Changing morphology of Ghana’s Accra coast

Kwasi Appeaning Addo

Received: 29 June 2010 /Revised: 5 October 2010 /Accepted: 14 October 2010 /Published online: 4 December 2010# Springer Science+Business Media B.V. 2010

Abstract Coastal features in Ghana's Accra coast reflectboth past and present processes that have been undergoingchanges. These changes are influenced by a range ofmorphogenic factors such as geology and climatic con-ditions. These regimes have shaped the coastal geomorphicfeatures through weathering processes that decompose anddisintegrate the coastal rock. Sea level rise due to climatechange is expected to increase coastal erosion and thusresult in rapid changes in shoreline positions. Historic rateof sea level rise in Accra coast is about 2 mm/yr (Ibe &Quelennec, 1989) which is predicted to reach approximate-ly 6 mm/yr in the next century since it conforms to theglobal change (Armah et al., 2005). This will result inflooding of vulnerable areas and enable waves to breakcloser inland. The effectiveness of the erosion process isaided considerably by the type of geology. Accra coastalzone has three types of rock in three identified geomorphicregions. They include unconsolidated and poorly consoli-dated rock along the western region, the Accraian seriesoccupying the central region and the Dahomeyan series inthe eastern region. The geology has thus influenced theextent to which the coastal features have changed and thetype of cliff that is formed as a result of erosion within theregions. Generally, soft rock coastal features decay morerapidly than those of hard rock and tend to act as sedimentsources. Human activities such as dam construction overthe Densu River, engineering interventions to check the

spread of erosion and sand mining has created sedimentdeficit which has exacerbated coastal erosion in Accra.Anthropogenic factors are estimated to account for 70-90%of coastal erosion problems in Accra.

Keywords Accra coast . Coastal erosion . Coastalmanagement . Coastal landforms . Geomorphology

Introduction

Coastal system along Accra coast operates within bothprevailing geology and artificial structures such as JamesTown Jetty, Tema Port Jetty and other defence structures.This has exerted controls on the range of landforms thathave been formed by coastal process through erosion anddeposition. The inherited geologic framework thereforeexerts an influence on the morphology of the coast.According to Inman et al. (2003), the present configurationof coastlines and their associated terraces and sea cliffsretain vestiges of the previous landforms from which theyhave evolved. Coastal evolution has thus been described asa Markovian process by Inman and Nordstrom (1971)where the present coastal features are dependent on thelandforms and processes that preceded them.

Changes in coastal morphological features are primarilycaused by climatic factors, physical processes and exacer-bated by human activities. They are aided significantly bythe geology. Wind action influence coastline geomorphol-ogy both directly and indirectly. Strong winds deflate finegrained sediment from beaches, lowering their surfaces, andcausing the movement of rock particles onshore, alongshoreand offshore (Bird 2008). Strong wind generated wave is amajor cause of shoreline change (Appeaning Addo 2009)and the greatest land loss normally occurs where there are

K. Appeaning Addo (*)Department of Oceanography and Fisheries, University of Ghana,P. O. Box LG 99, Legon-Accra, Ghanae-mail: kappeaning-addo@ug.edu.gh

K. Appeaning Addoe-mail: appeaning@yahoo.com

J Coast Conserv (2011) 15:433–443DOI 10.1007/s11852-010-0134-z

long fetches of open water (Morton 2003). Wave energy isdistributed over a range of shoreline elevations betweenlow tide and high tide. Tidal current affects the size, sortingand distribution of sediment over most of the sea floor. It isestimated that water moving half a mile an hour willtransport medium-sized sand grains, and at three miles anhour will carry gravel an inch in diameter (Gometal 2006).The sea level has fluctuated repeatedly in the last 2 millionyears (Davis 2005). IPCC (2001) estimated the globalaverage rate of sea level rise for the past century to bebetween 10 and 15 cm and suggested that the sea levelcould rise as much as 1 m over the next century. IPCC(2007) again approximated that the historic sea level rise isabout 2 mm/yr while the future rate is about 6 mm/yr. Theeffect will be localised depending on the prevailing localconditions. Sea level rise has thus enabled tides to movemore inland during high tides in relatively low lying areas.Continues increase in global sea level will exacerbate coastalerosion, especially in developing nations (Appeaning Addoet al. 2008).

Ghana’s Accra coast has changed over millions of yearsin response to changes in the natural environment. Historicrate of erosion is 1.13 m/yr±0.17 m/yr with about 82% ofthe shoreline eroding while the remaining 18% is eitheraccreting or stable (Appeaning Addo et al. 2008). Geo-morphologic change, continuous to the present day, hasbeen influenced by human activities such as engineeringworks and over exploitation of coastal resources thatinclude vegetation and beach sediment (Armah 1991;Alvarado 2003). Dam construction over the Densu Riverand the Kokrobite irrigation scheme are considered to beone of the main sources of shoreline erosion in the Accracoastal region (AESC 1988). This is because they havereduced the sediment supply to the littoral zone that hasresulted in imbalance in the sediment budget. The uniqueconfiguration of the shoreline has also been identified byArmah (1991) as a cause of erosion. The shoreline runs inan approximately east-west direction. The orientationenables incident waves to break obliquely and generatelongshore currents that facilitate littoral drift. The directionand intensity of the drift is from west to east. The WorldBank (1995) identified poor decision-making on infrastruc-ture location, inappropriate mitigation measures and re-source extraction as the major cause for shoreline erosion.This is largely due to inadequate and unreliable informationon recession trends in the Accra shoreline, which hasresulted in poor coastal management practices, and adhocand disjointed measures in solving perceived erosionproblems (Boateng 2006). The measures are implementedwith little consideration for geomorphological sensitivity ofthe site and results in creating more problems down-drift.Beach sediment mining activity has increased alongGhana’s shore due to the fast growing construction industry

and the readily available market for the product (Mensah1997).

Coastal erosion along the Accra coast, apart fromchanging the geomorphic state, has also affected the socialand economic life of the local population, threatenedcultural heritage and hindered coastal tourism development(Norley 2006). In the western portion of the Accra coast, 17coastal inhabitants have lost their buildings to coastalerosion over 26 years (Campbell 2006). This indicates onaverage of approximately 0.7 houses eroded annually in thewestern portion that is significantly high and thus results indisplacement of families. Coastal retreat has also erodedfish landing sites, collapsed the coconut oil business anddegraded the coastal environment. Erosion has thereforeaffected both the local and national economy.

This paper, which reviews the geomorphic state of theAccra coast in Ghana, also analysis how the climatic andhuman induced factors influence the observed changes. Thesecond section reports on the methodology adopted for thisstudy and the third session discusses the Accra coastalgeomorphology. The fourth section evaluates the climateand oceanographic processes as well as anthropogenicactivities, while the five section talks about the variousdrivers of shoreline recession along the coast of Accra.Conclusion drawn from the discussion is reported in sectionsix.

Methodology

Primary and secondary methods of data collection wereused for this study. Extensive site surveys were undertakento assess the coastal erosion situation along the Accra coast.Factors suspected to drive localized erosion were identifiedand their level of influence assessed. These include locationof constructed engineering intervention structures to controlerosion and their influence on the adjoining shorelines; lowlying areas that are likely to be flooded in the event of sealevel rise; locations with sandy beaches that are experienc-ing sand mining activities; and the various sources ofsediment to the littoral zone. Opinion leaders in the coastalcommunities were also engaged in an informal discussionto sample their view on coastal erosion. The interactionscentered mainly on whether they have noticed significantchange in the shoreline position over the years. Althoughthis method is unscientific since the results cannot bemeasured, it reveals the coastal dwellers perception abouterosion and what they suspect to have contributed to theerosion problem. Information on the wave climate, tidalregimes and the type of the geology along the coast wereobtained from different sources. Wave climate data wasobtained from the Svašek Hydraulics in the Netherlands;tidal data was received from the British Oceanographic

434 K. Appeaning Addo

Data Centre (BODC) hosted by POL, Liverpool; digitaltopographic map was obtained from the Mapping andsurvey section of the Ghana Lands commission; while theGhana geological survey department provided the coastalgeological information.

Geomorphology of the Accra coast

The Accra coastal zone is divided into three portions basedon the geomorphology namely the western, central andeastern portions. Figure 1 shows the locations of the threeportions labeled as 1, 2 and 3 respectively. The westernportion is eroding more relative to the central and the

eastern portions (Appeaning Addo et al. 2008). Studies byMuff and Efa (2006) reported that the western portionconsists of unconsolidated and poorly consolidated rocksmainly from sediments and superficial deposit, the centralportion is made up of the Accrain series which consist ofsandstones, while the eastern portion is made up of theDahomeyan system, which consist of metamorphic base-ment rocks overlay by laterite and clay.

The prevailing rock types have varying properties andconstituents that determine their strength and thus influencetheir response to pressures from sea wave energy. The rockswith weak strength erode easily while those with highstrength do not. The significant engineering properties ofthe rocks make them suitable raw materials for various

Fig. 1 Map of Ghana showingthe coastal zone of Accra andthe three geomorphic portions.(Source: Ghana Mapping andSurvey Department)

Changing morphology of Ghana’s Accra coast 435

construction works (Muff and Efa 2006). This phenomenonhas considerably increased sand and gravel mining activi-ties along the entire length of the coast. The unconsolidatedand poorly consolidated rocks deposit consists of marine,lagoonal or fluvial sediments. They are weak and looselybound together, which indicates a weak rock strength. Theyare therefore susceptible to erosion by the action of seawaves and wind. The sediments are indicators of recent sealevel change since excavations show layers that indicatepast sea levels (Geoconsultants 2005). The Accraian seriescomprises of three distinct formations. These are uppersandstone-shale formation, middle shale formation andlower sandstone formation (Muff and Efa 2006). Theproperties of the three formations and their overlayingweathering products are very variable. The upper sandstoneand the middle shale formation are more susceptible toerosion than the lower sandstone formation. Mining of therock for the building construction industry weakensbonding of the layers and thus reduces the ability of therock to withstand the eroding action of wave energy. Thestrength of the metamorphic basement rock in the Daho-meyan system is high but the overlaying soil of laterite andclay are easily eroded by the action of waves.

Evolution of the coastal landforms has been influencedby climatic oscillations. Accra coastal geological inves-tigations indicate that three beach levels are recognisable inseveral places (Muff and Efa 2006). According to Muff andEfa (2006), the mineralogical composition of the beachesindicates that their sediments are derived from existinggeological formations, which is evident from observedcharacteristics of geological features identified in the threerecognised beach levels. The observed lower roundnessindex of the older beaches in comparison with the youngerbeaches; having the dominance of grains with ferruginisa-tion in cavities and the presence of old scars corroborate theview that the younger beaches had been derived fromsuccessive re-working of the older beach materials (Muffand Efa 2006). This indicates a fast and strong geomorphicprocess along the Accra coast. Fault zones identified in theAccra coastal area, which played a major role in the historiccoastal evolution, are presently considered active. Studiesby Amponsah (2004) has reported that Accra is aseismically active area. Most of the earthquake in Ghanaoccurs in the western part of Accra at the junction of thetwo major fault systems namely, the coastal boundary faultand Akwapim fault zone. This indicates that an earthquakedisaster is imminent in the near future. This would damagethe coast (CoZSSA 2005) and seriously alter the shorelineposition, especially in the western portion where the faultsare prominent. Erosion rates will be high and theearthquake shocks will trigger landslides along the coast.The future erosion rate will follow the historic trend (Fig. 2)with high rate in the western portion, relatively low rate in

the central portion and moderately high rate in the easternportion.

Sediment fluxes

Various agents act as sources of supply of sediment into thesea. The main sources of sediment are cliff erosion(including erosion of mainland shores), river erosion andseabed erosion as illustrated in Fig. 3. Minor sedimentsources are from transport through tidal inlets, windtransport gradient, cross-shore transport, bio-geneous depo-sition and biological activity. From these sources, thesediment is re-distributed by the action of waves, currentsgenerated from breaking wave energy and to a lesser extentthe influence of tides.

Persistent rise in sea level increases near-shore waterdepth, which enhances the formation of notches in the rockfaces (especially in soft cliff areas). Continuous attackwidens the notches and initiates undercutting. Once thebase of the cliff is eroded, upper cliff soils and vegetationlose their support and collapse downward causing the cliffto recede landward. The debris form talus at the base of thecliff. The talus and other foreshore materials becomesediment for longshore transport. Studies by AppeaningAddo et al. (2008) estimated that in the next 250 years andat a sea level rise of 6 mm/year, the shoreline will migratebetween 270 m and 340 m inland in the central portion,between 280 m and 450 m of land will be lost in the easternportion, while in the western portion the barrier ridgeseparating the Sakumo wetlands from the sea will breacharound 2082. This will result in the flooding of thewetlands and the displacement of settlement on the ridge(Fig. 4).

0 5 10 15 20 25 30 35 40-3

-2.5

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Fig. 2 Recession trend along Accra coast from 1904 to 2002 in thevarious portions and sub-portions. (Source: Appeaning Addo et al.2008)

436 K. Appeaning Addo

Sediment supply from rivers in Accra is at their peakduring the rainy seasons and most of the rivers areconnected to the sea through lagoons. During the rainyseason, the rivers carry sediments into the sea through thelagoon inlets from their upland catchments area and causeserosion of the banks. Lagoons with their inlets openedthroughout the year facilitate a continuous supply ofsediment into the sea while those with blocked inlets allowsediment into the sea usually only during the rainy seasons(Gordon et al. 1998). The seabed usually acts as a basin inwhich fine sediment is trapped and thus gradually raisingthe platform’s elevation, while the coarse sediment istransported in the west–east direction alongshore. Figure 5shows example of offshore profiles in the three geomorphicportions. According to Bruun (1962), the volume of sandthat is lost from the cliff top due to sea level rise getsdeposited on top of the platform within the nearshore.Bruun (1962) formulated the notion of an equilibriumprofile and its response to sea level rise in part to explain

erosion rates. The Bruun rule asserted that the shorefacetends to maintain the same slope for a given depth,balancing the tendency of the wave motions to movesediment on shore and the tendency of gravity and watercurrents to move sediment off shore. The resulting shape ofthe shoreface is called the equilibrium profile. When sealevel rises, the shoreface adjusts itself to re-establish theequilibrium slope at each depth. Bruun (1962) proposedthat sediment is eroded from the upper shoreface to thelower shoreface to achieve this readjustment. Bruun (1962)found reasonable agreement between his model andobserved shoreline retreat rates along the coast of Florida.Various studies (Hands 1983; Leatherman 2001; Zhang etal. 2004; and Storms et al. 2002) confirmed the validity ofthe Bruun model for predicting future shoreline rate ofchange. However, other studies (Dubois 1992; Lowenstein1985; Pilkey et al. 1993; and Cooper and Pilkey 2004)criticized the Bruun rule. They questioned whether it is everapplicable in practice and referred to it as a “one model fits

Fig. 4 Flooding of wetlandsafter breaching of the barrierridge in Accra

Fig. 3 Sources of sediments(based on Pethick 1984)

Changing morphology of Ghana’s Accra coast 437

all” approach that makes it unsuitable for a highly complexsedimentary environment with significant alongshore sed-iment transport. The studies concluded that the concepttime has passed and should hence be abandoned since it hasno power for predicting shoreline behaviour under risingsea level.

As waves break, the energy it generates destabilisesediments, which are then carried away in the littoral drift.According to Wellens-Mensah et al. (2002), the availabletotal supply of sediment from the various sources to the surfzone of the Accra coast is about 200,000 m3/yr. However,the potential rate of sediment transport is approximately5.7×105 m3/yr and the rate increases from the westernportion towards the central portion (Appeaning Addo2009). Although the volume supplied by each of thevarious sources has not been determined, the bulk of thesupply is from erosion of cliff and mainland shores. This isbecause the largest river, Densu River, which could havesupplied significant sediment for littoral transport, isdammed and the upstream is further channeled for anirrigation scheme. The dam traps the fluvial sediment(Akuffo 2003) and the reduced water volume has resultedin periodic blocking of the inlet. Variations in the volume ofsediment supplied and transported have been identified as asignificant cause of increased erosion in the Accra shoreline(Armah 1991; AESC 1980).

Climate and oceanographic process

The local wind climate is characterised by a southwestmonsoon modified by land and sea breezes along the WestAfrica coast (Osei-wusuansa 2000). Computed wind speed,from 10 year period of wave climate developed in Takoradiby Svašek Hydraulics (2006) and measured from a location

about 10 km in-shore, indicates that the average wind speedis about 4.5 m/s (Fig. 6).

However, the wind speed measured onshore andreported by AESC (1980) is 2.5 m/s, which indicates thatthe wind velocity reduces as it approaches the shore. Theoffshore winds generate swell waves, which blow towardsthe coast of Accra and is influenced by the foreshorebathymetry. A local trade wind generates the local wavescalled ‘seas’, which are prominent along the West Africacoastline (AESC 1980). According to AESC (1988), typicalperiod for the ‘seas’ is between 2 and 4 s and the significantwave height is less than 0.3 m. A rough sea is veryinfrequent and if it occurs, the height and the period of the‘sea’ will not exceed 2 m and 5 s respectively.

Unlike the ‘sea’, swell approaching the shores of Accrahave constant characteristics during the year and aregenerated by waves associated with storms far beyond theimmediate influence of the coast (AESC 1988). Significantwave height (Hs) is about 1.4 m for 50% of the time andwave period (Tp) is between 10 and 15 s (AESC 1988).The swell wave characteristics estimated by AESC (1988)were confirmed by analysis of 10 years measured waveclimate data along Takoradi coast (refer to Fig. 1) bySvašek Hydraulics (2006) using a directional wave buoyfrom 1997 to 2006. Graphs of Hs and Tp are shown asFigs. 7 and 8 respectively for the 10 years of wave climate.

The relatively steep slope of the sea floor, Fig. 9, enablesswell waves to contact the headlands, which increase thewave energy. Continuous pounding of the cliff face bywave action leads to the formation of cracks, which widensfrom trapped air and eventually gets dislodged by the forceof the swell waves. The dislodged rocks are broken downfurther in the surf zone through attrition.

Fig. 6 Wind speed from wind climate data for 10 years period(source: Svašek hydraulics 2006)

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Fig. 5 Example of offshore profiles in the three geomorphic portions

438 K. Appeaning Addo

Tides on the coast of Ghana are semi-diurnal incharacter. The average Neap and Spring tides for the Accracoast are 0.62 m and 1.26 m respectively, and they increasefrom west to east (Wellens-Mensah et al. 2002). The tidalrange is approximately 1 m and thus the tidal current isweak most part of the year. This is influenced considerablyby the prevailing relatively weak wind activities along thecoast as a generating force and the effect of bottom friction.

Two other types of currents can be distinguished alongthe Accra coast. The first is the longshore current that isdriven by the breaking waves and reveals itself in thebreaker zone. The wave breaker type is plunging, whichbreaks obliquely due to the orientation of the shoreline. Theshoreline orientation is approximately 2530 (AppeaningAddo 2009). Since the sea floor slopes relatively steeperalong portions of Accra shore (refer to Fig. 9) swell wavesbreak closer to the shore at these points. The breakingenergy also generates longshore currents, which moveparallel to the shoreline and work in tandem with wavesto move significantly large volumes of sand (Davis 2005).The longshore current generated in the Accra surf zonemeasures approximately 1 m/s with variations between0.5 m/s and 1.5 m/s (AESC 1988). The second current isthe counter equatorial current, which is referred to as theGuinea current. Its direction moves from west to east. Thiscurrent is a specific ‘sea’ current off the coast of WestAfrica and shows a maximum from May to July during thestrongest southwest monsoon winds, when it can reachabout 0.5 m/s (AESC 1988). It is generally weaker and lesspersistent further offshore. Other locally generated currentsand wind usually disturb the Guinea current and explainwhy it is generally weak for most of the year. Its influencein shaping the shoreline is therefore limited due to its weakintensity. Changes in weather patterns and human activities

have resulted in widespread destruction of the Accra coastalvegetation and the loss of sediment being held by theirroots. This has led to significant increased erosion in theAccra shoreline.

Drivers of landform change along Accra coast

Coastal erosion in Accra is exacerbated by various activitiesengaged in by the coastal dwellers. The zone is inhabitedby 12% of the nation’s 20 million population and has agrowth rate of about 3% per year as compared to thenational growth rate of 2.5% (Amlalo 2005). The popula-tion density is 3,388 persons per km2 and urbanisation rateis 52% as compared to national urbanisation rate of 35%(Churcher 2006). The opportunity for employment contin-ues to attract an average of about 25,000 migrants per yearand according to Duedall and Maul (2005), this is expectedto increase by 82% in 2025. Increased in coastal populationhas resulted in unsustainable use of the coastal resources.Wetlands have been drained for settlement, coastal vegeta-tion (e.g. mangrove) has been destroyed through overcutting and lagoons reclaimed for settlement (Alvarado2003). The situation has also made planning and decisionmaking about infrastructure location difficult (Fig. 10).Development is close to the shore and thus created a coastal“land squeeze” situation. The World Bank (1995) identifiedinappropriate mitigating measures to this problem as acause of high rate of erosion.

Lack of employment for the increasing coastal popula-tion has made beach sand mining an attractive business.The resource is also abundant and extraction cost is low(Ibe and Quelennac 1989) and the demand for sand in theconstruction industry is high (Osterkamp and Morton2005). Although sand mining is banned on Ghana beaches,the activity continues to be a source of sand supply for the

Fig. 8 Ten year period wave period measured at 3 h intervals.(Source: Svašek Hydraulics)

Fig. 7 Ten year period buoy measured wave height data at 3 hinterval (source: Svašek Hydraulics)

Changing morphology of Ghana’s Accra coast 439

construction of houses (Armah and Amlalo 1998) andaccording to Mensah (1997), its contribution to theconstruction industrial output increased from 17% in 1986to 21% in 1993. The products are sold openly because ofnon-enforcement of the ban due to economic reasons. Sandmining activity is a cause of severe erosion along Accrashoreline, since the activity is not adequately controlled(CoZSSA 2005). Deposit beach sand, in addition to itsaesthetics, maintains a balanced sediment budget, whichensures there is available sediment for littoral transport andalso act as a buffer for the cliffs. Beach sand along theshores in the eastern and central portions of Accra was usedfor the construction of Tema Township, the industrial cityof Ghana built from scratch in the late 1950’s, and othermonumental structures such as the State House (EPA 2003).Removal of beach sand has reduced the available sedimentbudget for littoral transport and also reduced the beach

volume, thus enabling waves to attack the cliff face directlyand erode the cliff materials. This is the plausible cause ofsevere erosion experienced along these portions of the coastin these three geomorphic portions.

Coastal engineering structures constructed in the Accracoastal zone are management strategies adopted to stabiliseor reclaim lost beach, protect and stabilise navigationentrance channels, and protect the coastal properties fromwave-induced damage and flooding. All these structuresessentially interact with active waves and currents, andresult in sediment transport process in the vicinity of thestructure. Jetties, constructed in 1906 (Dickson 1965) toprotect and stabilise navigation entrance channels to the oldAccra harbour trap sediment along the up-drift side andstarve the down-drift side of sediment. This has changedthe shoreline position; the up-drift side has a beach withsand while the down-drift side is experiencing severe

Fig. 9 Contour map of part ofthe Accra shores indicatingrelatively steep slope

Fig. 10 Poorly planned settle-ment in the central portion(Jamestown) with developmentclose to the shore leaving noland for the coastal processes

440 K. Appeaning Addo

erosion (Appeaning Addo 2009). The accreted sand on theup-drift side of the jetties has attracted sand miningactivities, which initiated severe erosion by waves andcurrent actions, thus degrading the coastal environment anddestroying coastal infrastructure (AESC 1988). Groyneswere constructed to check erosion behind the RegionalMaritime Academy, while a combination of groynes andrevetment were constructed to reclaim lost coastal landsalong the Accra-Tema beach road. Although varyingdegrees of success have been achieved (Appeaning Addoet al. 2008), none has been able to stabilise sandy beachesthat are being eroded (Ontowirjo and Istiyanto 2003; Davis2005). This is because the interventions have created newproblems on adjacent shorelines, while in some situationsthey have exacerbated the problem (AESC 1988; Wellens-Mensah et al. 2002). This suggests that coastal defencestructures are inappropriate for management of the shore-line in the Accra coastal zone. Dam construction over theDensu River, the largest river in Accra, has significantlyreduced the volume of water and sediment that aredischarged into the sea.

Conclusions

Factors that cause coastal erosion in Accra, and thus changethe coastal geomorphology, can be categorised as eithernatural or human induced. These driving factors areinfluenced by the coastal geology and hydrological climate.The western portion is eroding more due to the presence ofconsolidated and poorly consolidated rocks, while theremaining portions are experiencing moderate erosion.The natural factors that include wave action, current andsea level rise depicts the normal behaviour of the coastalsystem. The relatively steep Accra ocean platform enablesswell waves to strike the cliff face and dislodge sedimentsfrom their original positions, especially in the easternportion (refer to Fig. 4). However, the metamorphicbasement rock slows down erosion while the overlayinglaterite and clay facilitate erosion process. This has resultedin exposed rock outcrops dotted along the shore. Sandmining practice in the central portion weakens bonding ofthe layers and thus reduces the ability of the rock towithstand the eroding action of wave energy.

Swell waves expel little energy during their propagationthrough the water and expend most of their energy at theshore. The energy released when the wave breaks generatesabout 1 m/s longshore current, which transports suspendedsediment in the west-east direction. Although swell waveshave uninterrupted approach to the continental shelf, veryrough wave activities are not experienced along the coast.As one of the principal causes of wave-energy dissipation isbottom friction, it is conceivable that the broad width of the

Accra coastal shelf confers considerable reduction in theenergy of the waves arriving on the coast. This explainswhy strong wave action is not experienced along the coast.The significant difference in wind speed as they approachshore probably explains why storms and storm surge arenot common along the Accra coast. The ‘sea’ produced bythe local trade winds have short period and wave height andthus their influence on shoreline change is minimal. Weakerline squalls with heavy rains and strong winds of shortduration occasionally occur. Sediment transport in thelittoral zone is mainly by longshore current since the tidalcurrent is low and hence considerably ineffective. The tidalcurrents are weak due to the prevailing small tidal range.Although this is generally true, for the long term erosionrate a small tidal range is very significant to shorelinemorphology since they form a higher elevation beach(Rosen 1977).

Relative sea level rise erodes the cliffs through undercutting. The increase in water depth, as a result of an annualsea level rise of 2 mm/yr, exposes sections of the uplandareas to wave attack as the water depth increases and thusshifting the littoral system landward. Future changes in theshoreline position will be influenced significantly by theincreasing sea level rise as a result of global climatechange. The barrier ridge along the shore in the westernportion will breach, which will result in flooding of landwith elevations below between 0.30 m and 0.48 m altitude,while the mean estimate will flood elevations below 0.36 m(Appeaning Addo 2009). The presence of relatively highhinterlands will prevent inundation further inland.

Anthropogenic factors, which have induced coastalerosion in Accra, include sand mining and coastal con-struction works. Their actions result in sediment deficien-cies in the littoral cell leading to coastal erosion. Activitiesof man, through construction of jetties and groynes, haveresulted in coastal erosion along the adjoining shoresthrough interception of sediment in the littoral transport.Revetments have also caused erosion in the adjoiningshoreline through energising waves. Clearing of coastalvegetation for development has loosened sediment bondingand thus made them more vulnerable to wave and currentattacks. Coastal sand mining has reduced the ability of thebeach to protect the dunes and also created sediment deficit,which has resulted in coastal erosion. Dam constructionover River Densu has reduced the volume of water andamount of sediment that is dislodged into the sea. This hasresulted in creating sediment deficit budget, which has ledto erosion. Activities of man, apart from initiating freshproblems of coastal erosion as in the case of sand mining,have also exacerbated erosion problems in several areas ofthe shore (Ibe 1996; Wellens-Mensah 1994). ACOPS(2003) therefore estimated that anthropogenic activitiesaccount for 70–90% of coastal erosion problems in Accra.

Changing morphology of Ghana’s Accra coast 441

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