Rip channel migration in the nearshore
Minetree, Courtney M.
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Video imaging data generated from the Naval Postgraduate School Imaging System (NAPSIS) during November 2004 to June 2006 was analyzed to determine the location of rip channels and track their morphology. During the study period, the rip fields constantly changed in shape, size, and location. Rip channels were found to have a mean migration southward at a rate of 0.16 meters per day with a standard deviation of 7.6 meters per day and maximum rates varying between approximately 30 meters per day north and 30 meters per day south. The migration exhibited a strong seasonal variation with southerly shifts in the fall and winter months, northerly shifts in the late winter and early spring months, and no significant shift in the late spring and summer months. Directional wave spectra measured every hour at the offshore NOAA buoy were refracted to the 10 meter depth contour at Marina and Sand City and compared with measured spectra at these locations. The significant wave heights at both locations exhibited a correlation of 0.94. Mean wave directions for Marina and Sand City were found to have correlations of 0.83 and 0.34, respectively. These refracted data were then used to calculate sediment transport rates at Stillwell Hall, Fort Ord. Rip channel migration and calculated sediment transport rates were correlated at 0.8, qualitatively confirming the hypothesis that the migration rate of rip channels is a function of modeled alongshore sediment transport. The sometimes rapid migration of these large scale morphological features is critical to the successful planning and execution of U.S. Navy and Marine Corps beach assaults and the operation of mine warfare. Because amphibious and special forces operate mainly in shallow areas, the modeling of rip current direction and magnitude contributes greatly to effective mission organization and accomplishment. In addition to causing mines to drift, rip currents transport sediment that can cause the underlying morphology to change, possibly covering bottom mines and creating a potential hazard for military forces operating in the area. Being able to predict where mines may be drifting and how much sediment has concealed them is a necessity in securing a littoral battlespace.
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