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Coastline of Gambia
 
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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 (see map). 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.

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