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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

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.