Blasting Bridges And Culverts: Water Overpressure And Vibration Effects On Fish And Habitat

Abstract

Water overpressures and ground vibrations from blasting may injure or kill salmonid fish in streams and embryos in streambeds. Explosives are used to remove failing structures in remote areas of the Tongass National Forest that impair watershed function. The State of Alaska Department of Fish and Game standards limit blast induced water overpressures to 2.7 lb/in 2 (18.6 kPa) and streambed vibrations to 0.5 in/s (13 mm/s) when embryos are present. Researchers, however, have reported salmonid mortality from pressures only as low as 12.3 and 19.3 lbs/in2 (85 and 133 kPa) and embryo mortality from vibrations as low as 5.75 in/s (146 mm/s). I recorded in-stream overpressures and streambed vibrations with hydrophones and geophones at various distances from log bridge, log culvert, and metal culvert blasts. Peak water pressures (lb/in2) were directly related to cube-root scaled distances with an attenuation rate of -1.51. Peak particle velocities in gravel were directly related to square-root scaled distances (SRSD, ft/lb 1/2) with an attenuation rate of -0.75. Water pressures were less than 7.1 lb/in2 (49.0 kPa) in all but one blast, and streambed vibrations did not exceed 5.5 in/s in gravel streambeds. State standards should be revised to reflect reported mortality and these observations of blasts in streams.