Accurate and timely determination of river-bed geometry is essential for discharge and sedimentation studies involving unstable river channels. Knowledge of stream-channel cross-sectional area is a critical component of any direct estimate of water discharge at flood stage, and, by extension, of any calculation of suspended sediment discharge or bedload transport rate for the same peak flow. Sequential profiles of channel geometry can help define the active zone of bedload transport (to assist in the design of a sampling program), as well as aid in the indirect computation of bedload transport rates by developing a relation between bedform volume and celerity, and bedload discharge (Mahmood and Mehrdad, 1991). Typically, however, the conditions that result in the greatest movement of sediment and the greatest variability in channel geometry are also those that most challenge standard measuring techniques. Rivers at flood stage are dangerous to gage because of high velocities and drift (logs, stumps, debris), and traditional depth-measuring equipment is increasingly subject to error and/or failure as stream depth, velocity, and bed instability increase (Sauer and Meyer, 1992). A technique that allows non-contact monitoring of stream bed changes during high flow conditions is needed to avoid such problems, and to provide a safe way to acquire flow data that would otherwise be impossible to obtain.
One technology that may help improve the acquisition of channel-geometry data during high flow periods is ground-penetrating radar (GPR). Ground-penetrating radar is a geophysical technique that produces continuous high-resolution profiles of the subsurface by measuring the travel time of an electromagnetic pulse between a transmitter, a reflective boundary, and a receiver. The velocity of an electromagnetic pulse varies with the dielectric of the penetrated material (Davis and Annan, 1989). It is therefore necessary either to calibrate signal travel time to a known distance traveled in order to infer distance from the recorded travel time, or to use a known velocity through a material to calibrate the instrument.
Ground-penetrating radar has been used extensively and successfully to investigate surficial deposits (Smith and Jol, 1992), but much less so in surface-water hydrology. Applications of GPR in hydrogeologic studies have shown that GPR can discern water-sediment boundaries. Ground-penetrating radar studies on lakes and rivers have shown that when a low-frequency (100 megahertz) (MHZ) antenna is towed across relatively low conductivity (400 S/cm) water, GPR can be used to map water depth up to about 10 m with resolution on the order of 10 cm. GPR has been used to detect the water-streambed interface on a river and a lake in Connecticut (Beres and Haeni, 1991), and to profile a lake bed in Arizona (Sensors and Software, 1994). These studies, and the fact that a radar pulse can travel through both air and water (Davis and Annan, 1989), suggest that GPR can be used to obtain estimates of water depth in streams and rivers when conditions make conventional stream gaging difficult or dangerous. The following report is a description of field tests conducted to assess this possibility.
