33
Hemisphere when the Southeast Trades noticeably weaken,
the Northeast trades are full developed. During this period
the Canary current intensifies bringing cool water to the
coast of Sierra Leone. This current generally flows in a south-
easterly direction at the surface in the near-shelf regions.
The Canary current is mainly southward from August to
April. When this current approaches the equator, it turns
westward as the North equatorial current. The monsoon
period generally lasts from July to August; during the
Northern Hemisphere summer. During this period, the
equatorial counter current is strongly developed and is
the source of much water joining the Guinea current. In
the winter months (December to February) however, the
equatorial counter current ceases to be of importance and
the canary current is the main source of water joining
the Guinea current. In the autumn the southeast trades
strengthen reaching maximum strength in August. During
the May – July period, the canary current moves northward
carrying low salinity Liberian surface waters to the north
as far as Senegal (Berrit, 1969).
6.4. Seasonal Upwelling and Productivity
The Sierra Leone coastal zone lies at the southernmost
extension of the upwelling system which occurs along the
entire northwest African coast Dakar – Freetown region.
In Sierra Leone, the continental shelf is isolated from the
seasonal coastal upwelling areas of North-West Africa and
central Gulf of Guinea by a complex of shoals (submerged
sand banks) that comprises the Bissagos Archipelago, off
Guinea-Bissau in the North and the shoals of Saint Ann
to the South. The area of the continental shelf up to a
depth of 200 m is about 30,000 km
2
.
The western tip of Sherbro Island delimits two contrasting
coastal waters. The narrow southern shelf has limited fish
resources and is influenced by the eastward flowing Guinea
current.The northern Sierra Leonean coast on the other hand
constitutes the productive shelf of Sierra Leone. Therefore,
most of the artisanal fishing activities in Sierra Leone occur in
the North. Here, there are three major estuaries: the Scarcies
River, the Sierra Leone River and the Sherbro River, as well as
the Yawri Bay. The continental shelf has good yield potential
for demersal and pelagic fish as well as shrimps.
According to Coutin (1989), a strong upwelling occurs
each year along the coast of North-West Africa between
December and April and especially between October
and February for the Dakar – Freetown region during
the Harmattan. These Harmattan winds cause aerobic
upwelling along the entire northwest African coast.
Longhurst (1983) indicated that the continental shelf
waters of Sierra Leone are influenced by the Cape Verde
divergence to the north and the convergence at the
northern margin of the Equatorial Counter Current (ECC)
further south of the equator. Upwelling is characteristic of
the former whilst down welling is a feature of the latter.
Analysis of limited hydrographic data suggests that
upwelling off the Sierra Leone coast is characterised by a
shallower thermocline and nutrient enrichment below the
surface at 20m depth (Johnson & Johnson 1996).
The upwelling phenomenon has not been studied in
detail with regards to causes, timing, persistence and
impacts climate and productivity of the Sierra Leonean
coastal zone. Productivity and upwelling are inextricably
interwoven.
6.5. Tides
At the northern mouth of the Sierra Leone River estuary,
a sand bar of considerable magnitude obstructs the flow
of water from and into the estuary and consequently the
velocity of the tidal stream is very great. In the dry season,
during spring tides, the flood stream runs from 1 to 1,5
knots lasting for about 5 hours, and the stream on the ebb
tide varies from 2 to 2,5 knots for about 7 hours. In the
wet season, however, when the tidal prism is augmented
by the flood discharge from the river, the ebb stream
attains velocities in the region of 5 knots. The tide brings
in fair quantities of sediments from the upper reaches of
the estuary.
The highest astronomical tide of maximum tidal range
goes up to 3.38 metres above the lowest astronomical tide
or the admiralty chart datum. Mean high water neaps are
3.0 meter above Chart Datum whereas mean low water
neaps are 1.0 meter above Chart Datum.
6.6. Waves and Swells
Almost a perennial swell of moderate height, coming
in from the Atlantic disturb ships at their moorings
within the estuary. This phenomenon is particularly
evident during the wet season when both frequency and
amplitude are greatest. The maximum amplitude is in the
region of 1.0 meter. Winds waves, usually not higher than
1.0 meter, only occur for brief spells usually just before
the change of seasons about May and October.
6.7. Sediment Transport Processes
Longshore drift current is the main mechanism by which
sediments are transported along the Sierra Leone coast. The
sediment transport takes place mainly within 1–10m water
depth. Three main longshore drift current directions can be
recognized along the Sierra Leone coastline. These currents
flow in a north eastern direction causing erosion of the
northern coastline around Yelliboya Island and Konakridee.
Similar south easterly flowing currents in the south carry
sediments from the Freetown Peninsula beaches and along
the entire southern coastline of Sierra Leone.
Tidal currents also influence the sediment transport
dynamics particularly those of very fine sand and mud
mainly at the entrances of bays and estuaries.