Rip current/cuspate shoreline interactions in Southern Monterey Bay

Abstract

The interaction between rip channels and cuspate shoreline was examined by analyzing data obtained by the Naval Postgraduate School Imaging System (NAPSIS) during the winter of 2004-2005 in Southern Monterey Bay. Video imaging data was used to determine rip channel locations. The rip fields had constantly changing shapes and sizes, and the beach underwent a transformation from a Transverse-barred-beach (TBB) to a Longshore-bar-trough (LBT) state. Mean rip spacing was determined to be 173 and 258m respectively for the two different beach states (TBB and LBT). Directional wave spectra measured at the offshore NOAA buoy in deep water were refracted to the 10m depth contour at the actual study site. Estimated alongshore sediment transport, Qs, was calculated using the refracted wave data. The hypothesis that rip channel migration is due to alongshore sediment transport is qualitatively confirmed. Little or no migration occurred when Qs values were close to zero. Migration rates were calculated over a three week period during a time of high rip mobility with an average migration rate of 3.2m per day. The rip channel orientations were constantly changing. Three distinct rip channel shapes were common: straight, slanted, or C shaped. The rip channels tended to slant in the opposite direction of the estimated sediment transport, since the rip channels migrated more rapidly at their base (nearest to shore) and more slowly offshore. The hypothesis that the mega-cusps on the beach are erosional features of rip currents was tested by crosscorrelating the 2m beach contour obtained using GPS beach surveys with an alongshore video pixel intensity line. During a time of steady rip channel migration, it was found on average that the cusps lagged the rip channels by 50m with a maximum correlation near one. Assuming the system is in steady state, a response time of 14.7 days was obtained by dividing the lag distance by the average migration rate.