A. B. Murray and M. LeBars

Observations of the behavior of waves in the presence of a rip
currents--rapid, concentrated offshore flows in and near the surf
zone--suggest that rip currents cause a local decrease in wave height.
Such an interaction could lead to the self-organization of rip
currents. In a steady-state, alongshore-uniform situation, the momentum
waves deliver as they dissipate in the surf zone (a gradient in the
radiation stress), would support an offshore surface slope (the set
up). If a perturbation, such as a local, transient decrease in
incident-wave height, initiates a weak offshore current, and that
current then caused a sustained decrease in local wave height, the
local radiation-stress gradient might no longer support the local
set-up slope. The resulting acceleration of the offshore flow could
produce a strong rip current, further reducing local wave heights. We
have developed a cellular numerical model to test this hypothesis. The
model simulates effects including those of: 1) wave momentum flux and
water transport; 2) incident-wave height variation in time and space;
3) surface-slope driven alongshore and cross-shore currents; 4)
alongshore mixing of cross-shore current momentum; 5) bed friction; 6)
and a hypothesized mechanism for the dissipation of wave energy in the
presence of current-generated, large-scale turbulence.

The model produces strong, narrow, widely spaced, dynamic rip currents
that can occur on planar as well as nonplanar beaches. Rip-current
velocities, widths, durations and frequencies of occurrence in the
model match field observations. While indicating that the model offers
a possible explanation for a wide range of rip-current behaviors, this
quantitative agreement results from the adjustment of two poorly
constrained parameters. We are testing model predictions that do not
depend on parameter adjustment, but arise robustly from the
interactions in the model: Rip-current activity (defined as average
rip-current duration times frequency of occurrence divided by the
alongshore distance considered) decreases with increasing temporal
irregularity of the incomming waves, and with increasing beach slope
(decreasing surf-zone width). We have made extensive video observations
of Torry Pines Beach in Southern California, and from these measured
rip-current durations and frequencies of occurrence under a range of
wave conditions. The direction of the trend of the relationship between
rip-current activity and temporal irregularity of the incident waves
matches the model prediction. Field work to test the model prediction
of rip-current dependence on beach slope is in progress.

Supported by the Andrew W. Mellon Foundation and the Office of Naval
Research, Coastal Dynamics.