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

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 reflect both past and present processes that have been undergoing changes. These changes are influenced by a range of morphogenic factors such as geology and climatic conditions. These regimes have shaped the coastal geomorphic features through weathering processes that decompose and disintegrate the coastal rock. Sea level rise due to climate change is expected to increase coastal erosion and thus result in rapid changes in shoreline positions. Historic rate of sea level rise in Accra coast is about 2 mm/yr (Ibe & Quelennec, 1989) which is predicted to reach approximately 6 mm/yr in the next century since it conforms to the global change (Armah et al., 2005). This will result in flooding of vulnerable areas and enable waves to break closer inland. The effectiveness of the erosion process is aided considerably by the type of geology. Accra coastal zone has three types of rock in three identified geomorphic regions. They include unconsolidated and poorly consolidated rock along the western region, the Accraian series occupying the central region and the Dahomeyan series in the eastern region. The geology has thus influenced the extent to which the coastal features have changed and the type of cliff that is formed as a result of erosion within the regions. Generally, soft rock coastal features decay more rapidly than those of hard rock and tend to act as sediment sources. Human activities such as dam construction over the Densu River, engineering interventions to check the

K. Appeaning Addo (*) Department of Oceanography and Fisheries, University of Ghana, P. O. Box LG 99, Legon-Accra, Ghana e-mail: [email protected] K. Appeaning Addo e-mail: [email protected]

spread of erosion and sand mining has created sediment deficit 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 . Coastal management . Coastal landforms . Geomorphology

Introduction Coastal system along Accra coast operates within both prevailing geology and artificial structures such as James Town Jetty, Tema Port Jetty and other defence structures. This has exerted controls on the range of landforms that have been formed by coastal process through erosion and deposition. The inherited geologic framework therefore exerts an influence on the morphology of the coast. According to Inman et al. (2003), the present configuration of coastlines and their associated terraces and sea cliffs retain vestiges of the previous landforms from which they have evolved. Coastal evolution has thus been described as a Markovian process by Inman and Nordstrom (1971) where the present coastal features are dependent on the landforms and processes that preceded them. Changes in coastal morphological features are primarily caused by climatic factors, physical processes and exacerbated by human activities. They are aided significantly by the geology. Wind action influence coastline geomorphology both directly and indirectly. Strong winds deflate fine grained sediment from beaches, lowering their surfaces, and causing the movement of rock particles onshore, alongshore and offshore (Bird 2008). Strong wind generated wave is a major cause of shoreline change (Appeaning Addo 2009) and the greatest land loss normally occurs where there are

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long fetches of open water (Morton 2003). Wave energy is distributed over a range of shoreline elevations between low tide and high tide. Tidal current affects the size, sorting and distribution of sediment over most of the sea floor. It is estimated that water moving half a mile an hour will transport medium-sized sand grains, and at three miles an hour will carry gravel an inch in diameter (Gometal 2006). The sea level has fluctuated repeatedly in the last 2 million years (Davis 2005). IPCC (2001) estimated the global average rate of sea level rise for the past century to be between 10 and 15 cm and suggested that the sea level could rise as much as 1 m over the next century. IPCC (2007) again approximated that the historic sea level rise is about 2 mm/yr while the future rate is about 6 mm/yr. The effect will be localised depending on the prevailing local conditions. Sea level rise has thus enabled tides to move more inland during high tides in relatively low lying areas. Continues increase in global sea level will exacerbate coastal erosion, especially in developing nations (Appeaning Addo et al. 2008). Ghana’s Accra coast has changed over millions of years in response to changes in the natural environment. Historic rate of erosion is 1.13 m/yr±0.17 m/yr with about 82% of the shoreline eroding while the remaining 18% is either accreting or stable (Appeaning Addo et al. 2008). Geomorphologic change, continuous to the present day, has been influenced by human activities such as engineering works and over exploitation of coastal resources that include vegetation and beach sediment (Armah 1991; Alvarado 2003). Dam construction over the Densu River and the Kokrobite irrigation scheme are considered to be one of the main sources of shoreline erosion in the Accra coastal region (AESC 1988). This is because they have reduced the sediment supply to the littoral zone that has resulted in imbalance in the sediment budget. The unique configuration of the shoreline has also been identified by Armah (1991) as a cause of erosion. The shoreline runs in an approximately east-west direction. The orientation enables incident waves to break obliquely and generate longshore currents that facilitate littoral drift. The direction and intensity of the drift is from west to east. The World Bank (1995) identified poor decision-making on infrastructure location, inappropriate mitigation measures and resource extraction as the major cause for shoreline erosion. This is largely due to inadequate and unreliable information on recession trends in the Accra shoreline, which has resulted in poor coastal management practices, and adhoc and disjointed measures in solving perceived erosion problems (Boateng 2006). The measures are implemented with little consideration for geomorphological sensitivity of the site and results in creating more problems down-drift. Beach sediment mining activity has increased along Ghana’s shore due to the fast growing construction industry

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and the readily available market for the product (Mensah 1997). Coastal erosion along the Accra coast, apart from changing the geomorphic state, has also affected the social and economic life of the local population, threatened cultural heritage and hindered coastal tourism development (Norley 2006). In the western portion of the Accra coast, 17 coastal inhabitants have lost their buildings to coastal erosion over 26 years (Campbell 2006). This indicates on average of approximately 0.7 houses eroded annually in the western portion that is significantly high and thus results in displacement of families. Coastal retreat has also eroded fish landing sites, collapsed the coconut oil business and degraded the coastal environment. Erosion has therefore affected both the local and national economy. This paper, which reviews the geomorphic state of the Accra coast in Ghana, also analysis how the climatic and human induced factors influence the observed changes. The second section reports on the methodology adopted for this study and the third session discusses the Accra coastal geomorphology. The fourth section evaluates the climate and oceanographic processes as well as anthropogenic activities, while the five section talks about the various drivers of shoreline recession along the coast of Accra. Conclusion drawn from the discussion is reported in section six.

Methodology Primary and secondary methods of data collection were used for this study. Extensive site surveys were undertaken to assess the coastal erosion situation along the Accra coast. Factors suspected to drive localized erosion were identified and their level of influence assessed. These include location of constructed engineering intervention structures to control erosion and their influence on the adjoining shorelines; low lying areas that are likely to be flooded in the event of sea level rise; locations with sandy beaches that are experiencing sand mining activities; and the various sources of sediment to the littoral zone. Opinion leaders in the coastal communities were also engaged in an informal discussion to sample their view on coastal erosion. The interactions centered mainly on whether they have noticed significant change in the shoreline position over the years. Although this method is unscientific since the results cannot be measured, it reveals the coastal dwellers perception about erosion and what they suspect to have contributed to the erosion problem. Information on the wave climate, tidal regimes and the type of the geology along the coast were obtained from different sources. Wave climate data was obtained from the Svašek Hydraulics in the Netherlands; tidal data was received from the British Oceanographic

Changing morphology of Ghana’s Accra coast

Data Centre (BODC) hosted by POL, Liverpool; digital topographic map was obtained from the Mapping and survey section of the Ghana Lands commission; while the Ghana geological survey department provided the coastal geological information.

Geomorphology of the Accra coast The Accra coastal zone is divided into three portions based on the geomorphology namely the western, central and eastern portions. Figure 1 shows the locations of the three portions labeled as 1, 2 and 3 respectively. The western portion is eroding more relative to the central and the Fig. 1 Map of Ghana showing the coastal zone of Accra and the three geomorphic portions. (Source: Ghana Mapping and Survey Department)

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eastern portions (Appeaning Addo et al. 2008). Studies by Muff and Efa (2006) reported that the western portion consists of unconsolidated and poorly consolidated rocks mainly from sediments and superficial deposit, the central portion is made up of the Accrain series which consist of sandstones, while the eastern portion is made up of the Dahomeyan system, which consist of metamorphic basement rocks overlay by laterite and clay. The prevailing rock types have varying properties and constituents that determine their strength and thus influence their response to pressures from sea wave energy. The rocks with weak strength erode easily while those with high strength do not. The significant engineering properties of the rocks make them suitable raw materials for various

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the central portion and moderately high rate in the eastern portion. Sediment fluxes Various agents act as sources of supply of sediment into the sea. The main sources of sediment are cliff erosion (including erosion of mainland shores), river erosion and seabed erosion as illustrated in Fig. 3. Minor sediment sources are from transport through tidal inlets, wind transport gradient, cross-shore transport, bio-geneous deposition and biological activity. From these sources, the sediment is re-distributed by the action of waves, currents generated from breaking wave energy and to a lesser extent the influence of tides. Persistent rise in sea level increases near-shore water depth, which enhances the formation of notches in the rock faces (especially in soft cliff areas). Continuous attack widens the notches and initiates undercutting. Once the base of the cliff is eroded, upper cliff soils and vegetation lose their support and collapse downward causing the cliff to recede landward. The debris form talus at the base of the cliff. The talus and other foreshore materials become sediment for longshore transport. Studies by Appeaning Addo et al. (2008) estimated that in the next 250 years and at a sea level rise of 6 mm/year, the shoreline will migrate between 270 m and 340 m inland in the central portion, between 280 m and 450 m of land will be lost in the eastern portion, while in the western portion the barrier ridge separating the Sakumo wetlands from the sea will breach around 2082. This will result in the flooding of the wetlands and the displacement of settlement on the ridge (Fig. 4). 1 A

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0.5 0 Rate of change (m/yr)

construction works (Muff and Efa 2006). This phenomenon has considerably increased sand and gravel mining activities along the entire length of the coast. The unconsolidated and poorly consolidated rocks deposit consists of marine, lagoonal or fluvial sediments. They are weak and loosely bound together, which indicates a weak rock strength. They are therefore susceptible to erosion by the action of sea waves and wind. The sediments are indicators of recent sea level change since excavations show layers that indicate past sea levels (Geoconsultants 2005). The Accraian series comprises of three distinct formations. These are upper sandstone-shale formation, middle shale formation and lower sandstone formation (Muff and Efa 2006). The properties of the three formations and their overlaying weathering products are very variable. The upper sandstone and the middle shale formation are more susceptible to erosion than the lower sandstone formation. Mining of the rock for the building construction industry weakens bonding of the layers and thus reduces the ability of the rock to withstand the eroding action of wave energy. The strength of the metamorphic basement rock in the Dahomeyan system is high but the overlaying soil of laterite and clay are easily eroded by the action of waves. Evolution of the coastal landforms has been influenced by climatic oscillations. Accra coastal geological investigations indicate that three beach levels are recognisable in several places (Muff and Efa 2006). According to Muff and Efa (2006), the mineralogical composition of the beaches indicates that their sediments are derived from existing geological formations, which is evident from observed characteristics of geological features identified in the three recognised beach levels. The observed lower roundness index of the older beaches in comparison with the younger beaches; having the dominance of grains with ferruginisation in cavities and the presence of old scars corroborate the view that the younger beaches had been derived from successive re-working of the older beach materials (Muff and Efa 2006). This indicates a fast and strong geomorphic process along the Accra coast. Fault zones identified in the Accra coastal area, which played a major role in the historic coastal evolution, are presently considered active. Studies by Amponsah (2004) has reported that Accra is a seismically active area. Most of the earthquake in Ghana occurs in the western part of Accra at the junction of the two major fault systems namely, the coastal boundary fault and Akwapim fault zone. This indicates that an earthquake disaster is imminent in the near future. This would damage the coast (CoZSSA 2005) and seriously alter the shoreline position, especially in the western portion where the faults are prominent. Erosion rates will be high and the earthquake 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

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-0.5 -1 -1.5 -2 -2.5 -3 0

Western 5

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15 20 25 Distance along shore (km)

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

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

Sediment supply from rivers in Accra is at their peak during the rainy seasons and most of the rivers are connected to the sea through lagoons. During the rainy season, the rivers carry sediments into the sea through the lagoon inlets from their upland catchments area and causes erosion of the banks. Lagoons with their inlets opened throughout the year facilitate a continuous supply of sediment into the sea while those with blocked inlets allow sediment into the sea usually only during the rainy seasons (Gordon et al. 1998). The seabed usually acts as a basin in which fine sediment is trapped and thus gradually raising the platform’s elevation, while the coarse sediment is transported in the west–east direction alongshore. Figure 5 shows example of offshore profiles in the three geomorphic portions. According to Bruun (1962), the volume of sand that is lost from the cliff top due to sea level rise gets deposited on top of the platform within the nearshore. Bruun (1962) formulated the notion of an equilibrium profile and its response to sea level rise in part to explain

Fig. 4 Flooding of wetlands after breaching of the barrier ridge in Accra

erosion rates. The Bruun rule asserted that the shoreface tends to maintain the same slope for a given depth, balancing the tendency of the wave motions to move sediment on shore and the tendency of gravity and water currents to move sediment off shore. The resulting shape of the shoreface is called the equilibrium profile. When sea level rises, the shoreface adjusts itself to re-establish the equilibrium slope at each depth. Bruun (1962) proposed that sediment is eroded from the upper shoreface to the lower shoreface to achieve this readjustment. Bruun (1962) found reasonable agreement between his model and observed shoreline retreat rates along the coast of Florida. Various studies (Hands 1983; Leatherman 2001; Zhang et al. 2004; and Storms et al. 2002) confirmed the validity of the Bruun model for predicting future shoreline rate of change. However, other studies (Dubois 1992; Lowenstein 1985; Pilkey et al. 1993; and Cooper and Pilkey 2004) criticized the Bruun rule. They questioned whether it is ever applicable in practice and referred to it as a “one model fits

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K. Appeaning Addo 2 eastern region central region western region

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

all” approach that makes it unsuitable for a highly complex sedimentary environment with significant alongshore sediment transport. The studies concluded that the concept time has passed and should hence be abandoned since it has no power for predicting shoreline behaviour under rising sea level. As waves break, the energy it generates destabilise sediments, which are then carried away in the littoral drift. According to Wellens-Mensah et al. (2002), the available total supply of sediment from the various sources to the surf zone of the Accra coast is about 200,000 m3/yr. However, the potential rate of sediment transport is approximately 5.7×105 m3/yr and the rate increases from the western portion towards the central portion (Appeaning Addo 2009). Although the volume supplied by each of the various sources has not been determined, the bulk of the supply is from erosion of cliff and mainland shores. This is because the largest river, Densu River, which could have supplied significant sediment for littoral transport, is dammed and the upstream is further channeled for an irrigation scheme. The dam traps the fluvial sediment (Akuffo 2003) and the reduced water volume has resulted in periodic blocking of the inlet. Variations in the volume of sediment supplied and transported have been identified as a significant cause of increased erosion in the Accra shoreline (Armah 1991; AESC 1980).

about 10 km in-shore, indicates that the average wind speed is about 4.5 m/s (Fig. 6). However, the wind speed measured onshore and reported by AESC (1980) is 2.5 m/s, which indicates that the wind velocity reduces as it approaches the shore. The offshore winds generate swell waves, which blow towards the coast of Accra and is influenced by the foreshore bathymetry. A local trade wind generates the local waves called ‘seas’, which are prominent along the West Africa coastline (AESC 1980). According to AESC (1988), typical period for the ‘seas’ is between 2 and 4 s and the significant wave height is less than 0.3 m. A rough sea is very infrequent 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 Accra have constant characteristics during the year and are generated by waves associated with storms far beyond the immediate influence of the coast (AESC 1988). Significant wave height (Hs) is about 1.4 m for 50% of the time and wave 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 wave climate data along Takoradi coast (refer to Fig. 1) by Svašek Hydraulics (2006) using a directional wave buoy from 1997 to 2006. Graphs of Hs and Tp are shown as Figs. 7 and 8 respectively for the 10 years of wave climate. The relatively steep slope of the sea floor, Fig. 9, enables swell waves to contact the headlands, which increase the wave energy. Continuous pounding of the cliff face by wave action leads to the formation of cracks, which widens from trapped air and eventually gets dislodged by the force of the swell waves. The dislodged rocks are broken down further in the surf zone through attrition.

Climate and oceanographic process The local wind climate is characterised by a southwest monsoon modified by land and sea breezes along the West Africa coast (Osei-wusuansa 2000). Computed wind speed, from 10 year period of wave climate developed in Takoradi by Svašek Hydraulics (2006) and measured from a location

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

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have resulted in widespread destruction of the Accra coastal vegetation and the loss of sediment being held by their roots. This has led to significant increased erosion in the Accra shoreline.

Drivers of landform change along Accra coast

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

Tides on the coast of Ghana are semi-diurnal in character. The average Neap and Spring tides for the Accra coast are 0.62 m and 1.26 m respectively, and they increase from west to east (Wellens-Mensah et al. 2002). The tidal range is approximately 1 m and thus the tidal current is weak most part of the year. This is influenced considerably by the prevailing relatively weak wind activities along the coast as a generating force and the effect of bottom friction. Two other types of currents can be distinguished along the Accra coast. The first is the longshore current that is driven by the breaking waves and reveals itself in the breaker zone. The wave breaker type is plunging, which breaks obliquely due to the orientation of the shoreline. The shoreline orientation is approximately 2530 (Appeaning Addo 2009). Since the sea floor slopes relatively steeper along portions of Accra shore (refer to Fig. 9) swell waves break closer to the shore at these points. The breaking energy also generates longshore currents, which move parallel to the shoreline and work in tandem with waves to move significantly large volumes of sand (Davis 2005). The longshore current generated in the Accra surf zone measures approximately 1 m/s with variations between 0.5 m/s and 1.5 m/s (AESC 1988). The second current is the counter equatorial current, which is referred to as the Guinea current. Its direction moves from west to east. This current is a specific ‘sea’ current off the coast of West Africa and shows a maximum from May to July during the strongest southwest monsoon winds, when it can reach about 0.5 m/s (AESC 1988). It is generally weaker and less persistent further offshore. Other locally generated currents and wind usually disturb the Guinea current and explain why it is generally weak for most of the year. Its influence in shaping the shoreline is therefore limited due to its weak intensity. Changes in weather patterns and human activities

Coastal erosion in Accra is exacerbated by various activities engaged in by the coastal dwellers. The zone is inhabited by 12% of the nation’s 20 million population and has a growth rate of about 3% per year as compared to the national growth rate of 2.5% (Amlalo 2005). The population density is 3,388 persons per km2 and urbanisation rate is 52% as compared to national urbanisation rate of 35% (Churcher 2006). The opportunity for employment continues to attract an average of about 25,000 migrants per year and according to Duedall and Maul (2005), this is expected to increase by 82% in 2025. Increased in coastal population has resulted in unsustainable use of the coastal resources. Wetlands have been drained for settlement, coastal vegetation (e.g. mangrove) has been destroyed through over cutting and lagoons reclaimed for settlement (Alvarado 2003). The situation has also made planning and decision making about infrastructure location difficult (Fig. 10). Development is close to the shore and thus created a coastal “land squeeze” situation. The World Bank (1995) identified inappropriate mitigating measures to this problem as a cause of high rate of erosion. Lack of employment for the increasing coastal population 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 the construction industry is high (Osterkamp and Morton 2005). 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)

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Fig. 9 Contour map of part of the Accra shores indicating relatively steep slope

construction of houses (Armah and Amlalo 1998) and according to Mensah (1997), its contribution to the construction industrial output increased from 17% in 1986 to 21% in 1993. The products are sold openly because of non-enforcement of the ban due to economic reasons. Sand mining activity is a cause of severe erosion along Accra shoreline, since the activity is not adequately controlled (CoZSSA 2005). Deposit beach sand, in addition to its aesthetics, maintains a balanced sediment budget, which ensures there is available sediment for littoral transport and also act as a buffer for the cliffs. Beach sand along the shores in the eastern and central portions of Accra was used for the construction of Tema Township, the industrial city of Ghana built from scratch in the late 1950’s, and other monumental structures such as the State House (EPA 2003). Removal of beach sand has reduced the available sediment budget for littoral transport and also reduced the beach

Fig. 10 Poorly planned settlement in the central portion (Jamestown) with development close to the shore leaving no land for the coastal processes

volume, thus enabling waves to attack the cliff face directly and erode the cliff materials. This is the plausible cause of severe erosion experienced along these portions of the coast in these three geomorphic portions. Coastal engineering structures constructed in the Accra coastal zone are management strategies adopted to stabilise or reclaim lost beach, protect and stabilise navigation entrance channels, and protect the coastal properties from wave-induced damage and flooding. All these structures essentially interact with active waves and currents, and result in sediment transport process in the vicinity of the structure. Jetties, constructed in 1906 (Dickson 1965) to protect and stabilise navigation entrance channels to the old Accra harbour trap sediment along the up-drift side and starve the down-drift side of sediment. This has changed the shoreline position; the up-drift side has a beach with sand while the down-drift side is experiencing severe

Changing morphology of Ghana’s Accra coast

erosion (Appeaning Addo 2009). The accreted sand on the up-drift side of the jetties has attracted sand mining activities, which initiated severe erosion by waves and current actions, thus degrading the coastal environment and destroying coastal infrastructure (AESC 1988). Groynes were constructed to check erosion behind the Regional Maritime Academy, while a combination of groynes and revetment were constructed to reclaim lost coastal lands along the Accra-Tema beach road. Although varying degrees of success have been achieved (Appeaning Addo et al. 2008), none has been able to stabilise sandy beaches that are being eroded (Ontowirjo and Istiyanto 2003; Davis 2005). This is because the interventions have created new problems on adjacent shorelines, while in some situations they have exacerbated the problem (AESC 1988; WellensMensah et al. 2002). This suggests that coastal defence structures are inappropriate for management of the shoreline in the Accra coastal zone. Dam construction over the Densu River, the largest river in Accra, has significantly reduced the volume of water and sediment that are discharged into the sea.

Conclusions Factors that cause coastal erosion in Accra, and thus change the coastal geomorphology, can be categorised as either natural or human induced. These driving factors are influenced by the coastal geology and hydrological climate. The western portion is eroding more due to the presence of consolidated and poorly consolidated rocks, while the remaining portions are experiencing moderate erosion. The natural factors that include wave action, current and sea level rise depicts the normal behaviour of the coastal system. The relatively steep Accra ocean platform enables swell waves to strike the cliff face and dislodge sediments from their original positions, especially in the eastern portion (refer to Fig. 4). However, the metamorphic basement rock slows down erosion while the overlaying laterite and clay facilitate erosion process. This has resulted in exposed rock outcrops dotted along the shore. Sand mining practice in the central portion weakens bonding of the layers and thus reduces the ability of the rock to withstand the eroding action of wave energy. Swell waves expel little energy during their propagation through the water and expend most of their energy at the shore. The energy released when the wave breaks generates about 1 m/s longshore current, which transports suspended sediment in the west-east direction. Although swell waves have uninterrupted approach to the continental shelf, very rough wave activities are not experienced along the coast. As one of the principal causes of wave-energy dissipation is bottom friction, it is conceivable that the broad width of the

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Accra coastal shelf confers considerable reduction in the energy of the waves arriving on the coast. This explains why strong wave action is not experienced along the coast. The significant difference in wind speed as they approach shore probably explains why storms and storm surge are not common along the Accra coast. The ‘sea’ produced by the local trade winds have short period and wave height and thus their influence on shoreline change is minimal. Weaker line squalls with heavy rains and strong winds of short duration occasionally occur. Sediment transport in the littoral zone is mainly by longshore current since the tidal current is low and hence considerably ineffective. The tidal currents are weak due to the prevailing small tidal range. Although this is generally true, for the long term erosion rate a small tidal range is very significant to shoreline morphology since they form a higher elevation beach (Rosen 1977). Relative sea level rise erodes the cliffs through under cutting. The increase in water depth, as a result of an annual sea level rise of 2 mm/yr, exposes sections of the upland areas to wave attack as the water depth increases and thus shifting the littoral system landward. Future changes in the shoreline position will be influenced significantly by the increasing sea level rise as a result of global climate change. The barrier ridge along the shore in the western portion will breach, which will result in flooding of land with 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 high hinterlands will prevent inundation further inland. Anthropogenic factors, which have induced coastal erosion in Accra, include sand mining and coastal construction works. Their actions result in sediment deficiencies in the littoral cell leading to coastal erosion. Activities of man, through construction of jetties and groynes, have resulted in coastal erosion along the adjoining shores through interception of sediment in the littoral transport. Revetments have also caused erosion in the adjoining shoreline through energising waves. Clearing of coastal vegetation for development has loosened sediment bonding and thus made them more vulnerable to wave and current attacks. Coastal sand mining has reduced the ability of the beach to protect the dunes and also created sediment deficit, which has resulted in coastal erosion. Dam construction over River Densu has reduced the volume of water and amount of sediment that is dislodged into the sea. This has resulted in creating sediment deficit budget, which has led to erosion. Activities of man, apart from initiating fresh problems of coastal erosion as in the case of sand mining, have also exacerbated erosion problems in several areas of the shore (Ibe 1996; Wellens-Mensah 1994). ACOPS (2003) therefore estimated that anthropogenic activities account for 70–90% of coastal erosion problems in Accra.

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