SHELF CIRCULATION PATTERNS OBSERVED FROM DAVIES DRIFTER OFF THE
EASTERN NIGER DELTA IN THE GULF OF GUINEA
By
Larry Awosika1
And
Regina Folorunsho2
Nigerian Institute for Oceanography and Marine Research
Victoria Island, Lagos.
1
larryawosika@yahoo.com, 2 rfolorunsho@yahoo.com
ABSTRACT
The eastern Niger Delta in Nigeria stretches from the Akasa River eastwards to the Calabar estuary
along the Nigeria Cameroon border. As part of a current measurement survey off the Niger Delta, a
Davies drifter was deployed off Sangana River. The Davies drifter drifted for a total of about 51
days and exposed a trajectory which traversed the inner shelf to the open shelf. The trajectory
showed several reversals and gyral patterns which indicate across and alongshelf circulation
system. Though the alongshelf seem to be more pronounced, the records depict the oscillatory
nature of the circulation and transport system in the Gulf of Guinea notch of the east central
Atlantic.
1. INTRODUCTION
The eastern Niger Delta consists of the barrier bar system in the west and the strand coast in the
east. Strand coast stretches from Imo River eastwards to the Cross River estuary (Figure 1). The
eastern Niger Delta shelf coastal system lies within longitude 040 25’ 30”N, latitude 080 22’50”E
and longitude 80 30’ 20”E, latitude 04024’ 20”E. The coastline is dissected by several rivers of the
Niger Delta (Akassa, Bonny, Brass, San Bartholomew, St Nicholas, the Cross, Qua Ibo, Imo
Rivers) and many small rivulets and tidal creeks which open to the sea.
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3°
3
4°
4
5°
5
6°
6
7°
7
MAP O
N IG E R IA N
ZON E S HO
CO A S T A L G E O M
ZO
8°
8
7°
7
7°
7
N
L ag o s
Ben i n C i ty
B A R RI E R C O A ST
6°
6
6°
6
LEG EN D
R
iv
er
er
cr
av
R
iv
do
s
H
T
O F
Fo
rc
a
G
Es
I
Uy o
B E N
I N
C al ab ar
5°
5
5°
5
Ow e r ri
os
M AH IN M UD
B
Be
ni
n
R
iv
er
Towns
Coastal D
Coastlin
Dr ainag
Int ernati
River
Cross
P o rt H arc o u rt
N I G ER D E L T A C O A S T
Riv
er
ST R A N D C O A S T
I mo
Bon
nyR
iver
Bo nny
4°
4
4°
4
G E O D E T IC I N
0
200 Kilomete rs
D A TU M :
MI
SP H E R O I D :
CLA
P R O JE C T I O N :
TR A
CEN TR A L
M E R ID I A N :
3
3°
4
4°
5
5°
6
6°
7
7°
8
8°
Figure 1. Map showing the different geomorphological zones along the Nigerian coast.
(Awosika et. al, 2002)
2. BATHYMETRY OF OFFSHORE AREA
The general bathymetry ranges from 0 to 120m (Figure 2) deep where there is an abrupt drop to the
continental slope. The nearshore areas have depths ranging between 0 and 40m with bathymetric
contours running parallel to the shoreline. Sand shoals are present especially off the Qua Iboe and
Cross River estuaries. The middle shelf has depth ranging between 40 to 80m while the outer shelf
has depth reaching 100m. The Calabar canyon is a major bathymetric feature grooving the
continental shelf off the study area.
2
8 .5
L A T . O F O R I G IN :
4 .0
FA L S E E A S T I N G :
670
FA L S E N O R T H IN G : 0 .0
S C A L E FA C T O R:
0.9
Western Niger
Delta
Mahin mud
coast
Benin River
River
Escravos River
Forcados River
Mahin canyon
Avon
canyon
Barrier-lagoon
coast
Ramos River
100
Calabar canyon
m
50m
700m
South westerly winds
South westerly waves
Longshore currents
direction.
DD
- Davies drifter
launching location
Figure 2. Bathymetric map of the study area (black circle)
with patterns of prevailing oceanographic forces (Awosika
2000).
3. GENERAL OCEANOGRAPHIC CONDITIONS
The tide along this coast and shelf is composed of south westerly semi-diurnal tides ranging from,
1-2m in the west to 3m in the Calabar. Tidal currents consist of flood currents arrive from the
south west and attain speed averaging 0.2m/s. Ebb currents are stronger and attain speed of
0.4m/s. The ebb tidal currents are stronger due to the imposition of fluvial currents. River
discharges are higher during the ebb tides. Waves are predominantly westerly due to the
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dominant south westerly winds (Figure 2). Due to the orientation of the coast waves arrive on the
coast at oblique angles of between 10 to 15 degrees with the waves opening to the east (Figure
3). As a result of waves breaking at oblique angles to the coast, longshore currents are generated
which move predominantly in a west- easterly direction. A west east littoral cell is hence generated.
Longshore currents attain speed averaging 0.2m/s to 0.4m along the coastline
4. OCEAN CURRENTS
The west - east Guinea current (Figure 3) is the dominant ocean current affecting the Nigerian
continental margin. The Guinea current, which is an extension of the north Equatorial Counter
current, attains speed of 0.3m/sec with some reversals. This current reversal seems to occur most
frequently at the beginning and end of the rainy season (Longhurst, 1964). The Guinea current
runs above an undercurrent which is thought to be a westward flowing extension of the northern
branch of the Equatorial Undercurrent which splits into two branches after impinging upon the
African continent at Sao Tome Island. The other important surface current in the Gulf of Guinea
is the South Equatorial current (SEC).
Figure 3. Gulf of Guinea showing prevailing ocean currents
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5. METHODS OF STUDY
On 24 July 2000, a Davies drifter with number 25620( Figure 4) was deployed at 11.00hr off
the Sangana river (Figure 5) onboard a research boat MV Calabar Carrier as part of the Shallow
water current measurement project (Figure 6). The Davies drifter recorded current speed
direction and time for a period of 24th July to 21 September 2000. Data was analysed and plotted
to show trajectory of the drifter from deployment in nearshore waters to the open shelf more than
100m water depth.
Fig 4. Davis Drifter before deployment (left) and after deployment
in water (right).
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Figure 5. Deploying Davies Drifter off Sangana River
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Figure 6. Survey team (MOBIL, EHI, NIOMR, UNICAL) during cruise No.1
onboard MV CALABAR CARRIER.
6. RESULTS.
The nearshore-open shelf trajectory of the Davies drifter (Figure 7) showed an
oscillating/reversal surface circulation pattern. Upon deployment, the drifter made a south
easterly movement for two days crossing the 100m water depth isobath with speed varying
between 20.8cm/s and 58.4cm/s. On 28th July 2000, it changed direction northward with current
speed ranging between 20.1cm/s to 33.2cm/s. On 31st July 2000 again the drifter changed its
direction southward crossing the 100m isobath with current speed ranging between 35.8cm/s to
49.9cm/s. On August 2nd 2000 the drifter continued on an easterly direction towards shallower
water from about 120m to 90m water depth. From here onwards, the drifter made a southward
drift with a speed averaging 58cm/s. The drifter continued this southward drift until September
7th
2000
when
again
it
made
a
northward
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drift
for
two
days
(08/09/2000).
Calabar
Estuary
Start:
07/24/20
11.00hr
Bonny Town
04oN
End:
09/01/2000
1800hr
06oN
08oE
06oE
Figure 7 Map showing trajectory of Davies Drifter No 25620 in the eastern Niger Delta Shelf
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after which it made a westward drift and eventually disintegrating on September 10th 2000.
In all, this drifter deployed in shallow water depth of about 25m made a net eastward directional
drift. The drifter traveled faster when it made a southward drift as against a northward drift.
Closer to the east, the drifter made a gyral drift which may indicate reversals in the current. It is
pertinent to also note the dominant drift from shallow waters nearshore to deeper waters
offshore.
7. DISCUSSION
The drifter trajectory observed in the short term study showed that the currents nearshore and
open shelf current are oscillatory in nature. According to several authors like NEDECO, (1961);
Longhurst (1962); Richardson and Reverdin, (1987), Awosika et al. 1994, amongst others, the
dominant ocean current in this part of the Gulf of Guinea had earlier been described as
predominantly west tom east flowing current. Recent studies of circulation system in the Gulf
of Guinea have revealed that the Guinea current consist of oscillatory or reversal which some
authors have attributed to the reversal in current and change of variations in the flow of the
North Equatorial Countercurrent, the Canary Current, and the Benguela Current and to the
weakening of the easterly winds. (Longhurst (1962), Ingham 1970 and Boisvert (1967. However,
Lemasson and Rebert (1968) suggested that the presence of a westward current at the surface
near the coast could be due to the surfacing of the Ivorian undercurrent and the seaward
displacement of the Guinea Current and not due to the reversal of the Guinea Current (Collin
1988). Another explanation, proposed by Ingham (1970), was that the apparent reversals are
actually caused by cyclonic eddies between the current and the coast.
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8. CONCLUSION
Despite all the proposed causes of these reversals no agreement has been reached on the exact
causes, spatial and time scale of the observed reversals. While no agreement has been reached on
the direct causes of the observed oscillating alongshelf surface currents offshore the western
Nigerian coast, the implications of the oscillating currents for management of oil spill and
nutrient circulation is apparent. Further studies of the oscillatory Guinea current will be required
to fully understand the met-ocean processes responsible for the current patterns.
9. ACKNOWLEDGEMENTS
The authors are grateful to the Mobil producing Nigeria Limited and the Oil producing Trades
Section of the Lagos State Nigeria Chamber of Commerce for funding this survey. The
assistance and cooperation of Evan Hamilton Incorporated is acknowledged.
10. REFERENCES
Awosika L. F., Folorunsho, R., Dublin-Green C. O and Imevbore V.O., 2000. Review of the
coastal erosion at Awoye and Molume areas of Ondo State. A consultancy report for
Chevron Nigeria Limited. 75p.
Binet, D., 1997. Climate and pelagic fisheries in the Canary and Guinea currents 964- 1993: The
role of Trade Winds and the Southern Oscillation. Oceanologia Acta, 20, 177-190.
Boisvert, W.E., 1967. Major currents in the North and South Atlantic Ocean between 64°N and
60°S, Tech. Rep. Hydrogr. Off. Wash., TR-193, 92 p.
Burke, K., 1972. Longshore drift, submarine canyons and submarine fans in the development of the
Niger delta. AAPG Bull. V.56, p. 1975 - 1983.
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Collin, C., 1988. Coastal upwelling events in front of Ivory Coast during the FOCAL program.
Oceanologia Acta, 11, 125-13.
Evan Hamilton, Inc., 2001. Nigeria OPTS Shallow Water Current Measurement Survey; Data
Reports 3 and 4.
Hisard, P., C. Henin, R. Houghton, Piton, B., and Rual, P., 1986. Oceanic conditions in the
tropical Atlantic during 1983 and 1984. Nature, 322, 243-245.
Ingham, M.C., 1970. Coastal upwelling in the northwestern gulf of Guinea. Bulletin of Marine
Science, 20, 1-34.
Longhurst, A. R., 1962. A review of the oceanography of the Gulf of Guinea. Bull. Inst. France
Afri. noure, V.24, p.633-663.
Richardson, P.L. and Reverdin, G., 1987. Seasonal cycle of velocity in the Atlantic North
Equatorial Countercurrent as measured by surface drifters, current meters, and ship drifts.
Journal of Geophysical Research, 92, 3691-3708.
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Dr. Larry Awosika is a Marine
geophysicist with the Nigerian Institute for
Oceanography and Marine Research Lagos
Nigeria. He is a member of the UN
Commission on the Limits of the
Continental Shelf (CLCS), member and
Vice Chairman of Joint Group of Expert
on Scientific Aspects of Marine Protection
(GESAMP). He has led several national
cruises and regional studies to understand
the marine Geology and ocean dynamics
in the Gulf of Guinea..
Dr. Regina Folorunsho is a Met- Ocean
expert with the Nigerian Institute for
Oceanography and Marine Research Lagos
Nigeria. She is a member of (Joint
Commission on Oceanography and Marine
Meteorology (JCOMM) Management
Committee. She has taken part in several
National and Regional cruises and carried
out studies in Met Ocean process in the
Gulf of Guinea.
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