A.C. Ibe and L.F. Awosika
Nigerian Institute for Oceanography and Marine Research, Lagos Nigeria
The African coastal zone, most of which is very low-lying, consists of the West, Central, East and Mediterranean coastal zones. Within these coastal zones are many cities: Dakar, Abidjan, Accra, Lagos, Dar es Salaam, Alexandria, Tripoli and Tunis. These coastal cities are characterized by teeming populations, industries, dense transportation and communication networks as well as extensive coast-based tourist resorts. At present, widespread erosion and flooding are devastating vast areas along the African coastline, causing severe ecological problems as well as creating a high level of misery for the people. A rise in sea level of say one metre, which in many places may be accentuated by the phenomenon of subsidence, would aggravate the already existing ecological problems through increased rates of coastal erosion, more persistent flooding, loss of wetlands, increased salinization of groundwater and soil as well as greater influx of diverse pollutants.
Other socio-economic impacts include uprooting human settlements, dislodging port and navigational facilities, upsetting coastal fishery as well as coast-based tourism. These adverse effects would impose unbearable pressure on the already hard-pressed African economy. This then calls for the establishment of coastal management policies including a phased disengagement from the coast, where practicable, and enforcement of set back lines. In already built-up areas, the use of low-cost, low-technology erosion and flood defense measures are advocated.
The African coastal zone is bounded in the west by the Atlantic Ocean, in the east by the Indian Ocean and in the north by the Mediterranean Sea. The coastal zone could be divided therefore into the West and Central African coastal zone, the East African coastal zone and the Mediterranean coastal zone. The West and Central African coastal zone stretches from Mauritania to Namibia and constitutes 29.5% of the whole area of the African continent. This coastal zone consists of four major basins which are bordered on the ocean side by low-lying coastlines which are sandy and muddy in some cases. General beach elevations range from 2-3 m above sea level.
Within the West and Central African region (WACAF) are the state capitals of Dakar, Banjul, Conakry, Abidjan, Lome, Accra, Lagos and Brazzaville, many of which are port cities. Industrial growth within these WACAF cities has intensified within the past five decades; for example, Lagos has more than 80% of the total industries in Nigeria. Most of the population within the WACAF region is concentrated within these major coastal cities.
The Eastern African coastal zone can be delimited by latitudes 18 deg. N to 27 deg. S and include coastal areas of the island states Madagascar, Mauritius, Reunion and Seychelles. A large part of the East African coastal plain which is low-lying is very variable in width. The coast is relatively unindented because of the absence of large rivers. The island states are generally volcanic in origin and are characterized by very narrow coastal plains. These coastal plains are almost absent in some areas, for example in Seychelles. The extensive coastal plains in Madagascar are associated with major rivers and are the result of deposition of soil from the upland plateau.
The Eastern African coastal zone is very heavily populated today because of its growing industrial infrastructure. It is estimated that 13% of the 62 million people in East Africa reside along the coast due to rapid development of coastal activities such as fishing, sea ports for imports and exports, coastal tourism and industries. The Mediterranean coastal zone though narrow, is of high economic value to the area. Many big cities are situated along the coast: Alexandria, Tripoli, Banghazi, Tunis, Algiers and Cairo.
Vital agricultural (including fishery resources), industrial, commercial and communication infrastructure as well as recreational beaches make the narrow Mediterranean coastal zone very important. This has resulted in high population density growth around the coastal cities. The Nile delta is very important for agriculture and supports a lot of other economic activities such as fishing.
The increasing emission of CO2 and other greenhouse gases such as chlorofluorocarbon, methane, nitrous oxide, tropospheric ozone etc have resulted in a general warming of the earth's surface temperature by 0.5deg.C in the past 100 years. These greenhouse gases are relatively transparent to sunshine but have the ability to trap some of the sun's returning radiation from the earth. Direct evidence linking greenhouse gases with the dramatic climatic changes comes from a study of bubbles of air trapped in the Antarctic ice sheet.
The warming of the earth will result in the melting of Alpine ice and thermal expansion of sea water which will ultimately result in a sea level rise. According to present knowledge, it is unlikely that the West Atlantic Ice Sheet will contribute to rising sea level due directly to greenhouse warming.
The UNEP/ICSU/WMO International conference in Villach, 9-25 October 1985 accepted increased temperatures of 1.5deg. to 4.5deg. C and sea level rise of 20-140 cm before the end of the 21st century. However, according to the March 1990 figures from Inter-governmental Panel on Climatic Change (IPCC) Working Group I, global mean temperatures will have risen 1.8deg. C and 3.5deg. C (best estimates) by 2020 and 2070 respectively.
The Workshop of the IPCC Working Group 3 on "Adaptive Options and Policy Implications of Sea Level Rise and Other Impacts of Global Climatic change" (Nov. 27-1st December, 1989) concluded that a world-wide rise in sea level would inundate wetlands and low lands; erode shorelines; exacerbate coastal flooding; increase the salinity of estuaries and aquifers and otherwise impair water quality; alter tidal ranges in rivers and bays; change the locations where rivers deposit sediments; change the heights, frequencies and other characteristics of waves; decrease the amount of light reaching the sea floor. It was also noted that local subsidence would exacerbate all the above impacts. The richly endowed coastal zone of Africa would be adversely affected by this eustatic rise due generally to the low-lying nature of the coastal zone and rapid rates of subsidence.
Wetlands in Africa consist of brackish mangrove swamps, forest swamps and marshes. The mangroves are very important ecologically because they provide spawning and nursery grounds for many coastal fish species. They also serve as the habitats for some of the crustaceans and molluscs. Increased salinity of mangroves and fresh water swamps will lead to the decimation of the mangroves not resistant to the high saline environment. The forest resource along the Eastern African coastal zone is mainly mangrove forest and constitutes up to 5,100 sq.km. Agriculture is practised along most of the coastal zone. Sea level rise will result in loss of these agriculturally rich wetlands.
The fresh water swamps which support immense agricultural practice for example, the Nile delta, could be polluted with saline water and soils and result in diminished food production for the already near to starving population in the area.
Beach erosion is one of the gravest ecological problems affecting the West and Central African region, parts of the low-lying East African coastline Including the island states and even the Mediterranean coastline. Analyses of historical hydrographic charts and aerial photographs as well as ongoing research by the Nigerian Institute for Oceanography and Marine Research (started in 1981) reveal widespread erosion along the Nigerian national coastline. Present typical rates established at erosion monitoring stations include more than 25-30 m annually at Bar beach in Lagos.
The beaches off Abidjan and Grand Bassam in Cote d'Ivoire have been reported to be receding as a result of beach erosion. Along the eastern port of Lome (Togo) harbour, an annual erosion rate of 20 m has been recorded.
The East African coastal zone is presently experiencing some coastal degradation due to erosion along some sandy and low-lying beaches. In Dar es Salaam, accelerated marine erosion and flooding in the last decade have uprooted settlements and resulted in the abandonment of luxury beach hotels.
Coral reefs in East Africa, especially near Malindi, Kenya, are being damaged due to excessive siltation caused by coastal erosion.
Although the values of sea level rise amount to only a few millimeters per year and may seem small, this phenomenon plays a big role in explaining erosion processes affecting most of the low-lying coastline in the world. The seriousness of accelerated sea level rise with respect to increased erosion and flooding can be deduced from the data of Brunn which showed that even a 0.3 m (ft) rise in sea level may cause a shoreline recession of more than 33 m (100ft). This recession rate may even be exceeded by more than hundreds or even thousands of times the vertical distance in such low-lying coastlines along the African coastal zones including the coasts of the island states.
The phenomenon of subsidence (relative sea level rise) is expected to exacerbate the adverse effects of sea level rise and aggravate the existing ecological problems associated with coastal erosion resulting in loss of wetlands and disorganizing the socio-economic infrastructure along the coastline.
Flooding of the low-lying beaches along the coasts of Africa and in particular the Eastern, West and Central African coasts has become an environmental headache in recent times. This phenomenon has become even more pronounced whenever storms coincided with spring tides.
The East African coast has about 5,000 sq. km of mangrove forest which help to trap sediments and prevent storm surges from eroding and flooding the hinterland. Sea level rise will result in the flooding of this important ecosystem as well as the rich agricultural fields e.g. rice paddies on the coastal plains in Gambia and Guinea.
With accelerated rise in sea level, the potentials for flooding and erosion of certain key transportation activities on barrier islands and others near the coast will increase. This will lead to a degeneration or interruption of social and economic services. With high sea levels sea ports, existing fishing facilities like jetties and fish storage centers built on the coastal fringes only a couple of feet above the mean high tide line will be subjected to more frequent tidal and storm inundation. Coastal tourism which is fast-growing' especially along the WACAF and East African coast as well as the productivity of agricultural lands on the flood plains would be adversely affected.
Salinity stress consequent upon the more persistent ingress of sea water may lead to the disruption of coastal fishery causing a disorganization of the faunal assemblages thereby resulting in the redistribution of species and failures in the reproduction and survival of their eggs/spores and larval/sporophytes.
Most of the water used for domestic and industrial activities within the African coastal zone is derived from groundwater sources. This is because of the absence of any large surface water supply.
Growing coastal activities have attracted large populations to the coast and this has exerted big strains on coastal groundwater resources. For example, Dar es Salaam is heavily populated with over 85% of the industries situated in and around the city. The depth to water-table in the coastal zone is often very shallow and is subject to saline sea water contamination and pollution. An increased global sea level rise is expected to raise the water-table along the coast and result in increased salinity of the groundwater. Many of the island states are already experiencing this phenomenon and the situation is expected to worsen with sea level rise.
The African coastal zone is richly blessed with vast areas of productive mangrove ecosystem. The Niger delta alone has a flourishing mangrove area of over 7,500 sq. km much of which is being decimated by erosion, flooding and increasing salinization of ground water and soil.
The Nile delta is of international importance from intensified land-use due to continued growth of population and the consequent need for food production. The already existing problem of deforestation is expected to worsen with sea level rise. Such plants that are not tolerant to this increased salinity will die. In Tanzania, for example, the mangroves near the mouths of large rivers such as Rufiji and other smaller rivers will be adversely affected. The savannah woodlands along Mozambique and Madagascar coastlines as well as the dry bushland of Acacia species along the Somalia coastline would be decimated as a result of sea level rise.
Transportation and communication along the African coastal zone consists of extensive networks of roads, railways, airports, seaports and canals.
The growth in coastal transportation and communication networks is due to the population and industrial growth within the coastal cities. Increasing sea level will result in flooding of these transportation and communication networks. The many port cities will be flooded and rendered unusable. Many airports situated on the coastal plains would be flooded, rendering them hazardous for air transportation.
It is suggested that sea level rise will cause a continued deepening of the continental shelf beyond the depth of closure and result in increase of effective wave heights due to the reduction on bottom friction as a result of greater depth.
It is thus expected that the present ocean dynamics (wave height, period, length, breaker angle, long-shore current direction and magnitude and tide levels) shaping the shore line will change. A change of ocean dynamics, particularly the near-shore dynamics, will adversely affect sedimentary fluxes and hence sedimentary budget. This budget controls the coastline evolution, through erosion, accretion or stability. For example, it has been suggested that sea level rise will result in inundation of the flood plains of the Rufiji delta; this will result in increased tidal currents in the estuary.
The net effect of all the above adverse effects of sea level rise on the African coastal zone is to cause a high level of frustration and misery because people and businesses would need to relocate and new infrastructure would need to be provided. The economy of the African countries can hardly absorb the high expenditures necessary to finance these expensive projects. Presently, these countries are still struggling to provide food and shelter for their teeming populations as well as cope with huge external debts.
Given the near certainty of an accelerated rise in sea level, the only hope left for Africa to save its cities and coastal infrastructure from total collapse is to embark on the implementation of low-cost, low-technology/technical measures as well as regulatory policy measures. Such policy options should . include a phased disengagement from the coast where practicable, prevention of new development in the coastal zone and the adoption and enforcement of the concept by set back lines.
In already built-up areas of cities or where the disengagement of coastal infrastructure is impossible due to the economics of such disengagement, the use of low-cost, low-technology erosion and flood defence measures are advocated. Methods include the raising of the height of the beach above the reach of the highest astronomical tides. The use of locally available raw materials like chicoco blocks, ripraps, and timber to construct coastal erosion and flooding combating structures is advocated.
1. Schneider, S.H., 1989. The changing climate. Scientific American, September, pp.
2. UNEP/WMO/ICSU, 1985. International assessment of the role of CO2 and other greenhouse gases in climate variations and associated impacts. Conference Statement, UNEP, Geneva, Switzerland.
3. Ibe, A.C., L.F. Awosika and E.E. Antia, 1984. Coastal Erosion Research Programme. Progress Report No.2, NIOMR Special Publication.
4. Brunn, P., 1962. Sea level rise as a cause of shoreline erosion. Journal of Waterways and Harbours, 1, 116-180.