2014-01-29_HYDROLOGY - P2009025 (8)

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Appendix D — SOP for Stream Flow Measurement — Hansen Project 10 3.1.4 Quality Assurance All flumes will be inspected prior to measurement of discharge to determine that the flume is discharging freely. Any problems observed during the inspection will be noted and reported to the Project Manager. 3.2 Velocity -Area Method using Price AA or Pygmy Meter Surface flow in stream channels and ditches that lack flumes will be measured by using the traditional stream gaging technique, the velocity -area method. 3.2.1 Current Meters 3.2.1.1 Vertical Axis Current Meter When using the velocity-area method to perform discharge measurements of flowing surface water streams, the vertical axis current meter has been selected for the following reasons: • This meter operates in lower velocities than the horizontal -axis meter • Bearings are well - protected from silty water • Rotor can be repaired in the field without adversely affecting the measurement • Single rotor serves for the entire range of velocities A common type of vertical axis current meter is the Price current meter, Type AA. The standard Price meter has a rotor fivem inches in diameter and two inches high with six cone - shaped cups mounted on a stainless steel shaft. A pivot bearing supports the rotor shaft. The contact chamber houses both the upper part of the shaft and a slender bronze wire ( "cat's whisker ") attached to a binding post. With each revolution, an eccentric contact on the shaft makes contact with a bead of solder at the end of the cat's whisker. A separate reduction gear (pentagear), wire, and binding post provide a contact each time the rotor makes five revolutions. A tailpiece keeps the meter pointing into the current. In addition to the type AA meters, the USGS and others use a Price Pygmy meter in shallow depths. The Pygmy meter is scaled two -fifths as large as the standard meter and has neither a tailpiece nor a pentagear. The contact chamber is an integral part of the yoke of the meter. The Pygmy meter makes one contact per revolution. The predominant flow conditions at the site indicate that either the Pygmy meter or the Price AA meter may be needed at particular sites, depending on the amount of runoff contributing to streamflow at the time. 3.2.1.2 Marsh - McBirney Velocity Meter Portable Marsh - McBirney flowmeters are a type of current meter that may be used in the velocity area flow measurement. Either Model 201D or Model 2000 may be used. The Marsh - McBirney meters measure flow velocity using the Faraday principle which states that as a conductor moves through and cuts the lines of a magnetic flux, a voltage is produced. The magnitude of the generated voltage is directly proportional to the velocity at which the conductor moves through the magnetic field. The selector switch of the Model 201D has five positions - these positions are OFF, CAL, FT /SEC, M/SEC and KNOTS. To check for electronics failure or discharged batteries, the selector is first switched to the CAL position and the time constant switch to 2. If this is not the case, change the batteries and check the CAL again. The time constant switch is used to help stabilize the velocity readings. General procedures are to begin with the smallest time constant and increase if the readout velocity does not stabilize. The sensor on the Marsh - McBirney meter can be connected to the universal sensor mount on the top- setting wading rod and used to determine flows using either the six - tenths depth method or the two- tenths and eight - tenths depths method. The minimum flow depth at which an average velocity can be measured with the Marsh - McBirney meter set at six - tenths of total depth is approximately 0. 18 feet. Velocities can be estimated 4153A.140129 Whetstone Associates •