Environmental Engineering Reference
In-Depth Information
Fig. 3
Gulf of Guinea showing
prevailing ocean currents
Canary Current
Gulf of Guinea Current
South Equatorial
Current
Benguela Current
undercurrent which is thought to be a westward-flowing
extension of the northern branch of the equatorial under-
current which splits into two branches after impinging on
the African continent at Sao Tome Island. Due to the fact
that the equatorial undercurrent carries cool and highly
saline water, the thermocline beneath the Guinea current is
particularly intense. The other important surface current in
the Gulf of Guinea is the South Equatorial Current (SEC).
input into the estuaries during ebb tides results in dilution of
the water, increase in the volume of water discharge and
increased current speed (Table
2
). While seasonal variations
affect the tidal range at the river mouth during dry and wet
seasons, the currents and estuary water fluxes vary with the
seasons. Significant tidal incursions exist in the Niger delta
estuaries during rainy and dry seasons. All these produce salt
wedge limits within the estuarine zones which are not fixed
but change as a result of variations in tidal spring-neap cycles,
atmospheric forcing, and river discharge. During rainy sea-
son, the salt wedge moves seawards while during the dry
season and high tide, the salt wedge moves inland. Variations
in the salt wedge could develop a velocity difference between
the two layers of fresh and salt water with the shear forces
generating internal waves at the interface resulting in mixing
the seawater upward with the freshwater.
The tidal variations superimposed by the freshwater
discharge create plume areas in the ocean off the mouth of
the estuary. These plumes, which are heavily laden with
sediments (Fig.
12
), are more prevalent especially during
low tides when fresh water input extends farther out into the
sea. These plumes are in the form of a fan that protrudes
from the estuary into the coastal ocean.
Estuarine Circulation
Circulation within the Niger Delta estuaries is driven
mainly by the tides from the sea, modified by the mor-
phologic and bathymetric configuration of the estuaries and
fluvial flow. Many of the estuaries consist of sand bars,
which in most cases are ephemeral and hence modify cir-
culation patterns. Typical current and water fluxes in some
Niger delta estuaries (Forcados in the west of the Niger
Delta and Brass in the east of the Niger delta) are shown in
Figs.
4
,
5
,
67
,
8
,
9
,
10
,
11
; Table
2
.
The river estuaries in the western part of the Niger Delta
enter the ocean at a westward oblique angle. The Forcados
river estuary, which is largest in the east, exhibit current
speeds of between 0.35 m/s to a high of 1.65 m/s with water
discharge in the ebb condition almost double that of the
flood (Table
2
). River estuaries in the eastern Niger Delta
enter the sea almost due south, with Brass being typical.
The Brass river estuary, though smaller than the Forcados,
has faster currents and total discharge during ebb condition
than during flood conditions.
The rainy season is characterized by more persistent and
strong winds as well as a larger tidal range compared with the
dry season when met-ocean forces are subdued. Fresh water
Ocean Circulation
Ocean circulation off the Niger Delta estuaries is usually
governed by tides, wind stress, bathymetry and salinity
gradient. According to Awosika and Folorunsho (
2005
),
among others, two major circulation patterns have now been
documented offshore of the Niger Delta. The across-shelf
patterns are driven and in phase with the tides, while the
along-shelf patterns alternate east to west almost fortnightly.
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