15. Longshore Currents
Obliquely approaching waves direct a portion of their momentum flux alongshore,
which generates a longshore current whose speed can range from 0.2 - 1.0 m/s.
Critical to our understanding of longshore sediment transport.
Mean Longshore Current
Longshore currents can be related to the longshore component of the cross
shore directed radiation stress, which is given by
where
sin α term - accounts for the longshore component of E,
cos α term - accounts for the increasing portion of coastline along which an
increasingly oblique wave spreads its energy flux.
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Estimates of Steady Longshore Current Velocity
Using radiation stress,
monochromatic waves, linear
wave theory, a planar-sloped
beach, and a saturated surf
zone, Longuet-Higgins (1970)
estimated the steady longshore
current velocity within the surf
zone to be
where tan β is the beach slope,
Cf is a friction coefficient, um is
the maximum orbital velocity
and αb is the wave angle to the
shoreline at the break point.
Empirically derived relationship
Cross Shore Distribution of Longshore Current Velocity
If horizontal mixing in the surf zone is considered NOT to exist, the cross shore
pattern of LS current velocity increases linearly to the break point, then drops to 0,
outside of the surf zone.
If horizontal mixing DOES exist, the degree of mixing will exert an influence on the
pattern, and a family of curves is defined based on the degree of mixing which
occurs.
Idealized (breaker line) and
realistic (breaker zone) cross
shore distributions of
longshore current velocity
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Comparison with Measurements (Thornton and Guza, 1986)
Examined a spectrum of waves
Longshore Currents on
a Barred Beach
Bars complicate longshore current
velocity prediction because of
bathymetrically induced irregularities
in wave breaking and set-up.
Measurements reveal that, compared
to predictions on a planar beach,
longshore currents were lower in the
outer surf zone and higher in the inner
surf zone.
Shown are measurements at storm
onset (B.) and during storm peak (C.)
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Shear Instabilities of the Mean Longshore Current
a.k.a. Shear Waves
Far infragravity oscillations O(200-400s)
result from instabilities in horizontal shear
of LS current; better developed on barred
coasts, where LS current flows in a trough.
Produce a wave/meander in LS current that
propagates alongshore at a speed of ~1/3
that of mean LS current
Movie on Nearshore Processes:
“A River of Sand”
http://www.youtube.com/watch?v=FqT1g2riQ30
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