EM 1110-2-1100 (Part II)
31 July 2003
Figure II-4-13.
Measured cross-shore and longshore flow velocities
M(Ud )
M(Vd )
(II-4-36)
'0
%
Mx
My
where
U = time- and depth-averaged cross-shore current
V = time- and depth-averaged longshore current
Fbx , Fby = cross-shore and longshore components of bottom friction
Lx , Ly = cross-shore and longshore components of lateral mixing
Rbx , Rby = cross-shore and longshore components of wave forcing
Rsx , Rsy = cross-shore and longshore components of wind forcing
(5) These equations include wave and wind forcing, pressure gradients due to mean water level varia-
tions, bottom friction due to waves and currents, and lateral mixing of the current. The primary driving force
is the momentum flux of breaking waves (radiation stress), which induces currents in both the longshore and
cross-shore directions. Radiation stress is proportional to wave height squared, so the forcing that generates
currents is greatest in regions of steep wave height decay gradients. Bottom friction is the resisting force to
the currents. Bottom roughness and wave and current velocities determine bottom friction. Lateral mixing
is the exchange of momentum caused by turbulent eddies which tend to "spread out" the effect of wave
forcing beyond the region of steep gradients in wave decay. Longshore, cross-shore, and rip current
components of nearshore circulation are discussed in the following sections.
Surf Zone Hydrodynamics
II-4-21
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