discussion by Oltman-Shay and Hathaway (1989)). The relative amount of infragravity energy and incident

wind wave energy is a function of the surf similarity parameter (Holman and Sallenger 1985, Holman 1986),

with infragravity energy dominating for low values of the surf similarity parameters (*ξ*o < 1.5). For low

values, the energy spectrum at incident frequencies is generally saturated (the spectral energy density is

independent of the offshore wave height, due to wave breaking), but at infragravity frequencies, the energy

density increases linearly with increasing offshore wave height (Guza and Thornton 1982, Mase 1988).

Storm conditions with high steepness waves tend to have low-valued surf similarity parameters, so

infragravity waves are prevalent in storms. Velocities and runup heights associated with infragravity waves

have strong implications for nearshore sediment transport, beach morphology evolution, structural stability,

harbor oscillation, and energy transmission through structures, as well as amplification or damping of

infragravity waves by the local morphology or structure configuration. Presently, practical questions of how

to predict infragravity waves and design for their effects have not been answered.

(1) The current in the surf zone is composed of motions at many scales, forced by several processes.

Schematically, the total current *u *can be expressed as a superposition of these interrelated components

(II-4-33)

where *u*w is the steady current driven by breaking waves, *u*t is the tidal current, *u*a is the wind-driven current,

and *u*o and *u*i are the oscillatory flows due to wind waves and infragravity waves. Figure II-4-13 shows long-

shore and cross-shore currents measured in the surf zone at the Field Research Facility in Duck, NC. The

mean value of the current in the figure is the steady current driven by breaking waves and wind, the long per-

iod oscillation is due to infragravity waves, and the short-period oscillation is the wind-wave orbital motion.

(2) Currents generated by the breaking of obliquely incident wind waves generally dominate in and near

the surf zone on open coasts. Strong local winds can also drive significant nearshore currents (Hubertz 1986).

Wave- and wind-driven currents are important in the transport and dispersal of sediment and pollutants in the

nearshore. These currents also transport sediments mobilized by waves. Tidal currents, which may dominate

in bays, estuaries, and coastal inlets, are discussed in Parts II-5 and II-7.

(3) Figure II-4-14 shows typical nearshore current patterns: a) an alongshore system (occurring under

oblique wave approach), b) a symmetric cellular system, with longshore currents contributing equally to

seaward-flowing rip currents (occurring under shore-normal wave approach), and c) an asymmetric cellular

system, with longshore currents contributing unequally to rip currents (Harris 1969). The beach topography

is often molded by the current pattern, but the current pattern also responds to the topography.

(4) Nearshore currents are calculated from the equations of momentum (Equations II-4-34 and II-4-35)

and continuity (Equation II-4-36):

M*U*

M*U*

Mη

%*V*

' &*g*

% *F*bx % *L*x % *R*bx % *R*sx

(II-4-34)

M*x*

M*y*

M*x*

M*V*

M*V*

Mη

%*V*

' &*g*

% *F*by % *L*y % *R*by % *R*sy

(II-4-35)

M*x*

M*y*

M*y*

II-4-20

Surf Zone Hydrodynamics

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