Technical Report Series
Number 77-2
32
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32
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CHARACTERISTICS OF WATER FLOW
AT THE NORTH END OF THE
WASSAW BARRIER ISLAND COMPLEX
Wassaw Island Erosion Study
Part II
by
George F. Oertel
31
31
Georgia Marine Science Center
University System of Georgia
Skidaway Island, Georgia
81
CHARACTERISTICS OF WATER FLOW AT THE
NORTH END OF THE WASSAW BARRIER ISLAND COMPLEX
WASSAW ISLAND EROSION STUDY
PART II
by
George F. Oertel
Skidaway Institute of Oceanography
P.O. Box 13687
Savannah, Georgia 31406
The Technical Report Series of the Georgia Marine Science
Center is issued by the Georgia Sea Grant Program and the Marine
Extension Serv i ce of the University of Georgia on Skidaway Island
(P.O. Box 13687, Savannah, Georgia 31406). It was established to
provide dissemination of technical information and progress reports
resulting from marine studies and investigations mainly by staff
and faculty of the University System of Georgia. In addition, it
is intended for the presentation of techniques and methods, reduced
data and general information of interest to industry, local, regional,
and state governments and the public. Information contained in these
reports is in the public domain. If this prepublication copy is cited,
it should be cited as an unpublished manuscript.
INTRODUCTION
This report supplements an earlier report on the ••sedimentary
framework of a channel margin shoal of an ebb delta, Wassaw Sound,
Georgia
11
•
The purpose of this report is to produce an overview of
water currents that may affect the sediment carpet described in the
earlier report.
Water flow was recorded at four different stations around the
marginal shoal (Fig. 1).
At each station, data was recorded for two
tidal cycles (approximately 25 hours) at six minute intervals.
In
all, approximately 2,000 data points were collected.
OBSERVATIONS
Stations A and B were located on the leeward side of the shoal
and stations C and D were located on the channelward side of the shoal.
Plotted data of water flow surveys illustrate the reversing flow of
tidal currents above the shoal and adjacent shoreface.
In all cases,
flow reverses corresponded to the predicted change in the semi-diurnal
tide (Figure 2,3,4,5).
Maximum current speeds were recorded at station C located at
approximately -6 meters (MLW) on the margin of the shoal and the channel.
Station D was located approximately one quarter mile closer to
the shoal than station C and at approximately -2 meters (MLW).
The
mean current speeds at stations C and D were 40 cm/s and 20 cm/s,
respectively.
Higher velocities at station C illustrate the channel-
ized flow of the main channel.
Increases in
caused a reduction in flow speed.
flow of water was
the main channel.
11
Shielding
11
11
bed drag
at station D
11
During the ebbing tide the offshore
by the marginal shoals and confined to
During the flooding tide,
11
Shielding caused water
11
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Figure 1.
Location map of study area and current meter stations.
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May 28, 1975
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Figure 2.
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Time in Hours
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Graphic representation of tidal current velocities and directions at Station A during a 26-hour
interval (2 cou1plete tidal cycles). Stippl ed area indicates the portion of the sample when
velocities were above the estimated threshol L. velocity for the sediment at the seabed.
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Figure 3.
Graphi c representation of tidal current velocities and directions at Station B during a 26- hour
interval (2 complete tidal cycles). Stipp l ed area indicates the portion of the sa111ple when
velocities were above the estit»ated threshol d velocity for the sediment at the seabed.
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Figure 4.
°
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Graphic representat i on of tida l current vel oc i t i es and directions at Stat i on C during a 26- hour
interval (2 comp l ete tida l cyc l es). St i pp l ed area indicates the portion of t he sa111p l e when
ve l ocities were above the estinlated threshol d ve locity for the sediment at th e seabed.
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Graphi c representat i on of t i da l current vel oc i t i es and di rect i ons at Stat i on C during a 26- hour
interva l {2 comp l ete tida l cyc l es). St i pp l ed area i ndicates t he portion of t he samp l e when
ve l ocit i es were above the est i mated t hres hold ve l ocity for the sed i ment at the seabed.
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Figure 5.
Graphic representation of tidal current velocities and dire ctions at Station D during a 26- hour
interval (2 complete tidal cycles). Stippled area indicates the portion of the samp l e when
velocities were above the esti111ated threshold velocity for the sediment at the seabed.
7
to flow northeast around the distal end of th e shea l and "spi 11
into the main channel.
11
In the channe l , l andward flo od in g wat er was
confined by channel wa l ls and fl0w wa s t o the northwest.
Flow vectors
were determined at each station by obtaining the products of the
mean speed and duration, and plotted in the mean di rections for the
ebb and flood.
Vectors and the graphic resultants of vectors are
illustrated in figure 6.
The resultant of the ebb and flood vectors at station 0 was
oriented toward the central axis of the channel.
The resultant at
station C was very sma ll and oriented away from the channel ax i s.
After two complete tidal cyc l es, stat io n 0 appeared to il lu strate
a residual flow of water "spilling
of the main channel.
11
over the shoals toward the axis
This was produced by the flow of flood water
toward the channel axis and not directly toward the inlet throat .
The
flow at station C was parallel to the axis of the main channel and
directly toward the inlet throat during flood tide and directly away
from the inlet throat during ebb tide.
Although stations A and B were shie lded from the main channel by
marginal shoals, they al so illu st rated semi- diurn al reverses in flow
direction.
The speeds were 25 cm/s and 19 cm/s fo r stations A and 8,
respectively.
At station A, the flood vector was oriented toward the end of
Wassaw Island, and the ebb vector was oriented in a southeast direction.
The resultant of the ebb and flood vectors was oriented toward
a 11 spi 11 over 11 channe 1 in the shea 1.
Observations indicate that water
flows over the entire shoa l surface during the earl flood and is issued
through
11
11
Spill-over 11 channels during the l ate flood.
The l argest
Spi11-over 11 channel on the marginal shoa l was located adjacent to
Stat ion A.
-2m
-2m
.uili.._
...ili...
...uit.._
~L..
.Jlt...
.ulu...
..ill£.__
..uJu.
dlu._
0
,o~
...lili.....
\c;_,
0~
~
c;_,
~oc;_,
-2m
Nautical Miles
~
0
I
1/2
Vector Seale
0
Figure 6.
3000m
6000m
Vector map for all current velocities recorded lm above the seabed. Vectors are the product of
the n~an direction and speed for the ebb and f lood portions of the two canplete tidal cycles.
The resultant is the graphic average of the ~b b and flood vectors.
00
9
At station B, water is transported toward the axis of the shoal
during the flood, however, ebb currents are southeasterly and parallel
to the axial flow of the main channel.
The resultant flow at station
B was toward the axis of the main channel but in an offshore direction.
/
The general scheme of circulation is similar to the models pre-
sented by Oertel (1974) and Hayes (1975).
Reversing tidal currents
are the major agents of water transport at Wassaw Inlet.
On the inlet
side of the shoal reversing tidal currents were bipolar and parallel
to the channel axis as a result of confinement by the channel walls.
On the leeward side of the shoal, ebbing currents paralleled the axis
of the shoal and channel, however, flooding currents were oblique to
the axis and flowed toward the channel.
The resultant flow from stations
outside of the main channel is toward the channel axis and not necessarily toward the inlet throat.
While the data described above yields information on the total
velocity field, only a portion of these velocities is competent enough
for sediment transport.
A gross estimate of the sediment-transporting
currents is made by plotting the resultant of the duration and mean
velocity of currents greater than 25 cm/s (estimated threshold velocity).
In figure 7, ebb, flood and resultant vectors were plotted for
produtts of the duration and speeds greater than 25 cm/s.
The resul-
tant vectors representing these currents were considerably smaller than
the vectors for the total velocity field.
resultant flow was toward the channel axis.
Leeward of the shoal, the
In the channel (station C)
the sediment flow was small and away from the channel axis.
The resultant pattern of potentially competent currents is toward
the axis of the marginal shoals.
Observations of bedform orientations
appear to confirm this pattern.
On the channelward side of the shoal,
-2m
- 2m
_ill!L
_illiJ._
....ultL._
...ili.._
...~~~.__
...1L.
.-DlLL
-2 m
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...ili,_
~
,(}~
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\C:J
(}~
..illkt.....
··.
C:J
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-2m
Nautical Miles
~-
0
1/2
I
Vector Scale
0
Figure 7.
3000m
6000m
Vector map for current velocities greater than 25 em/sec recorded lm above the seabed. Vectors
are the product of the mean direction and speed of velocities greater than 25 an/sec for the
ebb and flood portions of two crnnp lete tida l cyc les. The resultant is the graphic average of
the ebb and flood vectors.
t-'
0
megaripples appeared to be migrating toward the shoal axis in an oblique
offshore direction.
On the leeward side of the shoal, megaripples
appeared to be migrating toward the shoal axis in an oblique onshore
direction.
Spill-over channels that transect the shoal surface are
important structures necessary for maintaining this type of sand and water
circulation.
he spill-over channel closest to the shore is particu-
larly important for bringing sediment to the north end of Wassaw Island.
Events that produce the closure of this channel, would consequently
cause stagnation of sediment in the floor of the spill-over channel
and decrease the quantity of sediment being transported to the adjacent
beach and shoa l .
The sediment starved area at the north end of Wassaw
would erode until the spill-over channel was reopened.
12
ACKNOWLEDGEMENTS
This project was sponsored (in part ) by the Georgia Sea Grant
Program, supported by N.O.A.A., Office of Sea Grant, Department of Commerce,
under grant no. 04-5-158-4.
and distribute reprints for
The
u.
S. Government is authori zed t o produce
governmen t a~
purposes not withstanding any
copyright notation that may appear hereon.
13
REFERENCES
Hayes, M.O., 1975, Morphology of sand accumulations in estuaries,
.ill Cronin, L.E. (ed. ), Estuarine Research, 2, Academic Press,
New York, N.Y.
Hoyt, J.H., R.J. Weimer and V.J. Henry. 1964.
Late Pleistocene and
Recent sedimentation, central Georgia coast, U.S.A., p. 170176, .l!J. Straaten Van, L.M., J.V. (ed.}, Deltaic and Shallow
Marine Deposits, Elsevier, Amsterdam.
Oertel, G. F. and C. F. Chamberlain. 1975.
Differentia l rates of
shoreline advance and retreat of coastal barriers of Chatham
and Liberty Counties, Georgia.
Trans. Gulf Coast Assoc.
Geol. Soc. XXV: 383-390.
•