Environmental Engineering Reference
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season, the west-to-east direction is highly reinforced;
however, south-easterly and south-westerly directional
components are observed. The dry season months of
October to December exhibit a reversal with a dominant
westerly component (Figs.
13
,
14
).
The oscillating along-shelf currents have also been
captured by the trajectories of two Davies drifters launched
in the Niger Delta in 2000. Drifter number 25,629 was
deployed offshore the Sangana River (western Niger Delta)
on July 24, 2000, at 11.00 h (GMT) and showed the
oscillating pattern in a west-east direction (Fig.
15
). The
trajectory (Fig.
16
) of another Davies drifter (25,620)
launched offshore the Nun River (Eastern Niger Delta)
showed
Site S2. Apr. 19 - 25 Jul. 2000
Bin 22
Bin 13
again
an
oscillatory
pattern
with
some
gyres
circulation.
The oscillatory circulation patterns described above seem
to be different from those earlier described by several authors
like NEDECO (
1961
), Longhurst (
1962
), Richardson and
Reverdin (
1987
), Awosika et al. (
1994
), who described the
dominant ocean current in this part of the Gulf of Guinea to
be predominantly a west-to-east flowing pattern.
The causes of these reversals have been the subject of
debate. Richardson and Reverdin (
1987
), Longhurst (
1962
)
and Ingham (
1970
) concluded that the seasonal instability
of the North Equatorial Countercurrent (NECC) and the
Canary Current affect the seasonal variability of the Guinea
Current and hence the oscillatory current patterns. Long-
hurst (
1962
), Ingham (
1970
), and Boisvert (
1967
) attributed
the reversal in current direction during the minima to var-
iation changes in the flow of the NECC, the Canary Current,
and the Benguela Current and to the weakening of the
easterly winds. Another explanation, proposed by Ingham
(
1970
), was that the apparent reversals are actually caused
by cyclonic eddies between the current and the coast.
While no agreement has been reached on the direct
causes of the observed oscillating along-shelf surface cur-
rents offshore the Niger Delta, the implications of the
oscillating currents for management of oil spill and nutrient
circulation are apparent.
Bin 3
Fig. 13
Current polar plots for station S2 (15.8 m water depth)
between 19 April 2000 and 25 July 2000. (Evan Hamilton Inc
2001
).
Bin 3 is 2.3 m from sea bottom, bin 13 is 7.3 m from seabed, and bin
22 is 11.8 m from sea bed
Mean velocities of 12 cm/s (0.12 m/s) are dominantly
east to west (Fig.
13
). Percentage occurrence of these
directions is often over 20 %. The north-south across-shelf
oscillating currents are much more subdued and less fre-
quent. Near-surface currents (Bin 22), as expected, are much
stronger and more consistent with regard to the general
current directions. Frequency analysis of the data, however,
shows energy peaks at several periods including those of
daily tides, fortnightly, and approximately 20 days.
The tides, which create currents reaching a maximum of
approximately 20 cm/s, provide the most consistent cross-
shelf currents and may be responsible for the majority of
cross-shelf mixing and stirring of coastal waters. However,
these do not appear to be of as much significance as along-
shelf net transport.
Implications of Circulation Current Patterns
on Oil Spill Management in the Niger Delta
Along-Shelf Circulation
Oil movement in the coastal and marine area is influenced
by a combination of dynamic forces, including wind speed
and velocity, air and water temperature, tides, and currents.
To effectively respond to an oil spill, scientists must gather
critical chemical, oceanographic, climatic, and on-scene
environmental data. Although various types of remote
sensing techniques are available for detecting and mapping
oil distribution, it is important to understand the charac-
teristics of surface currents in the area.
The along-shelf-period oscillations dominate the along-shelf
transport which is mainly east to west. Records of ADCP data
(Fig.
14
), from station S5 offshore the Kwa Ibo estuary in the
eastern Niger Delta, shows near-surface oscillating currents
(Bin 26). Reversals along this east-west direction are very
easily observable in the feather plots (Fig.
14
).
In April 2000, during the onset of the rainy season, the
dominant direction is west to east. In July during the rainy
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