Water Depth = 6.1cm
Figure 22. Measured and predicted wave spectra at Gauge 9 for bimodal sea
state shoaling on a constant slope beach
Rip Currents on Barred Beaches
Rip currents are narrow, jet-like currents that flow out from the surf zone
towards the open ocean. Rip currents are typically generated when there are
alongshore variations in the wave breaking location. Alongshore variations could
be associated with bathymetric effects (e.g., gaps in sandbars), the presence of
structures (piers, jetties, etc.) or edge waves, generated by the trapping of
reflected waves near the shoreline. One characteristic feature of rip currents is
their unsteady nature. The currents tend to occur episodically as well as oscillate
in the horizontal plane (Smith and Largier 1995).
Haller, Dalrymple, and Svendsen (1997) carried out laboratory experiments
to investigate the generation of rip currents on a barred beach with rip channels.
The bathymetry consists of a 1:30 constant slope beach with a superimposed
longshore bar. The bar has two 1.8-m-wide gaps as shown in Figure 23. The
water depth is 40 cm at the seaward boundary and 5 cm at the top of the bar. The
Boussinesq model was applied to a regular wave test case with height
H = 4.5 cm, and period T = 1 s. Numerical simulations were carried out using
grid spacings ∆x = ∆y = 0.06 m and time-step size ∆t = 0.015 s.
Figure 24 shows a snapshot of the instantaneous water-surface elevation 70 s
into the simulation. Waves propagating over the bar break on the bar while those
propagating through the gap break closer to the shoreline. This sets up a spatial
variation of the mean water level and drives a time-varying circulation pattern.
Figure 25 shows a 2-D map of the mean currents averaged over 50 wave cycles
(from t = 150 s to t = 200 s). As observed in the laboratory experiments, there are
two pairs of counterrotating circulation cells. The primary circulation consists of
Chapter 5 Model Validation