HR: 15:20h
AN: OS13C-07    [Abstracts]
TI: Rip-Currents and Large Beach Cusps
AU: * Thornton, E B
EM: thornton@nps.edu
AF: Naval Postgraduate School, Oceanography Department, Monterey, CA 93943 United States
AU: MacMahan, J H
EM: macmahan@nps.edu
AF: Naval Postgraduate School, Oceanography Department, Monterey, CA 93943 United States
AU: Reniers, A J
EM: ad@dutcvmm.ct.tudelft.nl
AF: Delft University of Technology, Civil Engineering and Geoscience, Stevinweg 1, Delft, 2628CN Netherlands
AU: Stanton, T P
EM: stanton@nps.edu
AF: Naval Postgraduate School, Oceanography Department, Monterey, CA 93943 United States
AB: Well-developed, large-scale beach cusps O(200 m) associated with rip currents are studied along 18 km of Southern Monterey Bay, California. Low-tide terraced bars incised by rip channels characterize the nearshore morphology. There is a large increase in wave height going from small wave heights at the southern most part of the bay in the shadow of a headland, to the center of the bay where convergence of waves owing to refraction over Monterey Bay Canyon results in increased wave height. The strongly refracted waves result in near normal incidence along the shoreline and well-developed rip currents. The large alongshore gradient in wave climate results in a concomitant alongshore gradient in morphodynamic scale. The alongshore spatial and temporal variations of cuspate shoreline, measured by a Kinematic GPS equipped ATV, and rip channel spacing, as determined from opportunistic shore parallel jet-ski runs and video image analysis, are cross-correlated. Approximate bi-monthly shoreline surveys were initiated in Dec 03, along with directional wave spectra measured at two alongshore locations in 12 m depth. Rip current morphology and cuspate shoreline are significantly correlated. Decorrelation times and migration rates are calculated from cross-correlations of the shoreline spatial series. Between shoreline surveys on yearday 343, 2003 and 7, 2004, a major storm occurred (7 m significant wave height offshore on yearday 344 during time of spring tides) and the decorrelation time was less than the time between surveys. However, the decorrelation time thereafter (yeardays 7 - 100, 2004) exceeded 90 days, from which it is determined that bi-monthly surveys are sufficient. Based on the cross-correlations, the cusp system at the southern extreme of the bay in the shadow of the headland was stationary and stable. The cusps in the middle were initially stationary for 22 days, migrated rapidly at ~2 m/day to the south for 24 days and then returned to stationary. It is noted that the largest waves (> 8 m) occurred on yearday 91 during a time of neap tides, and the shoreline remained correlated between surveys. It is concluded the bi-monthly surveys are sufficient under "normal waves", but the additional surveys need to be performed after storms. The migration direction is found correlated with the net alongshore sediment transport. A research version of the Delft3D morphodynamic model is used to explain the relationship between alongshore length scale of cusps and wave climate of the incident waves. It is shown that the length-scales of the large cusps are associated with both the directional bandwidth and spectral energy of the incident waves.
DE: 4546 Nearshore processes
DE: 4560 Surface waves and tides (1255)
DE: 3020 Littoral processes
DE: 3022 Marine sediments--processes and transport
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting