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Costal Inlets Research Program
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> Concluding Discussion - MorphoTimeICCE040011
igure 6. Simulated offshore losses and associated shoreline locations adjacent to lateral groins in a groin compartment with periodic, instantaneous change in wave direction.
Figure 8. Comparison of shoreline evolution in enclosed groin compartment at steady-state
MorphoTimeICCE04
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2630
COASTAL
ENGINEERING
2004
50
1000
m
3
40
Q
off
/Q
g
(%)
30
20
10
0
1
2
3
4
5
6
7
8
ξ
Figure 7.
Offshore
transport losses,
expressed
in 10
3
m
3
and
in
percentage
of
the
gross
longshore
transport
rate
Q
g
half-way
between
the
groins, in a groin
compartment
as a function of
the
morphodynamic
response
factor
ζ
.
the
offshore
losses.
Thus, as
the
shoreline
orientation
is preserved
over
time,
the
offshore
loss
through
rip
currents
per
wave
cycle
will
not
decrease
with
time. It
should be
noted,
however,
that
diffraction
from
the
groin
tips
is not
considered
in
these
simulations.
If it
were
considered,
there
would
probably
be a gradual
reduction in
loss
rate
as erosion
continues
and
the
distance
between
the
shoreline
and
the
outer
groin
tips
increase.
6.
Concluding
Discussion
The
present
study
is
based
on
the
observation
that
rip
currents
are
typically
induced in
the
vicinity
of groins
and
jetties
in
situations
where
there
is a large
difference
in
orientation
between
the
shoreline
and
the
breaking waves.
Analytic
and
numerical
solutions
were
applied
to illustrate
and
quantify
this
phenomenon.
Previous
studies
have
shown
that
not
only
the
net
longshore
transport
rate
but
also
the
gross
rate
is
important
for
the
shoreline
response
to groins
and
jetties.
These
rates
and
their
relation
are
primarily
associated
with
variability
of
wave
directionality.
The
present
study
takes
this
discussion
further
in
introducing
the
time
scale
or
frequency
of
these
changes.
The
difference
in
orientation
between
the
hydrodynamic
forcing
(breaking
waves)
and
the
morphodynamic
response
(shoreline)
may
be interpreted as a
phase
shift
between
two
systems
that
may
have
drastically
different
temporal
scales.
For
an
open-coast
system
with
a single
groin
or jetty
exposed
to an
instantaneous
change
in
the
direction
of
the
forcing,
the
critical
time
scale
is
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