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Costal Inlets Research Program
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Conclusions from Turbulence Scale Effects Experiments
Figure 50. Experiment setup for harbor dredging planform configuration
TR-03-6
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6 Impact of Dredging Planform
Introduction
At
present,
the
upstream
and downstream
ends
of the dredged
region
of the
Port
of
Anchorage
have
vertical
sidewalls
cut
out
of hard,
nonerodible
material.
The
dredge
transitions
are
oriented
at a 30
deg
angle
to the
shoreline.
The
abrupt
vertical
transition
may
cause
quiet
flow
regions
which
allow
suspended
sediment
to
deposit
in the
harbor.
The
objective
of
this
task
was
to
test
the hypothesis
that
sloping
dredge
planform
transitions
on
both
ends
of the
harbor
would
decrease
areas
of
flow
reduction
and
increase
harbor
flushing.
Comparisons were
made
between
existing
dredging
practice and the
proposed
dredging
planform
featuring
sloping
transitions.
Both
alternatives
were
fabricated
and placed on the
precision
flow
table
for
testing.
Flow
fields
near
the
transition
were
quantified
using
the
laser
Doppler
velocimeter,
and
dye
injection
was
used
as a
qualitative
tool
to
evaluate
the
two
dredge
planforms.
Experiment
Setup
A
rectangular
flow
channel of
width
380
mm
(14.9
in.)
was
created
on the
flow
table
using
Plexiglas
blocks
as
illustrated
in
Figure
50.
The
water
depth
was
set
to 44 mm
(1.7
in.),
and a
flow
rate of
1.7
L/sec
3(.5
pints/sec)
was
used
for
the experiments.
Average
flow
velocity
over
the channel
cross
section
was
approximately
10
cm/sec
(3.9
in./sec).
Relevant
scaling
factors
(prototype-to-
model
ratios)
for
the dredge
transition
models
are
listed
in
Table
9
along
with
approximate
model
equivalences.
Model
geometric
distortion
was
Ω
= N
X
/N
Z
=
5
for
this
simplified
model
of the
Port
of
Anchorage
dredging
transitions.
Two
dredging
transition
models were
fabricated
out
of
Plexiglas
by the
ERDC
Model
Shop.
The
first
model,
shown on the
left
side
of
Figure
51,
featured
a
vertical
transition
along
the dredge cut
that
is sloped at 30 deg to the
shoreline.
This
represented
the
present
dredging
practice.
The
second
model,
shown on the
right
side
of
Figure
51,
is
similar
except
for
the
sloping
transition
from
the
deeper
dredged
area
up to the
existing
bottom
elevation.
In the
first
tests
each
transition
was
oriented
as shown in
Figure
50
with
the
deeper
portion
upstream
of the
shallower
portion.
This
represented
flow
leaving
the
harbor
over
the
transition.
Next,
the
transitions
were
reversed
on the
flow
table
to
represent
tidal
flow
from
the
shallower
area
entering
the
harbor
over
the
transition.
Velocities
on the
horizontal
portion
adjacent to the
transition
were
67
Chapter
6
Impact
of
Dredging
Planform
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