Hydroacoustic Current Meters for the Measurement of Discharge in Shallow Streams and Rivers

Wading Discharge Measurements

The U.S. Geological Survey (USGS) commonly uses the velocity-area method to measure discharge in streams and rivers (Wahl, Thomas, and Hirsch, 1995). The velocity-area method involves measuring the channel area and water velocities of a stream at a cross section that is perpendicular to the main flow of the channel. Discharge is computed by multiplying channel area by the mean channel velocity. The methods of measuring discharge, using the velocity-area method, are summarized below.

• It usually is not feasible to measure the entire channel area and mean channel velocity from one point in the stream; thus, the channel is divided into a number of subsections.
• The area and mean velocity in each subsection is measured and the subsection discharge is computed. The total discharge within the stream is the sum of the individual subsection discharges.
• The area of each subsection is determined by multiplying the measured width of the subsection by the water depth measured within the subsection.
• The mean velocity within a subsection is measured by placing a current meter at a depth where the mean velocity is expected to occur. In shallow streams, studies have shown that this depth is about 0.6 of the distance from the surface of water to the streambed. In deeper streams, the mean velocity is best estimated by measuring velocities at 0.2 and 0.8 of the surface-to-streambed distance and averaging the velocities measured at those depths. Velocities are sampled for 40 to 70 seconds or more.

In shallow streams, personnel measure discharges by wading in the stream.

• A tag-line with marks at known distance increments is strung across the channel perpendicular to the flow. The tag-line distance marks are used to determine subsection widths.
• Depths are measured, using a standard top-setting wading rod. A top-setting rod has a main rod marked with 0.10-foot increments for measurement of depth.
• A current meter is attached to a second sliding rod that is attached to the main rod. "Top-setting" refers to the fact that the second rod can be slid up and down, and marks at the top of the rod allow a current meter to be set at .6, .2, or .8 of the water depth.

An important equipment item for making discharge measurements is an electronic interface. Before the advent of electronic interface units for discharge measurements, USGS personnel making measurements would wear headphones that would receive a "click" every time the propeller of a mechanical current meter completed one revolution. The number of clicks counted in 40 to 70 seconds was used to compute velocity, which was recorded on a note sheet. In recent years, electronic interfaces have become available that allow personnel to input depths and subsection distances through a key pad or touch screen and record the meter revolutions and compute discharges. These interfaces can store a discharge-measurement file containing all of the pertinent measurement data so that the need for manual note keeping and the subsequent chance of an error is minimized.

A detailed discussion of conventional USGS wading-discharge-measurement techniques is given in Nolan and Shields, 2000. The following photographs are from Nolan and Shields, 2000.

USGS hydrographer making a discharge measurement with a top-setting wading rod.

Measuring water depth, using a top-setting wading rod.

Top of a top-setting wading rod.

Electronic discharge-measurement interfaces

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• The use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the United States Government.
• All information herein is provisional and subject to revision.
• Emphasis on the instruments described in these pages does not imply that these are the only instruments that can be used for this purpose. The instruments were selected for the following reasons: the instruments are readily adaptable to this purpose, the instruments are on Federal purchase schedules, and the instrument vendors have voluntarily committed resources to this application. Other instruments will be investigated within the limits of project funding and schedules.

U.S. Department of the Interior
U.S. Geological Survey
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URL: http://sunrise.er.usgs.gov/hydroacoustics/acoustic_wading_rod_measurements.shtml
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