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Environment and development in coastal regions and in small islands |
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Environment and development in coastal regions and in small islands |
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Study of Main Drainage Channels of Victoria and Ikoyi Islands in Lagos Nigeria and their Response to Tidal and Sea Level Changes |
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| Clogged drains- Obalende - Ikoyi |
Increasing concentrations of carbon dioxide and other gases are expected to warm the earth several degrees in the next century, which would raise sea level a few feet and alter precipitation patterns. Both of these changes would have major impacts on the operation of coastal drainage channels. However, because sea level rise and climate change resulting from the greenhouse effect are still uncertain, most planners and engineers are ignoring the potential implications. Rather than ignore the greenhouse effect until its consequences are firmly established, engineers and planners should evaluate whether it would be worthwhile to insure that new systems are not vulnerable to the risks of climate change and sea level rise.
Assessments of climate changes have generally focused more on sea level than precipitation. Existing climate models cannot yet forecast precipitation changes for specific regions. Because sea level depends primarily on global and latitudinal temperature increases, which the models can forecast, rough predictions of sea level rise have been possible. Available estimates generally imply a rise on the order of one meter in the next century.
Should the assumption of future sea level rise be incorporated into the designs of projects initiated today, and if so, how much of a rise? Although all major assessments of the greenhouse effect have concluded that sea level and precipitation will change significantly, most engineers and planners implicitly assume that these changes will not occur.
Engineers have ignored the consequences of the greenhouse effect for several reasons:
The design of a coastal drainage channels depends on the amount of runoff expected during a major storm and the elevation of the area being drained. Although the amount of rainfall and the severity of the worst storm vary from year to year, it has been reasonable to assume that historical weather records provide a reliable guide to future precipitation and runoff over the design life of the project. Recent developments in climatology, however, suggest that design conditions that have been fixed in the past may change substantially in the future.
Analysis of the leveling and heightening of the 8 major drainage channels revealed many problems. These problems include:
5.2.1. Drainage Channel Routes
In planning the routes of the storm drainage channels consideration should be given to coordinate the structures of other utility services ducts namely, water mains, electricity, telephones and street lighting ducts. Very often the utilities as above are laid in and along the drainage channels. It is however, recommended that the storm water drainage channels should be separated from the other utilities. This is for safety of these utilities against damage, due to storm water drainage during intense rainfall. Also, for health reasons, water supply conduits shall be kept separate from the storm drainage. Widening of some drainage channels could also reduce flooding. In some developed countries tidal gates are used to contain tidal waters, while excessive floodwaters are pumped out to sea. This involves high capital and maintenance cost and such actions impracticable for developing countries like Nigeria due to efficient and reliable electricity, equipment and maintenance.
5.2.2. Earthen Channels
Some drainage channels have earthen floors and broken walls and need urgent repairs. Such repairs should include:
a. Lining of all earth channels.
b. Adequate cover for rectangular concrete conduits.
c. Bar screens at selected inlets and in some culverts.
d. Fencing or right of way boundaries of trunk and outfall open channels.
Since the existing drainage lines are inadequate particularly those with flat or reverse slopes, it may be necessary to replace them. Where the existing system can be retained the modifications involve either providing cover or repair to sidewalls or surface protection or grading of inverts or any combination of foregoing work.
5.2.3. Flooding as a Result of Storm Surges
Many flooding mitigating measures have been taken on local level mainly by state agencies, local governments, private companies and even local people. Such measures include beach nourishment like in Victoria Island, Lagos. Probably the most widely used soft method to combat beach erosion is beach nourishment and increasing beach height above astronomical high tide levels. However, this method has not been very successful due to failure to reduce foreshore slope and the inability to raise the beach height above astronomical high tide levels. A good example of this, is featured along the Victoria beach in Lagos (Awosika et al 1992). Though nourishment of the Victoria beach had been implemented many times, it has failed to curb flooding because the nourished beach is always too low to contain the high waters associated with storm surges. The recently nourished Victoria beach is presently too low and should be raised to about 1-1.5m above the present level to abate any episodic event like the swells and surges, which normally occur between the months of April and August.
5.2.4. Blocked Drains
Blocked drains should be periodically cleared while adequate disposal points should be put in place to stop the dumping of waste and refuse in drain. Construction of new drains should have adequate drainage heads to accommodate projected rising sea levels.
The perennial flooding of Ikoyi and Victoria Islands call for action. The destabilisation of socio-economic activities as a result of the annual flooding puts a strain on the economy of the nation. With population explosion, climate change and concomitant sea level rise the hazard of flooding is expected to worsen. Action in this regard is needed.
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