Introduction:
The Gambia's coastline
runs along the West African coast from the mouth of
the Allahein River (13”4’N) to 13’3 1’56”N between Buniadu
Point and the Karenti Bolon; a total length of 80 km.
The west coast, between the Allahein River and Cape
Saint Mary (56 km), faces the North Atlantic Ocean.
The sections of Cape Saint Mary to Banjul
( 13km) and Barra Point to Buniada Point (1 km) border
the bar area of the Gambia River estuary which enters
the picture between Banjul Point and Barra
Point.
The coastal region is a flat and monotonous area of
loose marine sands; low dunes being being common.
These Holocene mineral deposits are underlain by tertiary
ferruginous sandstone of the "Continental Terminal",
which is occasionally laid bare along the coast as rocky
platforms or cliffs. The continental shelf is relatively
narrow, the 200 m isobath being about 80 km offshore.
 The
Saloum River, with its tributaries, debouches directly
North from the northern border with Senegal, forming
the delta-shaped north-eastern shore of the Gambia estuary;
the latter enters its bay-mouth between Banjul and Barra
Point. A tidal channel runs North and West across the
bar with Shallow sandy banks North of Banjul. The channel
and the eastern bars of the bay have a muddy bottom.
The adjacent coastal arc between Buniadu Point and Barra
Point consists of sandy beach barriers based on finer
estuarine deposits, partly sheltered from the ocean
waves by the shallow bar of the estuary. This equally
holds for the fine sandy beach barriers of the western
bay-shore between Banjul an Cape Saint Mary.
 Southward
of Cape
Saint Mary, the coast is fully exposed to the Atlantic
Ocean and displays an echelon-like arrangement which
is related to the differential erosion of the beaches
and the cliffs and rocky platforms, cut into the sandstone
of the “Continental terminal”. A few small streams have
their mouths along this north-south oriented coastal
stretch. The San Pedro or Allahein River, a bit larger,
forms the southern border with Senegal.
Marine Climate Conditions:
The climate
of the coastal zone of The Gambia belongs to the tropical
boreal type, with a long dry season in winter and rains
concentrated in summer (July-September); August is normally
the rainiest month. During the wet season 762-l 143
mm of rain may be expected. The country had experienced
droughts, with the rainfall for 1970-1980 dropping to
only 345 mm/yr.
On the coast, the intensity of the dry season is lessened
by heavy dew, by high relative humidity, lower temperatures
and by sea breezes. Marine trade winds account for 43
% of the average readings at Banjul, the (north-easterly)
Harmattan for 17%, and the rain- bearing westerly or
north-westerly for 18%, the rest being calms.
Tide is of a semi-diurnal type, with two daily maxima
and minima. The tidal range is relatively small; at
Banjul the range is 1.6 m in spring tides and 0.7 m
in neap tides. As a consequence, tidal currents along
most of the coast are weak (< 0.1 m/s) except for
the Gambia estuary. Here tidal filling and emptying
causes tidal currents to be well over 1 m/s.
With respect to wave action from the Atlantic Ocean,
it can be observed that the Gambia coast is situated
in between the exposed coasts of Mauritania and Senegal.
Affected by northerly to north-westerly swell regimes
- and the more sheltered coasts of Guinee- Bissau and
Guinea - affected by south-south-westerly to west north-westerly
swells. Consequently, along the Gambian coast the north-westerly
swell greatly prevails over the south-westerly ones;
its period varies between 8 and 12 seconds. This swell
is strongest during the dry season.
Littoral Regime:
 The
NW swell is, thus, the main factor controlling the regional
coastal morphology. It induces a major south-going sedimentary
transit owing to the genegal coast orientation, running
in a north-south direction.
The west-east oriented coast in between Cape Saint Mary
and Banjul, however, experiences an east-going littoral
drift which is strengthened by the flood-dominated tidal
flows running close to the shore. These two mechanisms
thus promote a littoral drift directed from Buniadu
Point in the North and Cape Saint Mary in the West towards
the estuary mouth, which acts as a sediment sink. This
is due to the fact that the Gambia estuary constitutes
a drowned valley still in the process of being filled
up, both from the river and the sea.
 To
the South of Bald Cape, the north-south coast again
experiences a predominant south- going littoral drift,
as evidenced by the distinct zeta shape of the beaches
in between the various headlands. Along the SW-NE running
coast in between Cape Saint Mary and Bald Cape, a divergence
point in the littoral drift system is thus apparent,
sediment being transported in opposite directions towards
Banjul Point in the north-east, with the estuary acting
as sink, and Kartong
Point in the South, where the accumulation development
across Allahein River forms the terminus of the south
going drift.
 As
a consequence, it may be concluded that the whole of
this coastal region has for a long time been in a state
of erosion, owing to its natural development, as induced
by the littoral regime and by sea level rise. Over the
past decades, human interventions also play a significant
role. The building construction boom in the coastal
zone which is related to urban developments and particularly
the tourist industry, has led to widespread removal
of the natural vegetation cover and dune formation (rendering
the coast more vulnerable to erosion) and mining of
significant quantities of sand from the beaches, which
has now been halted by the authorities.
Estimates of the littoral drift along the open Atlantic
coast have been made on basis of the extensive spit
development over some 20 km at the mouth of the Saloum
River (Sangomar spit) and amount to some 100,000 to
250,000 m”/yr. These figures may equally be applicable
to the coast South of Bald Cape. The mechanism of sediment
transport along the coast to the East of Cape Saint
Mary are complicated by the fact that along this bay
coast sheltering of ocean waves takes place concurrently
with increased tidal influence by flood-dominated flows.
From the distinct offshore bar bypassing at Oyster Creek
it may be inferred on the basis of governing rules for
tidal inlets that, the littoral drift along this coastal
stretch must be in the order of 30,000 to 100,000 rn3/yr
at most.
Possible sources of sediment for this littoral drift
system are the coast itself through erosion, and the
sediment yields from watersheds into the coastal system.
In the latter respect it can be noted that the Gambia
River constitutes a sink rather than a source of sediments.
The remaining rivers debauching at the coast have too
limited catchments to produce any significant sediment
yield. The only major river, the Allahein River, debouches
its sediment yield to the South of Kartong Point, at
the terminus of the coastal system.
The above appraisal of the littoral drift system and
yields of watersheds imply that on a yearly basis some
200,000 to 300,000 m” of sediment is lost through erosion
of the Gambian coast line in between Banjul Point and
Sanyang
Point (the more southern stretch towards Kartong Point
being relatively stable), i.e. an average erosion of
some 1 to 1.5 m/yr along the entire coastal stretch
of 20-25 km. The loss of land is 2.5-3 ha per year.
As the cliffs and rocky platform cut into the sandstone
of the “Continental terminal” are relatively hard (erosion
rates 0.5 m/yr), the “production” of sediment through
coastal erosion is even more important along the vulnerable
and low sandy beaches. This is evidenced by the inventory
of coastal erosion problems as given further on.
The above coastal erosion resulting from the overall
sediment balance, is further aggravated by the sea level
rise (1 to 1.5 mm/yr) over the past century. Strictly,
a change in sea level does not change the quantity of
sediments forming the coastal system. However, the coastal
system reacts with a redistribution of the sediments,
maintaining the cross-sectional profile with respect
to the sea level. At the present rate of sea level rise
the apparent loss of sediments along the Gambian coast
line is preliminary estimated at some 75,000 m”/yr,
not yet accounting for the sediment loss via raising
of the coastal plains. The effect of sea level rise
on the coastal sediment balance is thus significant. |
