In the past few months, radar technology has been applied to streamflow measurement. Surface velocity can be measured with devices similar to those used on the nation's highways to enforce speed limits. The basic physics behind the systems is the same as wellthe frequency of a radar signal changes as it is successively reflected by a moving object. The instruments convert the changes in frequency to a velocity. The instruments used for flow determination were originally designed for measurements of ocean currents. In April 1999, two versions of the radar-based velocity-measurement system were tested in Washington. A vehicle-mounted device appropriate for bridges over large rivers was used on the Skagit River to make the first-ever non-contact measurements of streamflow. A handheld instrument for use on smaller streams obtained surface-velocity values in a few seconds. Comparisons with conventional current-meter data showed the new techniques to be quite accurate. William Keller of the Applied Physics Lab at the University of Washington was involved in the design of the system, and John Costa of the USGS Cascades Volcano Observatory was involved in the field testing.
Surface velocities are hardly sufficient to determine discharge, of course. Studies are underway by the U.S. Geological Survey to improve estimates of average velocity from surface velocity beyond the old eight-tenths rule-of-thumb. And, for the cross-sectional area, there's a new use for ground-penetrating radar. This application was developed a few years ago by the USGS. The antenna for this type of radar can be moved back and forth across a river suspended from a bridge or cableway. The instrument produces a profile of the streambed and the water depth. Besides being much safer and faster to use than conventional techniques, the new technology allows measurements during major floods when bedload and debris transport is high and can obtain repeated measurements of channel geometry when the bed is unstable. A preliminary report on the ground-penetrating radar application was published in 1997: Spicer, K. R., J. E. Costa, and G. Placzek, Measuring flood discharge in unstable stream channels using ground-penetrating radar, Geology 25(5):423-426. Details about the velocity measurements were found in a newspaper article. The systems are still experimental, so don't call your local Walmart quite yet.