Figure 22. Measured and predicted wave spectra at Gauge 9 for bimodal sea state shoaling on a constant slope beach

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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