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<br />Acoustic Doppler and Marsh McBernie Velocity Meters <br /> <br />For model calibration it is important to have observed measurements of depth and <br /> <br />velocity at known flows. Whi Ie depth can be gathered using the same technologies that are <br /> <br />used in determining bathymetry, velocity measurements requires another set of instruments. <br /> <br /> <br />Two different technologies were used for measuring velocities in this project. <br /> <br /> <br />In 1998, a Marsh McBernie Flo-Mate Portable Flowmeter was used in conjunction <br /> <br /> <br />with the total station to determine point velocities. The Marsh McBemie has a published <br /> <br />accuracy of l.Scm/s +/- 2% of reading and is based on the electrical principle known as <br /> <br />Faraday's Law, where the flow rate of the fluid can be determined by passing a conductive <br /> <br />fluid through a magnetic field. A wading rod used to hold the meter head which is placed at a <br /> <br /> <br />depth chosen to represent average velocity (usually .6 of total depth) Locations were recorded <br /> <br /> <br />by shooting the point with the total station and then recording the average of three 10-second <br /> <br /> <br />readings. <br /> <br />In larger rivers with high velocities and deep areas, it is often not practical to use a <br /> <br />wading rod to measure velocities. In May of 2000, a 3MHZ Sontek River Surveyor Acoustic <br /> <br />Doppler Profiler (ADP) was purchased to gather calibration data on the IS-mile reach of the <br /> <br />Colorado River. The ADP measures the velocity of water using a physical principle called the <br /> <br />Doppler shift. This states that if a source of sound is moving relative to the receiver, the <br /> <br />frequency of the sound at the receiver is shifted from the transmit frequency. By determining <br /> <br />the Doppler shift using three beams, it is possible to determine the relative speed and direction <br /> <br />of the flow. Using the Doppler shift from the river bottom to determine the boat speed and <br /> <br />direction, it is possible to compute absolute velocity and direction. The ADP measures <br /> <br />velocities in IScm vertical increments down to the river bottom. These velocities are <br /> <br />averaged over a specified time interval. <br /> <br />16 <br /> <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />1 <br />