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A ground-penetrating-radar system, a black-and-white fathometer, a color fathometer, and a tuned transducer were evaluated for their ability to measure riverbed scour at the Bulkeley, Charter Oak and Founder's Bridges in Hartford, Connecticut. Cross sections and some lateral sections were run at each bridge from June through July 1987, and significant scour at piers supporting each of these bridges was observed. Each of the four geophysical systems proved to have characteristic advantages and disadvantages. A ground-penetrating-radar system with single and dual 80-megahertz antennae floating in the water was used. It was successful in water less than 25 feet deep and in resistive earth materials. With this geophysical method, the geometry of existing scour holes was clearly defined as was infilling in some holes. Penetration ranged from 1 foot to 20 feet, depending on the physical properties of the riverbed materials, and the resolution of the subbottom layers was 2 feet. This system was effective at distances greater than 3 to 5 feet from the piers. The black-and-white fathometer was useful for defining existing scour holes. However, at a signal frequency of 200 kilohertz, it offered virtually no penetration except in soft muds. This system was effective for obtaining rapid and accurate depth determinations. The black-and-white fathometer was useful in conjunction with other methods for correlation and interpretation of geophysical data, because it substantiated the depth data obtained with other methods. The color fathometer, operating at a signal frequency of 20 kilohertz, digitized the reflected seismic signals and assigned a color for every 6decibel change in the acoustic impedence of the reflected signals. It delineated the existing scour-hole geometry, detected infilling of scour holes, and provided qualitative information about the physical properties of bottom and subbottom sediments. Side echo became a problem within 5 feet of the piers, and penetration ranged from 2 feet to greater than 20 feet, depending on riverbed materials. This method could not be used in water less than 5 feet deep. The tuned transducer used in this study operated at a signal frequency of 14 kilohertz. It defined the existing scour-hole geometry and the extent of infilling of soft sediments. Penetration ranged from 2 feet in coarseor hard-packed materials to greater than 30 feet in softer, fine-grained sediments. As with the color fathometer, side echo was a problem within 5 feet of the piers, and the method could not be used in water less than 5 feet deep.

Use of surface-geophysical methods to assess riverbed scour at bridge piers