81
32
Technical Report ~ries
Number 74-2
32
i
\
PATTERNS OF SEDIMENT DISPERSION
ON THE SHORELINE OF AN ERODING
BARRIER ISLAND
)
by
George F. Oertel
31
31
Georgia Marine Science. Center
University System of Georgia
Skidaway Island, Georgia
81
Patterns of Sediment Dispersion on the Shoreline of an Eroding Barrier Island
by
George F. Oertel
The Skidaway Institute of Oceanography
P. 0. Box 13687
Savannah, Georgia 31406
January, 1974
The Technical Report Series of the Georgia Marine Science Center is issued by
the Georgia Sea Grant Program and the Marine Extension Se.r vice of the University of
Georgia on Skidaway Island (P. 0. 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 st"ate
governments and the public. Information contained in these reports is in the public domain. If this preplhlication copy is cited, it should be cited as an unpublished manuscript.
ABSTRACT
A study of the sediment budget of the shoreline at Tybee Island illustrated
some relationships between sediment transport patterns and patterns of e r osion.
Sediment eroded from the northeast portion of the Tybee shoreline accumulated at
the beach adjacent to ninth street and at the beach along the northwest portion of the
island.
Most of the shoreline responded to seasonal changes in energy conditions.
During the fall and winter the beaches lost sand.
IAlring the spring and summer,
the beaches gained sand. Between Third Street and Fourteenth Street, the sediment losses during the fall and winter were generally completely compensated for
during the spring and summer recovery period. North of Third Street and South
of Fourteenth Street, the sediment recovered during the spring and summer was
insufficient to balance the losses of the previous fall and winter.
2
INTRODUCTION
Tybee Island is a barrier island located immediately south of the Savannah
River entrance.
The barrier is approximately 3! miles long and is generally be-
lieved to be Holocene in age (Hoyt and Hails, 1967). Structually the island is composed of beach-dune ridges that have developed across accreting shorelines. Aerial
photographs illustrate the trends of these beach ridges although portions of some beach
ridges have been altered or destroyed by construction and land development.
The
ridges vary in elevation from 10 to 18 feet above mean low water.
The development of beach-dune ridges illustrated shoreline advance during
the Holocene history of Tybee Island; however , the shoreline history during most of
the 20th century has illustrated erosion. In considering the sediment budget of a barrier
island, erosion in one place generally implies a transfer of sand away from that area
and sediment accumulation in another area.
Sediment transfers could take place in
several different directions, onshore (into estuaries), offshore, and longshore to the
north or south.
Each of these processes have occurred in portions of Tybee Island;
however, the continuous erosion problem at Tybee Island appears to be related to offshore transfers that produce the lowering of the beach and shoreface profiles. However, sediment is depleted from the beach at a rate equal to the lowering of the adjacent
shoreface.
The lowering of the beach and shoreface profiles is very apparent adjacent
to seawalls, particularly near the north end of the island. Whereas, erosion appears to
be relatively continuous at Tybee Island, periods of "peak" erosion are associated with
3
northeast storms .
Periods of beach build-up (accretion) are during low energy summer
conditions. However, over the duration of a year there is generally a net loss of sediment from the beach.
Several methods were used to evaluate the patterns of erosion and accretion along
the shorelines of Tybee Island.
The occurrence and recent de velopment of new beach-
dune ridges on the northwest and south shorelines of Tybee Island illustrates where at
least some of the eroded sediment has been transported and deposited. Sequential development of beach-dune ridges illustrate the pattern of modern and late Holocene development (Fig. 1).
The sequential development of beach ridges (Fig. 1, SQ) documents
the accreting and constructional phases of barrier development.
Truncations (Fig. 1, T)
on the northern most end of Tybee Island illustrate the initial period of constructional
modification to barrier development.
cene.
This apparently took place during the Late Holo-
The truncations of northeast trending beach-dune ridges were followed by the
development of beach-dune ridges along the southern shoreline of the island, and on
the northwest shoreline of the island (Fig. 1, ridge A through ridge F).
During this
period, northeast accretion was accomplished adjacent to an extensive sand shoal, as
sand from this shoal periodically renourished the northern end of t he island following
periods of erosion.
This process of sand nourishing from offshore shoals is apparently
still active today on several of the other barriers along the Georgia coast (Oertel, 1973).
At present, the sand shoal that was adjacent to the north end of the island has been reduced to a fraction of its former size and it no longer se rves as a major reservoir of
sand for this portion of the island.
Severe erosion on the northeast portion of the island
Figure 1.
Aerial photograph of Tybee Island. The (T) illustrates the approximate
position of the beach ridge truncation during the Holocene. The white
arrows (SQ) illustrate the directions of sequential beach-ridge development. Beach ridges A- F represent the late Holocene development of
Tybee Island in a west and northwest direction. Ridges Sl, 82, S3 and
S4 are relatively recent dune ridges that illustrate modern accretionary
trends of the island.
5
has caused a great deal of concern, and protective measures in the form of jetties
and seawalls have been installed along approximately all of the ocean shoreline that
is exposed to waves and strong currents.
The northwestern and southern portions
of the island continued to accrete as they did previously; however, retreat of other
portions of shoreline was stopped by the installment of rock and concrete sidewalk.
Presently, several portions of the shoreline north of approximately Third Street
have eroded to the base of the seawall where waves and currents are continually
dissipating energy.
CHARACTER OF THE BEACH FACE
The beach along the seaward side of Tybee Island is defined as the sand-surface area between the low water line and a man-made seawall that parallels the shoreline adjacent to the Atlantic Ocean.
The beach is broadest at the middle and southern
portions of Tybee Island, but becomes very narrow north of Third Street. In some
areas on the northern portion of the island the low water line is present at the base
of the seawall and the beach is non-existent.
Bimonthly measurements and observa-
tions were made on the b each between First Street and Sixteenth Street (Appendix A).
In general, the average slope of the beach was very low (1-2
0
); however, the
slope did vary with respect to the width of the beach (Table 1). North of Sixth Street
the beach was generally one degree or less and the beach width varied from 10 to 55
meters. At Sixth Street, the beach slope was approximately one degree and the
average beach width was 55-80 meters. Between Ninth Street and Sixth Street, the
6
Table 1.
Average Beach Slope and Number of Beach Ridges
Oct 4, 1972
Oct 20, 1972
Nov 1, 1972
Nov 17, 1972
Nov 30, 1973
Dec 15, 1972
1:56
1:46
1:48
1:54
1:50
1: 64
0
0
0
0
0
0
1:32
1:17
1: 4 0
1:32
1:41
1:34
0
0
0
0
0
0
1:25
1:22
1:32
1:28
1:46
1:39
1
0
0
1
0
0
1:30
1:23
1:41
1:37
1:40
1:35
3
2
1
1
1
2
Jan 5, 1973
Jan 19, 1973
Feb 3, 1973
Feb 16, 1973
Mar 2, 1973
Mar 16, 1973
1:64
1:46
1:76
1:52
1:64
1:60
0
0
0
0
0
0
1:32
1:35
1:26
1:41
1:55
1:38
0
0
1
0
0
1
1:30
1:33
1:26
1:35
1:29
1:40
1
0
1
0
1
2
1:29
1:27
1:25
1:45
1:37
1:36
1
1
1
1
1
1
Mar 30, 1973
Apr 16, 1973
Apr 27, 1973
May 12, 1973
May 28 , 1973
June 8, 1973
1:68
1:66
1:62
1:66
1:56
1:56
0
2
0
0
1
0
1:35
1:34
1:35
1:32
1:28
1:24
0
2
2
1
1
1
1:30
1:35
1:32
1:29
1:34
1:28
1
2
1
1
1
1
1:25
1:25
1:31
1:32
1:31
1:24
1
1
0
2
1
1
June 25, 1973
July 6, 1973
July 20, 1973
Aug 3, 1973
Aug 20, 1973
Sept 6, 1973
1:56
1:53
1:36
1:28
1:45
1:41
1
1
1
1
1
0
1:23
1:29
1:36
1:29
1:25
1:23
1
1
0
1
2
1
1:31
1:26
1:29
1:30
1:31
1:30
1
1
0
1
2
2
1:28
1:42
1:34
1:36
1:37
1:27
1
1
0
1
1
1
Sept 25, 1973
Oct 8. 1973
1:38
1:45
0
0
1:26
1:32
0
0
1:29
1:37
2
0
1:34
1:44
1
0
Average Slope
1:54.2
Number of Beaches with
Ridges / total number of ridges
Ave rage Slope in
Degrees
1:31.3
7/8
1:31 . 4
11/ 14
1:32. 5
11/ 23
22/ 28
0
1° 9'
1
50'
1° 50'
1° 45'
7
average beach slope increased to approximately two degrees and the average width
of the beach was approximately 80-100 meters.
Wider portions of beach had a more
undulating topography. By counting the number of swash bars that transected a profile, an estimate was made of how flat the various portions of the beach were for the
period between October 4, 1972 and October 8, 1973. In general, a larger number
of ridges were present on the southern and wider portion of the beach than were on
the northern portion of the shoreline (Table 1, Appendix A). On a seasonal basis,
there were exactly twice as many swash bars observed during the spring and summer
than were observed during the fall and winter (Table 1).
From these trends it would
appear that increasing numbers of swash bars are generally associated with spring and
summer beaches and relatively wide beaches.
Following trends of increasing or de-
creasing numbers of beach ridges may be a barometer to the stability of a beach, that
is whether the future development of the beach follows a pattern of erosion or accretion.
During the study period, the number of beach ridges per season (fall, winter, spring,
summer) at Sixteenth Street decreased continually; whereas, the amount of erosion
increased.
BEACH PROFILES
On October 4, 1972 beach-profile surveying was begun on Tybee Island.
two weeks, beach profiles were made from the seawall to the low water line.
Every
Profiles
were made at Sixth Street, Ninth Street, Twelfth Street, Sixteenth Street and single
beach elevations were made adjacent to Third Street and Tilton Street.
These profiles
8
were made over the duration of a year, and were an attempt to see how the various
portions of the shoreline responded to seasonal changes (Appendix A).
OBSERVATIONS
Cumulative volumes of sand (based on beach profiles) were plotted against
time for one complete season (Fig. 2, 3, 4, 5).
These plots illustrate the seasonal
fluctuations in the sediment budget of various portions of the Tybee shorelines (Sixth,
Ninth, Twelfth and Sixteenth Streets). The shoreline in the central portion of Tybee
Island between Sixth street and Twelfth Street (Fig. 3, 4, 5) had remarkably similar
seasonal trends. Initially during the fall and winter, erosion lowered the beach face
in response to a "drain" in the sediment budget. However, during the summer and
spring the beach face began to build up as the sediment budget responded to sediment
input.
Although relatively large deficits in the sediment budget produced severe erosion
during the fall and winter, build ups during the spring and summer produced complete
recovery of all of the lost sediment. However, after a one year sampling period a
small deficit (-5, 842 cu m) was calculated for this area (Fig. 6 ).
North of Sixth Street, erosion also lowered the beachface during the fall and
winter; however, recovering during the spring and summer was slow and a relatively
large seasonal deficit was illustrated by the lack of any high-tide beach and only a very
small low-tide beach east of the seawall.
Between Twelfth Street and Sixteenth Street the beach illustrated a large net
deficit (-20,484 cum) in the sediment budget.
This area began eroding during the fall
9
Sixth Street
60
40
20
;)'
~
ot-------------rr~~--------~--------~L---~r------------------------
Q)
E
~
0
:::a. 20
-~
60
FALL
Figure 2.
WINTER
SPRING
SUMMER
Graph illustrating cumulative volumes of sand at a one-meter wide
t r ansect across the Sixth Street beach. Spring and summer accretion
more than compensates for fall and winter erosion.
10
Ninth Street
60
40
20
-
I t)
-g
E
0
Ql
E
.:::s
20
·c::
Ql
01
c::
0
..c:
(.)
40
60
FALL
Figure 3.
WINTER
SPRING
SUMMER
Graph illustrating cumulative volumes of sand at a one-meter wide
transect across the Ninth Street beach. Spring and summer accretion
just compensates for fall and winter losses.
11
Twelfth
60
Street
20
-
..,
~ o t---------------------------------~~~~~--------------­
Cb
E
~
0
::::.. 20
.~
60
FA L L
Figure 4.
WINTER
SPRING
S UMMER
Graph illustrating cumulative volumes of sand at a one-meter wide
transect across the Twelfth Street beach. Spring and summer accretion
just compensates for fall and winter losses.
12
Sixteenth Street
6
40
20
-
I f)
~
ot----------------------------------------------------------
60
FALL
Figure 5.
WINTER
SPRING
SUMMER
Graph illustrating cumulative volumes of sand at a one-meter wide
transect across the Sixteenth Street beach. A relatively continuous
erosion trend was present during fall and winter. An accretionary
trend in the early spring was balanced by an erosional trend in late
spring. An accretionary trend in early summer was also erased by
an erosional trend in late summer. The result is an erosional trend
for the entire year.
Qc;t . 73
Oct.72
(-) 6,519 m'
Figure 6.
Box diagram illustrating the yearly pattern of erosion and accretion
for portions of Savannah Beach (not to scale).
14
and winter, but had only a small recovery period during the spring and summer.
Accretion during the spring and summer was insufficient to balance the
severe periods of erosion. During the season between October 4, 1972 and October
8, 1973 the central portion of Tybee Island suffered extreme eros ion during fall and
winter, but reco vered almost completely during the spring and summer pe riods of
accretion (Fig. 6). The beaches north of Third Street and south of Fourteenth Street
also suffered extreme erosion during the fall and winter; however, recovery during
the spring and summer was somewhat smaller. At the northern end of the island,
this trend may be an indication of the effect of the sediment loss into the Savannah
River channel
SAND DISPERSION
Although continuous beach-profile surveys are useful in determining the net
gain or loss of sediment from portions of beach, supplementary data is often necessary
to gain an understanding of the mechanics and patterns of sediment dispersion that is
associated with erosion or accretion . Wave currents and tidal currents are considered
to be the prevailing and predominant agents of sediment transport along the Georgia
shoreline (Oertel, 1972, 1973). Flooding tidal currents entrain beach sediment toward
the Savannah River entrance; whereas, ebbing currents entrain beach sediment offshore
at the north end of the island and have little effect on the center and southern shorelines
of Tybee Island. Onshore waves generally refract around a "break point" in the shoreline at approximately Sixth Street. As waves break along the shoreline they produce
longshore currents that flow to the north or to the south, respectively, from this point.
15
The point of flow reversal is generally south of Sixth Street because of a topographic
high in offshore topography.
The patterns of sediment transport produced by wave currents and tidal
currents were evaluated by utilizing several tagged- grain studies. Sand taken from
several potential test sites was dryed and coated with a commercial, fluorescent
paint. Two different hues were used in differentiating the grains used at each test
site. Color-coded grains were returned to their respective test-run sites and released on the sea bed. The duration of interaction with physical processes was
carefully determined so that grains were exposed to equal durations of ebbing and
flooding tidal currents. Two test sites used are illustrated as A and B in Figure 7.
On February 26, 1973 two tracer experiments using fluorescent sand were
conducted on Tybee Island. Adjacent to Tilton Street, there is no high-tide beach,
and only a very narrow low-tide beach. Sand coated with a yellow-fluorescent acrylic
and weighing approximately 150 pounds was released on the beach face at point R
(Fig. 8). Northward flowing flood currents initially interacted with the tagged grains
for four hours, after high water, tidal currents reversed and ebbed southward for 3-!
hours. Sampling was conducted after grains interacted with the sedimentary environment for a total of seven and one half hours. Although winds were out of the northeast,
the refraction of swells at the beach produced a northerly longshore current within the
breaker zone. The sample grid illustrated on Figure 8 shows the pattern of fluorescent
sand dispersion for the seven and one half hour period described. Although grain recovery was modest, transport
patterns were consistantly in a northerly direction
along the shoreline. Fluorescent grains apparently penetrated through the "breaks 11 in
16
---- -------
------- --
....
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. ''
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' '
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'' '''
' ',
'
'
' ~' ,
'o•
\~~',
1
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A
'
' ...,,
, ct.'
,,_, '
' , ''
...,o., ,
N
~- '\
''
F
'
'
'
'
'
'
'
'
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H ·
/ .1
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j '\
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·f··•
. I .
\. . i . .
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- - - - "'' ·":. ·- .. - ; d1 • !/' T ·
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ri-""'·
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Figure 7.
""'''
1000
.
.... ~;......
•
2000
Index map of sample grids for fluorescent-sand tracing experiments.
'
..... '..."9; . .•. ......
:?~>
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17
Beach
Ocean
Figure 8.
Map of fluorescent-grain dispersion of sample grid A (February 26, 1973 ).
Approximately 150 lbs of sand coated with a yellow-fluorescent acrylic
were released on the beach face at point R. Sampling was made after an
initial flood duration of 4 hours followed by an ebb duration of 3! hours.
Contours are in grains per 100 sq em.
18
the old steel groin.
The very modest recovery may be an indication that sand was
moving offshore, and out of the sample grid.
Another sand-tracer experiment was simultaneously conducted between
Seventh and Eighth Streets (fig. 9). Sand coated with an orange-fluorescent acrylic
and weighing approximately 150 pounds was released on the sea bed of the foreshore.
Grains were allowed to interact with agents of transportation for approximately 6~
hours.
Grains interacted with ebbing currents for two and one half hours and with
flooding currents for four hours.
The highest concentration of fluorescent grains
was recovered just south of the release point, and adjacent to a wood jetty (fig. 9).
This high concentration is believed to be related to the onshore transport of sediment associated with "swash currents".
Sediment dispersion was both northward
and southward along the shoreline; however, the predominant trend of transport was
in a northward direction.
Ebbing and flooding currents are apparently both responsible
for transporting some sediment in opposite directions during respective periods of
the tide.
During the ebbing tide, tidal currents enhance the flow of sediment toward
the south, and during the flooding tide, tidal currents enhance the transport of sediment
in a northerly direction.
In that recovery extended a greater distance north of the re-
lease point it may be assumed that on this day there was a net transport of sediment to
the north.
The relatively high concentrations found on the upper portions of the beach
also illustrate the effectiveness of swash currents in building the beach during this
period. Our profiling records show that the beach adjacent to Ninth Street was indeed
building in late February and early March.
19
Beach
,
. ·..
. .
.
.
.
:. ... ·. ·. ·. : .· .· . . ... . . .. .. ·.
. . . . .
·. : : : : : .. :.... ·... ·. ·.
.
. . . . ; : .· ... :
·.
: : ... ·. : . ·... · .·
. ·. . . · .
: -: .· .· . : ·. .. . ·. .
. : .. . ..
. . .
Ocean
Figure 9.
Map of fluorescent-grain dispersion at sample-grid B (February 26, 1973).
Approximately 150 lbs of sand coated with an orange-fluorescent acrylic were
released on the beach face at point R. Sampling was made after an initial
flood duration of 2i hours followed by an ebb duration of 4 hours. Contours
are in grains per 100 sq em.
20
On May 22, 1973 tracer experiments were repeated at the two experimental
sites described above.
The interaction period again "straddled" high water, having
four hours of exposure to ebbing currents and four hours of exposure to flooding
tidal currents.
The winds were variable with low speeds and the sea condition was
essentially smooth.
The experiment conducted adjacent to Tilton Street produced results that were
very similar to the results determined for February 26, 1973; however, the amount of
recovery was much greater (fig. 10).
ward of the release point.
The maximum recovery was landward and north-
Fluorescent sand was again able to penetrate the old, dilapi-
dated, steel groin and isopleths illustrated strong tendencies for sediment transport to
the north.
The relatively high recovery and onshore transport of sand in figur e 9 may
be interpreted as an accretional trend.
This is in contrast to the possible offshore
transport trend that was depleting the sediment budget on February 26, 1973.
The May 22, 1973 tracer experiment conducted between Seventh and Eighth
Streets produced results similar to the earlier experiment conducted at this site (fig. 11).
The dispersion patterns appeared to illustrate a general transport trend to the north;
however, the dispersion patterns were locally very complicated.
The greatest re-
covery of fluorescent grains was slightly northward and onshore of the release point
(R ).
This onshore trend was interpreted as an indication of beach building, and was
later substantiated by the volumes calculated from profiles taken in late May. "Rip"
currents along the margins of the wooden jetties produced channels that transported
sediment offshore and depleted the concentration of tagged sand adjacent to the jetties.
21
Beach
.
.
. .:·
.
.
~
..
.
.
...
.
.
. :. .
. ....
':,.
.
I
I
I,
Ocean
Figure 10. Map of fluorescent-grain dispersion of sample-grid A (May 22, 1973).
Approximately 150 lbs of sand coated with a yellow-fluorescent acrylic
were released on the beach face at point R. Sampling was made after
an initial flood duration of 4 hours followed by an ebb duration of 3-!
hours. Contours are in grains per 100 sq em.
. •
Beach
~
. .. .. . ·. .
.
.
. . ..
. :
.
:·-
:: . . :·..... ..... . . . . .. : : .
: .· .. . . . . .
.
. ..
·..
. ' •.
.
... . . .
5--I
Ocean
Figure 11. Map of fluorescent-grain dispersion at sample-grid B (May 22, 1973 ).
Appr oximately 150 lbs of sand coated with an orange-fluoresc ent acrylic
were released on the beach face at point R. Sampling was m a de after an
initial flood duration of 2! hours followed by an ebb duration of 4 hours.
Contours are in grains per 100 sq em.
23
Tagged sand also was able to move around the ends of the wooden jetties in northerly
ani southerly directions. On the southside of the Eighth Street jetty, the recovery of
tagged sand attenuated significantly 30 meters away from the jetty, and the greatest
recovery of tagged sand was in the upper portion of the beach.
Swash currents associat-
ed with wave refraction are believed to be responsible for the a c cumulation of sediment
in the upper portion of this beach.
In that this experiment was conducted on a relatively calm day during mean
tides, it is expected that magnitudes of sediment transport would be very different
during spring tides or storms. However, the prevailing influence of the mean tidal
currents (flowing to the north) must be considered an important mechanism of sediment
transport.
CONCLUSIONS
Wave- induced currents and tidal currents both play important roles in affecting the sediment budget at Tybee Island.
North of Ninth Stre et, flooding tidal currents
appear to be a major mechanism contributing to sediment depletion and to the longshore
transport of sediment in a northerly direction.
Wave-induced currents also have a significant affect on the sediment budget of
Tybee Island.
There is generally a seasonal trend corresponding to different directions
of wave approach.
During the spring and summer months, wave appr oach is generally
from the southern quadrants.
These waves generally produce longshore currents that
flow northward and transport sand in a northerly direction.
During fall and winter months,
24
wave approach is generally out of the northern quadrants.
During this period, the
resultant currents and sediment transport are generally toward the south or west.
In response, sand bars are "welded" onto the northwest and southern portions of the
island.
During a major portion of the year, wave approach i s essentially parallel to the
shoreline and longshore sediment transport is bi-polar away from Eighth Street (as
described above).
Offshore swell is also oblique to the shoreline for a major portion
of the year and as waves approach the shoreline, the orientations of the shoreline and
the nearshore topography become very significant.
The shoreline of Tybee Island is
approximately oriented along a north-south bearing; however, north of Sixth Street,
the shoreline bends slightly toward the northwest. Also, north of Third Street the
water is relatively deep adjacent to the shoreline; whereas, south of Third Street the
water stays shallow for a considerable distance offshore. Wave r efr action around this
shallow area produced a southerly longshore current (south of Sixth Street) and a northerly
longshore current (north of Sixth Street).
At the north end of the island, the combina-
tion of the wave-induced longshore currents and flooding tidal currents transported sand
toward the navigation channel of the Savannah River. Strong ebb tidal currents in this
channel transported the sand offshore.
Wave-induced longshore currents also transport-
ed sand around the northern tip of the island where it was deposited on a recurved spit
(fig. 1, S1 and S2).
Deposition also occur red in spit s at the southern end of the island
(fig. 1, S3 and S4).
This accretionary trend pe rmits the ends of the barrier island to
maintain their widths as the shoreline retreats landward.
25
At the southern end of Tybee Island a large ebb delta is present adjacent to the
Tybee River entrance.
for the Tybee shoreline.
This ebb delta is a large sand shoal that is a reservoir of sand
Distributary tidal channels play important functions in trans-
porting sand to portions of the shoreline •
North flowing currents and onshore flowing
currents generally nourish the adjacent beach with sand during the spring and summer
months.
During the 1972-1973 study period, sand from the ebb delta apparently reentered
the shoreline adjacent to Ninth and Tenth Streets. A submarine spit from the ebb delta
recurved toward a sand shoal extending offshore from Tenth Street.
The onshore trans-
port of sediment from this spit produced a good spring and summer recovery period for
the adjacent portion of the shoreline.
In terms of a sediment budget, the shoreline of Tybee Island has two apparent
sources of sand, viz. the ebb tidal deltas at Tybee Inlet and the Ninth Street offshore
shoal.
The dunes are generally not a reserve source of sand for the shoreline because
of the restrictive nature of the seawall.
There are also several drains in the sediment budget.
The north end of the island
(north of Third Street) appears to have a sediment drain toward the navigation channel and
toward the recurved spits on the northwest side of the island.
During the fall and winter,
the profile of the shoreface is lowered, as sediment is apparently lost in an offshore direction.
During major storms sediment was also transporte d away from the northern
tip of the island and toward the southern and northwestern ends of the island.
During the spring and summer months, a combination of wave-induced currents
and residual tidal currents transported sediment toward the navigation channel of the
26
Savannah River.
Strong tidal currents in the navigation channel transport
sediment away from the Tybee Island sediment system.
ACKNOWLEDGMENTS
This report is a description of a port io n of th e results of a project
for the University of Georgia Sea Grant Program.
The project, entitled
"Patterns of sedimentation affecting the sediment budget of Savannah Beach,"
U. S. Department of Commerce grant number 04-03-158-6, involved the assistance
of many techni cians whose work was appreciated.
S. Arpadi, M. Chafey,
B. Edwards, S. Greet, T. Hansson, G. Holloway and M. Larsen were student
techn icians that assisted in various phases of the research .
D. Perlmutter
assist ed in dra f t i ng and in the photographic preparation of figures for the
manuscript.
27
REFERENCES
Oertel, George F.
1972.
Sediment transport on estuary entrance shoals and the
formation of swash platforms.
Oertel, George F.
1973 .
Jour. Sed. Pet. Vol. 42(4): 858-863.
Patterns of Sediment Transport at Nearshore Zones
Influenced by Wave and Tidal Currents: A Study Utilizing Fluorescent
Tracers . Southeastern Geology. 15(2): 77.
Hoyt and Hails.
1967.
Pleistocene Shoreline Sediments in Coastal Georgia:
Deposition and Modification.
1541-1543.
Reprint.
Science, March 24 Vol 155(3769):
28
APPENDIX A
Bimonthly Beach Profiles (1972-1973)
for Sixth, Ninth, Twelfth and Sixteenth Streets
29
Sixth Street Beach Profiles
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