EM 1110-2-1100 (Part II)
31 July 2003
Figure II-4-16.
Equation II-4-37 compared with field and laboratory data (Komar
1979)
c. Cross-shore current. Unlike longshore currents, the cross-shore current is not constant over depth.
The mass transport carried toward the beach due to waves (see Part II-1) is concentrated between the wave
trough and crest elevations. Because there is no net mass flux through the beach, the wave-induced mass
transport above the trough is largely balanced by a reverse flow or undertow below the trough. Figure II-4-18
current may be relatively strong, generally 8-10 percent of %&& near the bottom. The vertical profile of the
shows field measurements of the cross-shore flow below trough level on a barred profile. The undertow
gd
undertow is determined as a balance between radiation stresses, the pressure gradient from the sloping mean
water surface, and vertical mixing. The first quantitative analysis of undertow was given by Dyhr-Nielsen
and Sorensen (1970). The undertow profile is solved by Dally and Dean (1984), Hansen and Svendsen
(1984), Stive and Wind (1986), and Svendsen, Schffer, and Hansen (1987).
Surf Zone Hydrodynamics
II-4-25
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