HR: 0800h
AN: H51I-0900    [Abstracts]
TI: Cobble Cam: Grain-Size Analysis of Gravel Bars from Digital Photographs
AU: * Warrick, J A
EM: jwarrick@usgs.gov
AF: USGS Coastal and Marine Geology, 400 Natural Bridge Dr., Santa Cruz, CA 95060, United States
AU: Rubin, D M
EM: drubin@usgs.gov
AF: USGS Coastal and Marine Geology, 400 Natural Bridge Dr., Santa Cruz, CA 95060, United States
AU: Ruggiero, P
EM: pruggiero@usgs.gov
AF: Oregon State University, Geosciences Department, 104 Wilkinson Hall, Corvallis, OR 97331, United States
AU: Draut, A
EM: adraut@usgs.gov
AF: USGS Coastal and Marine Geology, 400 Natural Bridge Dr., Santa Cruz, CA 95060, United States
AU: Todd, E
EM: etodd@usgs.gov
AF: USGS Coastal and Marine Geology, 400 Natural Bridge Dr., Santa Cruz, CA 95060, United States
AB: Here we report on our efforts to apply the autocorrelation techniques of Rubin (2004) to measure grain-size of fluvial and coastal gravel bars using digital photographs. Digital photos were obtained from approximately 1 m above the ground surface of granule- to cobble-sized sediment. Physical measurements of the intermediate and long dimensions of clasts in the field closely resembled the short and long axes, respectively, measured from the digital photos on a computer. Calibration curves for the autocorrelation technique were generated from a series of the best-sorted samples in the digital photographs. Optimal results occurred when the analyses were discontinued at an autocorrelation threshold of 0.25, for which r-square between the actual grain size and the autocorrelation estimate of grain size was 0.94 and the r.m.s. error was 0.3 phi-units. We also evaluated the effects of wet/dry conditions and illumination on these results. Replicate photos were obtained of dry gravel and the same gravel that was wet artificially. The mean difference in the estimated grain-size from the wet and dry conditions was 4%. Patchy wetness, however, introduced an error on the same length scale of the patchiness. Illumination variability also introduced error (up to 10%) shown by repeat photographs at a single station over the course of a day. These illumination effects can be minimized by the use of shades and a camera flash, although these additional pieces of field equipment may introduce
DE: 1861 Sedimentation (4863)
SC: Hydrology [H]
MN: 2007 Fall Meeting