EM 1110-2-1100 (Part II)
31 July 2003
Figure II-4-17. NMLONG simulation of longshore current (Leadbetter Beach, Santa Barbara,
California, 3 Feb 1989 (Thornton and Guza 1986))
d. Rip currents.
(1) The previous sections on longshore current, cross-shore current, and wave setup focused on processes
that are two-dimensional, with waves, currents, and water levels changing only in the cross-shore and vertical
directions, but homogeneous alongshore. Rip currents, strong, narrow currents that flow seaward from the
surf zone, are features of highly three-dimensional current patterns. Rip currents are fed by longshore-
directed surf zone currents, which increase from zero between two neighboring rips, to a maximum just before
turning seaward to form a rip current. Rip currents often occur periodically along the beach, forming
circulation cells (Figure II-4-14b,c). High offshore-directed flows in rip currents can cause scour of the
bottom and be a hazard for swimmers.
(2) Rip currents and cell circulation can be generated by longshore variations in wave setup. Breaking
wave height and wave setup are directly related; thus ,a longshore variation in wave height causes a longshore
variation in setup. The longshore gradient in setup generates longshore flows from the position of highest
in the term M&/My in the longshore momentum equation (Equation II-4-35). The longshore variation in wave
waves and setup toward the position of the lowest waves and setup (Bowen 1969b). This effect can be seen
η
setup may be caused by convergence or divergence of waves transforming across bottom topography (Sonu
1972, Noda 1974) or the sheltering effect of headlands, jetties, or detached breakwaters (Gourley 1974, 1976;
Sasaki 1975; and Mei and Liu 1977). Edge waves can interact with incident waves to produce a regular
variation in the breaker height alongshore, and thus generate regularly spaced rip currents (Bowen 1969b,
Bowen and Inman 1969). Interaction of two intersecting wave trains can similarly generate regularly spaced
rip currents (Dalrymple 1975).
II-4-26
Surf Zone Hydrodynamics
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