream, Granite Creek near Tulameen.

D.2.1.3 Equipment Selection and Testing

If the metering section is very narrow or shallow, or if most of the verticals have depths of <0.15 m, use a small current meter. The Price meter tends to over-register if the buckets are only partially submerged; preferably, use it only where average depths are >0.15 m. Do not use an unmodified Price Pygmy meter in velocities under 0.5 m/s.

Test the current meter's electrical circuit before making the measurement:

1. Attach the meter to the wading rod.
2. Connect the electrical lead on the rod to one of the terminals on the meter contact.
3. Attach the headset, beeper, or counter to the receptacle on the handle of the rod.
4. Rotate the bucket wheel. If a headset or beeper is used, a series of sharp clicks or beeps should be heard. With a counter, the rotor revolutions will register in the viewing window. (Note: See Section C.4.4.)

D.2.1.4 Locating the Wading Section

If the gauging station has been in operation for some time, a wading section for making measurements will already have been established (Figure D-4). Inspect the wading section, and the reach immediately above and below it, to make sure it is still the most suitable.

Figure D-4. Conducting a discharge measurement with tagline across the wading section, Bridge Creek below Deka Creek.

D.2.1.5 Setting Up the Tagline and Establishing Verticals

To begin the discharge measurement, a tagline must first be placed across the watercourse.

1. Make a preliminary crossing before stringing the tagline. Use the wading rod as a support when crossing the watercourse. Turn the rod so that the meter is on the high end, or remove the meter from the rod so that it will not be damaged if a slip or fall occurs.
   Try to obtain an overall impression of the depths and velocities while wading. This is also a good time to look for rocks and debris that might be removed from the channel bed to improve the metering section. Be certain, particularly for very small watercourses, that removing rocks will not affect the control.
2. Anchor the tagline with the zero referenced to the initial point. The initial point is a permanently marked point at the start of a cross section, normally located above the high water mark on the right bank.
3. Wade across the watercourse, stringing the tagline at a right angle to the direction of the current.
4. Secure the tagline on either shore, and determine the overall width of the metering section.
5. Assess the approximate spacing of the verticals, according to the flow pattern. Follow the guidelines in Section B.2.4.

D.2.1.6 Discharge Measurement Procedure: Mid-Section Method

The mid-section method of discharge measurement is described below, and illustrated in Figure D-5. Refer to Appendix II, Form AQU-03.

1. Record the starting time on Form RIC AQU-03.
2. Record the tagline distance for the edge of the water. If there is a steep drop at the edge of the stream, the first "vertical" depth and velocity observation should be taken close to the edge.
3. Move to the next vertical. Record the distance indicated by the numbered marker on the tagline. Observe and record the depth.
4. Set the current meter to the correct depth to obtain the velocity.
5. To obtain the velocity, count and record the number of revolutions the bucket wheel makes for a duration of time between 40 and 70 seconds.
6. Observe and record the time to the nearest 1/2 second.
   To use the current meter rating table, the number of revolutions counted should be one of the 13 that are listed. Current meter rating tables are designed so that the velocity in meters per second can be obtained directly, for a given number of revolution within the required time frame. The 13 choices of pre-selected revolutions are 5, 10, 15, 20, 30, 40, 50, 80, 100, 150, 200, 250, and 300.
7. If the procedure described above is not used, a double interpolation of both time and count is necessary to use the table to compute velocity.
8. Repeat the above procedure until the watercourse is traversed and the measurement is completed.
9. After completing the measurement, note and record the time and water level.

Figure D-5. Mid-section method of discharge measurement.

D.2.1.7 Precautions and Tips

To obtain accurate measurements by wading, the technician must pay attention to detail and technique. If followed carefully, these suggestions will help obtain reliable results:

1. Position the tagline correctly. Take the time to ensure that the tagline is placed in a position that is perpendicular to the direction of the current.
   Even when this precaution is taken, there will still be instances where angular flow occurs. When this happens, record the cosine of the horizontal angle.
2. Improving the metering section. Where necessary, take the time to improve the metering section by removing boulders and debris from the metering section and the area immediately above it. Remove weeds for a distance of about three times the depth from the area upstream and downstream from the section. On smaller watercourses it may be possible to construct small dikes to cut off sections of shallow flows and dead water.
   After the modifications are made, be certain to allow sufficient time for conditions to stabilize before proceeding with the measurement. Note if the modifications have an influence on the gauge reading. All improvements to the metering section should be completed before starting the measurement, i.e. do not make changes to the metering section (such as by moving rocks) during the course of the discharge measurement.
3. Spacing of Verticals. Obtain 20 - 25 observations of both depth and velocity for one complete measurement (Section D.3.1, Errors Affecting Accuracy). If the cross section is narrow, do not space the verticals closer than 0.15 m when using the Price 622AA meter because the distance between verticals must be greater than the diameter of the current meter bucket wheel.
   If the cross section is very narrow, use a small meter and space the verticals more closely. The small propeller meters are usually supplied with 50-mm diameter propellers, and 30-mm interchangeable types are also available.
4. Position of the technician. The field technician's position with respect to the current meter is very important when making a discharge measurement by wading. The technician should stand to the side and downstream from the meter so as not to influence the velocity (Figure D-6). Studies show that the following position has the least effect on the operation of the current meter: stand in a comfortable and safe place facing either shore, and no less than 0.4-m downstream and to the side of the current meter.
5. Position of the current meter. Hold the wading rod in a vertical position and the current meter parallel to the direction of flow while making the velocity observation.
   Vertical axis meters - if the axis of the meter is not kept vertical, the meter will tend to under-register.
   Horizontal axis meters - many propellers are designed to compensate for angular flow. Consequently any deviation from the vertical position of the rod will introduce an error in velocity.
6. Observing Velocities. If depths are sufficient, the 0.2 and 0.8 method should be used for observing velocities. It is quite easy to make the settings on the top setting wading rod.
• set the 0.2 depth position on the rod, simply double the value of the observed depth. Set the 0.8 depth position as one-half of the observed depth.
  Example: Observed depth = 0.96 m.
  For the 0.2 depth, set 1.92 on rod.
  For the 0.8 depth, set 0.48 on rod.

Note: The 0.2-0.8 method is not entirely satisfactory if the channel bed is very rough, irregular, or covered with aquatic growth. These conditions will often produce erratic results for the observation at the 0.8 depth. In some situations, more reliable results will be obtained by computing the average velocity on the basis of the 0.2 and 0.8 depths and averaging the computed value with the velocity from the 0.6 depth. This is known as the three-point (3 Pt.) method.

7. Uneven Channel Bed. Sounding a channel bed that is extremely soft or strewn with boulders requires a great deal of extra care and attention.
   Be careful not to over-sound by allowing the bottom of the wading rod to sink into soft channel bed material. If the channel bed is very rough, take time to adjust the observed depths so that they reflect both the tops of the boulders and the depths between them. Measuring verticals should be equidistant around the vertical line which define the breakpoint on the edge of the submerged obstruction (Figure D-7).
Sometimes there may be a near-vertical boundary separating zones of different depth or velocity. In this case, position the adjacent measuring verticals equidistant from this boundary, so that the boundary coincides with the common boundary of the partial sections.

Figure D-6. The technician is positioned downstream and to the side of the meter.

Figure D-7. Defining the breakpoint.