West African Coastal Area: Challenges and Outlook - The Land/Ocean Interactions in the Coastal Zone of West and Central Africa - page 14

Fig. 3 Continental shelf from
Mauritania to Benin (blue
shading from 0 to 300 m)

estuaries indicates an east-west orientation for a large part
of the coast of Liberia and Sierra Leone.
In the portion of coast between Guinea Bissau, Guinea,
and the north of Sierra Leone, called the Southern Rivers
region, sediment circulation and redistribution is primarily
governed by tidal removal currents, combined with river
spates in these regions with high seasonal rainfall. The role of
the mangrove estuaries in trapping sediment and building
sediment accumulations, which are partially expelled in
periods of spate, contributes strongly to modelling the facies
of this portion of the West African coast. It should be noted
that the tidal ranges are very wide in this zone, exceeding 5 m
in places, while the average for the whole coastal area studied
is in the order of 1 m. In these regions where rainfall deter-
mines important annual spates, the dams built across rivers
can reduce these spates and restrict the expulsion of the mud
plugs, an important source of sediment supply usually put
into circulation in the coastal waters during these episodes.

A Narrow Continental Shelf

The landform is, on the whole, not very rugged. The conti-
nental shelf is narrow in the main, around 30 km on average,
except from Guinea Bissau to the Sherbro islands in Sierra
Leone, where it widens considerably to 200 km. This conti-
nental shelf is marked by some major deep features: the
Khayar canyons in Senegal to the North of the Cape Verde
Peninsula, and the deep canyon (''Trou sans fond'' (Bot-
tomless pit)) that cuts through the shelf perpendicular to
Abidjan in Côte d'Ivoire. For certain authors, these bathy-
metric features contribute to trapping the sediment trans-
ported by the coastal drift current parallel to the coast (Fig. 3 ).

Circulation and Redistribution of Sediment

The propagation of ocean waves affects the whole of the
two major sea fronts along this coastline, west and south,
with an orientation that is generally oblique, which con-
tributes to the generation of a significant coastal drift cur-
rent more or less parallel to the coast.
The circulation and redistribution of sediment is gov-
erned on the major part of the coast by this coastal drift
current, which is subject to annual variations, but the
resultants of which are globally north-south all along the
western sea front (from Mauritania to Guinea Bissau) and
west-easterly along the Gulf of Guinea. In certain cases,
these variations are considerable: the drift is reversed sea-
sonally on the Grande Côte of Senegal, and the resultant
observed through the physiography of river mouths and

Five Major Coastal Profiles

There are five distinct major coastal profiles from north to
south:
• The straight coastal regions from Mauritania to the Cape
Verde peninsula composed for the most part of sandy
formations subject to the direct action of the coastal drift.
In the immediate proximity of and behind the ridge/
sandbar, there are vast expanses of low-lying salt marshes
situated below sea level in places.
• A coastal region with headlands and softened coves from
Cape Verde peninsula to Basse Casamance structured by