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Wise Coastal Practices for Sustainable Human Development Forum

Seeking solutions to flooding / Lagos-Nigeria

Posted By: Larry Awosika and Regina Folorunsho
Date: Wednesday, 21 June 2000, at 4:49 p.m.

In Response To: Questioning the nature of a 'wise practice' (R. Sudarshana)


A recent study has shown that flooding of the Victoria and Ikoyi Islands in Lagos, Nigeria, is predominantly caused by excessive rains which are very prevalent during the months of June to July when rainfall usually exceeds 100 mm per day. A second rainfall maxima is experienced during the months of September to October.

The situation is complicated by the fact that mean sea level is higher during the months of September and October (based on historical analysis of tidal data from 1992-1996). Furthermore, storm surges are experienced during the months of April to May and August to September, when surge heights may exceed 4 m above low low water levels, also resulting in flooding.

The project, conducted by the Nigerian Institute for Oceanography and Marine Research Lagos, examined the efficiency of the major drainage channels by:

- surveying 8 main drainage channels to estimate their capacity to discharge storm waters to the lagoon, - analysing historical tide gauge data to determine sea level rise variations, - conducting a bathymetric survey off the drainage channel outfalls, - collecting and analysing meteorological and storm surge data, - determining the efficiency of the present channels.

The project also provided recommendations to government, and organized a public enlightenment campaign through the media and public forum.

Analysis of the drainage channel leveling data revealed that many channels are blocked by refuse and sediment, or are obstructed by buildings. Some channels also have reverse gradients, which could cause back-flow of storm waters. Many of these drainage channels also lack enough drainage head (gradient) to drain off storm waters. When heavy rains coincide with high tides, tidal waters flow back into the channels through the outlets causing excessive flooding.

Residents were surveyed by questionnaire as to their opinion on the flooding issues. Analysis of their answers showed that most residents are not well prepared to tackle the problem. The drainage channels are not efficient enough to drain off storm waters around their houses. Also there are insufficient refuse containers, with the result that refuse is dumped/discharged into the drainage channels by residents.

Based on the results of this study it is recommended to:

- conduct a complete and comprehensive survey of all drainage channels in the two islands, - re-route some drainage channels to increase their efficiency, - repair some channels and replace some earthen channels with concrete, - install bar screens along some drainage channels and culverts, - provide fencing or right of way boundaries to some channels, - increase beach height above astronomical high tides, - construct new channels and rebuilding of old ones with enough drainage heads, - undertake a sustained public enlightenment campaign to discourage residents from dumping their refuse into drainage channels.

During a media and public forum held on 6th June 2000 in Lagos, the results of the study were presented to government, media, local residents and other stakeholders. The forum raised the awareness of government, residents and stakeholders on the severity of the problem and initiated response measures.

Mr Larry Awosika and Ms. Regina Folorunsho Nigerian Institute for Oceanography and Marine Research, Lagos.

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