|Environment and development
in coastal regions and in small islands
Study of Main Drainage Channels of Victoria and Ikoyi Islands in Lagos Nigeria and their Response to Tidal and Sea Level Changes
Within the overall objectives of the UNESCO Project on Environment and Development in Coastal Areas and Small Islands and specifically its component related to sustainable development of continental coastal regions, and as a contribution to the Disasters Reduction Programme, the Coast and Small Islands (CSI) Division of the United Nations Educational, Scientific and Cultural Organisation (UNESCO) funded a project to study the drainage channels in Victoria and Ikoyi Islands, Lagos, Nigeria. The study also includes examining the impacts of flooding and conduct public awareness campaign on the reduction of flooding in Lagos (Victoria and Ikoyi Islands), Nigeria.
Victoria and Ikoyi Islands in Lagos State, Nigeria, constitute the two main barrier islands (Figure 1.1) complexes made up of both residential, commercial and tourist facilities. The Victoria beach in Lagos, is the most popular tourist beach in Nigeria. The beach is made up of a low-lying sandy beach. Ikoyi Island on the other hand is surrounded wholly by the Lagos Lagoon and the Five Cowrie creek (Figure 1.2). Ikoyi Island is mainly composed of residential buildings and some commercial buildings. Victoria Island is located immediately east of the eastern mole on the downdrift side of the natural inlet into the Lagos harbour. The Islands lies specifically north of latitude 6°25'N and longitudes 3°24'30"E to 3°26'30"E.
Several authors have described the morphology of Victoria beach. Notable ones amongst others include Ibe, Awosika et al (1986), Ibe (1987), Ibe and Awosika (1993), Awosika et al (1993). Victoria Island beach is the eastern part of a system of barrier-lagoon complex, which stretches from Cote d' Ivoire to the Niger delta in Nigeria. The foreshore of Victoria Island is a low coastal plain with beach ridges farther inland.
Behind the beach ridges is the Lagos lagoon, the largest coastal lagoon in western Africa. The barrier system is fronted by a very narrow beach with foreshore gradient of about 1:50. Beach crest elevation is between 3-4m above mean low-low water. The beach is backed by narrow and sandy beach ridges, which are, aligned parallel with the modern coast. Coastal vegetation along the barrier system is very sparse with scattered mangroves and swamps. Coconut trees especially along Badagry and Lekki beaches dominate the coast. According to RPI (1986), coastal vegetation consists of the sedges Cyperus sp; the herb Euphorbia hypossopifolia; the succulent herb Dioda vaginalis and the trees Parinari robusta.
Behind the Barrier bar is the Lagos lagoon. Lagos Lagoon is one of the largest lagoons along the Gulf of Guinea covering an area of about 350square kilometres. Ogun, Oshun, Ona, Shasha and other ephemeral smaller rivers feed this coastal lagoon with fresh water.
Victoria beach has been the most popular beach and most widely used by tourists since the colonial years of 1900 to 1960. It has also been the site of the fastest erosion rates in Nigeria and one of the fastest eroding beaches in the world. The rapid erosion rates, which characterise this beach, were sparked off with the construction between 1908 and 1912 of two stone moles (east and west moles). The moles were built to protect the dredged deep entrance into the Lagos harbour from intense wave action and silting of the entrance by the west east drifted sand.
|Channel Outfall to Five Cowrie Creek|
Many huge drainage channels built about three decades ago, serve as receptacles for surface runoff and smaller channels which discharge into the Lagos lagoon or the Kuramo waters. Recent observations have shown that many of these drainage channels lack enough drainage heads to discharge runoffs into the lagoon. This is because these drainage channels were built without proper information on tidal parameters and sea level rise. Rising sea level, increased rainstorm and ocean storm surges associated with astronomical high tides are now making these drainage channels ineffective. This often results in the backup of runoffs in the drainage channels and eventual flooding of the Islands. Such occurrences are most apparent and disastrous during the rainy seasons or when storm surges coincide with high tides resulting in astronomical high tides.
The dumping of domesticated refuse directly into the drainage channels also exacerbates the problem of seasonal flooding of the islands. This human activity results in the blockage of the channels resulting in eventual flooding of the residential and commercial areas of the islands.
Considering the above, the main objective of this study is to examine the drainage channels and the natural and anthropogenic forces responsible for flooding arising from tidal oscillations, sea level rise, excessive rainfall and storm surges.
The scope of this survey include the following:
Flooding could be linked to both natural and anthropogenic activities. Some of the natural causes include: low lying coastal topography, intense wave and tidal climate, vulnerable soil characteristics and storm surge. Sea level rise as well as localised subsidence also exacerbates the rates of coastal erosion and subsequently flooding.
1.5.1. Natural Causes
(a) Low lying topography
Many of the sandy beaches as well as the muddy beaches along the entire coastline are very low-lying with nowhere greater than 3m above mean low water. Some areas along the Lekki barrier bar complex are almost close to sea level. The low-lying topography makes the coastline highly susceptible to flooding by the waves especially during astronomical high tides. The effect of this is to subject the coastline to heavy pounding by the waves.
Tides describe the periodic and regular variations in sea level, which have a coherent amplitude and phase relationship to some periodic gravitational forces. The moon and to a lesser extent, the sun create ocean tides by gravitational forces. The changing position of the Sun and Moon causes very small changes in gravity that produces ocean tides. At the Earth's surface, the change in gravity due to the Moon is about twice as great as that of the Sun. Sea level tends to be highest on the parts of the Earth's surface directly toward and directly opposite the Moon, creating tidal bulges. As the Earth rotates, the tidal bulges move across the oceans causing two daily high and two daily low tides (semidiurnal) or one high and one low tide (diurnal) or mixed tide. The time when daily high and low tides occur changes as the Moon orbits around the Earth. During any month, the biggest range between daily high and daily low tide happens at new moon and full moon. Astronomical high tides could cause flooding especially when they coincide with storms and heavy rainfall. Such astronomical high tides also intensify wave regimes.
1.5.2. Anthropogenic Causes
(a) Urbanisation and poor drainage system.
Rapid population growth as a result of urbanisation and accompanying poor landuse may lead to blockage of drainage channels. Alterations of the channel (channelisation, dikes, dams, and bridges) change the overall conveyance system of a catchment area. These activities have a noticeable effect on flow volume, peak magnitude, and timing of the peak. Increased population also lead to increased generation of solid waste. Ineffective waste disposal facilities cause residents to dispose their waste in the channels, with the believe that the channels will transport the refuse to the ocean or lagoon. Blocked drainage channels hence result in flooding of adjacent areas.
The earth's climate is predicted to change because human activities are altering the chemical composition of the atmosphere through the buildup of greenhouse gases-primarily carbon dioxide, methane, CFCS and nitrous oxide. Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Since the beginning of the industrial revolution, atmospheric concentrations of carbon dioxide have increased by nearly 30%, methane concentrations have more than doubled, and nitrous oxide concentrations have risen by about 15%. These increases have enhanced the heat-trapping capability of the earth's atmosphere.
With respect to the past, recent analyses suggest that: global mean sea level has risen 10-25 cm over the last 100 years. This range is slightly higher than that reported in IPCC (1992) (i.e., 10-20cm). The various models, from simple upwelling diffusion models to complex coupled atmosphere-ocean GCMs, all agree that oceanic thermal expansion is one consequence of global warming. The thermal expansion over the last 100 years is estimated to be 2-7 cm. Large-scale observations of changes in sub-surface ocean temperatures are beginning to support these estimates.
Observational data indicates that, globally, there has been a general retreat of glaciers during this century. Global warming should on average, increase the melting rates of glaciers and ice caps, causing sea level to rise. Based on both observations and models, recent analyses suggest that this enhanced melting may have increased sea level by about 2-5cm over the last 100years. The changes in future sea level will not occur uniformly around the globe. Recent coupled atmosphere-ocean model experiments suggest that the regional responses could differ significantly, due to regional differences in heating and circulation changes. In addition, geological and geophysical processes cause vertical land movements and thus affect relative sea levels on local and regional scales. Finally, extreme sea level events - tides, waves and storm surge - could be affected by regional climate changes, which are, at present, difficult to predict.
Global sea level rise of about 80 to 100cm has been said to have occurred since the beginning of the Holocene. The predicted sea level rise will do nothing less than aggravate the already high rates of erosion and inundation especially along low-lying coast. Though the values of sea level rise may seem small, inundation effects of sea level rise by Brunn (1977) show that a sea level rise of 0.3m (1ft) could cause an inundation of more than 35m (100ft).