Technical Report Series 32 Number 77-2 CHARACTERISTICS OF WATER FLOW AT THE NORTH END OF THE WASSAW BARRIER ISLAND COMPLEX

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1 32 r Technical Report Series 32 Number 77-2 J CHARACTERSTCS OF WATER FLOW AT THE NORTH END OF THE WASSAW BARRER SLAND COMPLEX Wassaw sland Erosion Study Part by George F. Oertel Georgia Marine Science Center University System of Georgia Skidaway sland, Georgia 81

2 CHARACTERSTCS OF WATER FLOW AT THE NORTH END OF THE WASSAW BARRER SLAND COMPLEX WASSAW SLAND EROSON STUDY PART by George F. Oertel Skidaway nstitute of Oceanography P.O. Box Savannah, Georgia 3146 The Technical Report Series of the Georgia Marine Science Center is issued by the Georgia Sea Grant Program and the Marine Extension Serv ice of the University of Georgia on Skidaway sland (P.O. Box 13687, Savannah, Georgia 3146). t 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. n 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. nformation contained in these reports is in the public domain. f this prepublication copy is cited, it should be cited as an unpublished manuscript.

3 NTRODUCTON 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. n all, approximately 2, data points were collected. OBSERVATONS 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. n 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 4 cm/s and 2 cm/s, respectively. Higher velocities at station C illustrate the channelized flow of the main channel. ncreases in 11 bed drag 11 at station D caused a reduction in flow speed. During the ebbing tide the offshore flow of water was 11 Shielding 11 by the marginal shoals and confined to the main channel. During the flooding tide, 11 Shielding 11 caused water

4 Soutl1 Carolina N J ~ ATLANTC OCEAN Noulicot UUes c = Noullcol Milt5 ~=- 1/2 B.c......_... '~,"':>\.,. slolion loco lion -2m m Figure 1. Location map of study area and current meter stations.

5 2 5 Meter A May 28, ' 11 ~ 14 Cl) "' Cl) ' Cl) 2 c 23 c.;: 26 Cl) -~ ,, \./'""' \ r", 1 ''., ' ~ ".J!. \ \ \ ) \~,_/ Time in Hours 4 /,' \~ ~ "!, ,., : ' 16 ~ _,"''',, \ '/',,. \ ~ \ _/', 1,... 1,,,,.. r,-,,.. ' 1---\ ,, 6 < (1) 4 - g 2 '< ::l 3 2 ;;;- 4 (1) sot... VJ Figure 2. Graphic representation of tidal current velocities and directions at Station A during a 26-hour interval (2 cou1plete tidal cycles). Stippled area indicates the portion of the sample when velocities were above the estimated threshol L. velocity for the sediment at the seabed.

6 "'... (/J G> Q)... 1 G> 9 6 -:1/"'\ c 27 c: -~ 24 -u... G> t A ~ tl t t j /' 11 f \ ~,.,, " 1/ ~ 1 '._.. r l ""'p;:..._j_ Meter B r l '', 1 1 '/, n ~ ' / ( ' J r 11 1 ~ 1 1 t r ll 1 ~ U ~ 1 ll l _! ~ A P Jl til \ A 1 l11l \ 1 \1 1.! ~, 4 ~ 1 1 \ 1 # 1 J 1 \" ~ A, ~ \ - June 5, 1975 ~ j r l ' /d /' " "~, tl "-J \ tl A '' ' ~ ' '' v, u 1: f.~ \ ' ' l-4 ~-,3 ~ 2 g 1 -- '< - ::::J (/J CD 2 T Time in Hours Figure 3. Graphi c representation of tidal current velocities and directions at Station B during a 26- hour interval (2 complete tidal cycles). Stippl ed area indicates the portion of the sa111ple when velocities were above the estit»ated threshol d velocity for the sediment at the seabed. ""'

7 Meter c 2-1 t t,, April 28, :\ 9,... 1 J, \ 11 :J.... (/) Q) 4> 14 ~ C1) 17 c: 2 1/~.~:~~j,.:,l. : : : : : : : : : : :-: : : : : :-: : : : : : : : g 23 1\ ~ _}l - ~ 26 '- - :~ ; \ 29 /t /\1 /\ "), -"'',., f \ \.--" 1" \ 32i " ' v \ 35-i / \ \ /\ ~- / \J \ - "'' -"" ' \/ ~ r < (1) - 4 g '< -:J 3 2 ~ (1) 4, Time in Hours V1 Figure 4. Graphic representati on of tida l current vel oc i t ies and directions at Station C during a 26- hour interval (2 comp lete tida l cyc l es). Sti pp l ed area indicates the portion of t he sa111p l e when vel ocities were above the estinlated threshold ve locity for the sediment at the seabed.

8 ,......!., (/) C1) C1) a... Ol Cl) ,, ' :\ 1 1 \ ~. ~.~,..., - ~ Meter C April < (1) 4 g... 2 '< :l c: 2 3 ~ s -... Cl) ',-' " r -"'' /,,...- ' \ \-"'.... J,., \J " ' ' " 1\1 \ " ) \..-""' t" \ v \ \ ~ 2 ;n- 4 (1) T Time in Hours ljl Figure 4. Graphi c representation of t i da l current vel oc i t i es and di rections at Station C during a 26- hour interval {2 comp lete tida l cyc l es). Sti pp l ed area i ndicates t he portion of t he samp l e when ve l ocities were above the estimated t hreshold ve locity for the sed iment at the seabed.

9 ~,, Meter D June 5, 1975 ~--~~----~r---~~~~~----~ ~. A k t 1 1 ' 1 r ll / 1111 lr 11 u,1,, tfj < (J) 2 - '< :l 3 ;;,. 1 (J) Time in Hours \ Figure 5. Graphic representation of tidal current velocities and directions 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.

10 7 to flow northeast around the distal end of the shea l and "spi into the main channel. n the channe l, l andward flo od in g water was confined by channel wa l ls and flw 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 was oriented toward the central axis of the channel. The resultant at station C was very small and oriented away from the channel ax i s. After two complete tidal cyc l es, station appeared to il lustrate a residual flow of water "spilling 11 over the shoals toward the axis of the main channel. 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 shielded from the main channel by marginal shoals, they al so illust rated semi-diurnal 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 sland, 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 Spill-over 11 channels during the l ate flood. The l argest 11 Spi11-over 11 channel on the marginal shoa l was located adjacent to Station A.

11 -2m -2m.uili.._...ili......uit.._ ~L...Jlt.....ill..ulu.....uJu....lili... dlu._ ~ ~ c;_, ~oc;_,,o~ \c;_, Nautical Miles ~ 1/2-2m Vector Seale 3m 6m Figure 6. 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 flood portions of the two canplete tidal cycles. The resultant is the graphic average of the ~b b and flood vectors.

12 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 presented by Oertel (1974) and Hayes (1975). Reversing tidal currents are the major agents of water transport at Wassaw nlet. 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). n figure 7, ebb, flood and resultant vectors were plotted for produtts of the duration and speeds greater than 25 cm/s. The resultant vectors representing these currents were considerably smaller than the vectors for the total velocity field. Leeward of the shoal, the resultant flow was toward the channel axis. n 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,

13 -2m - 2m _ill!l _illij._...ultl._...~~~....ili.._...1l..-dlll -2 m...ili,_..illkt......illu_..,_\lu..._ (}~ C:J ~(}C:J ~,(}~ \C:J Nautical Miles ~- 1/2 B~. ~ -2m Vector Scale 3m 6m Figure 7. 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 tidal cyc les. The resultant is the graphic average of the ebb and flood vectors. t-'

14 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 particularly important for bringing sediment to the north end of Wassaw sland. 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.

15 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 The u. S. Government is authori zed t o produce and distribute reprints for governmen t a~ purposes not withstanding any copyright notation that may appear hereon.

16 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 Late Pleistocene and Recent sedimentation, central Georgia coast, U.S.A., p ,.l!J. Straaten Van, L.M., J.V. (ed.}, Deltaic and Shallow Marine Deposits, Elsevier, Amsterdam. Oertel, G. F. and C. F. Chamberlain Differential rates of shoreline advance and retreat of coastal barriers of Chatham and Liberty Counties, Georgia. Trans. Gulf Coast Assoc. Geol. Soc. XXV:

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