|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
|Scene of flooding at Ahmadu Bello Way (Beach Drive)|
As 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 in Lagos, Nigeria. The study includes examining the impacts of flooding and conducting public awareness campaign on the reduction of flooding in Lagos (Victoria and Ikoyi Island) Nigeria. The project was implemented by scientists at the Nigerian Institute for Oceanography and Marine Research, Lagos.
Victoria and Ikoyi Islands in Lagos State Nigeria constitute the two main barrier islands complexes (see map), which are heavily built up with residential, commercial and tourist facilities. 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. Ikoyi Island is separated from Victoria Island by the Five Cowrie Creek. (see map)
The scope of this survey included the following:
Historical meteorological data were collected from the archives of the Nigerian Meteorological Department, the Nigerian Institute for Oceanography and Marine Research (NIOMR) and Nigerian Ports PLC.
Levelling along Channel 4
8 Drainage channels (3 at Ikoyi and 5 in Victoria Island) were surveyed by levelling from the head of the channels to the outfall using a surveyor's level. A zero datum was chosen at the head of each channel. This zero height was then used to level the channel from the head to the toe or outfall. In areas where water flow was observed, the velocity of the flow was recorded. The flow velocity was calculated by timing the flow rate within a 3- 5m length of channel. In areas where sediment or refuse was observed to accumulate in the bottom of the channel, the thickness of such sediment or refuse was measured.
A questionnaire was administered to local residents to collect information about flooding, refuse disposal and drainage channel patterns from local residents along flood prone areas. The answers to the questionnaire were statistically analysed and use to decipher resident's opinion on the problem of flooding.
Bathymetric survey starting from the outfall of all the levelled channels to the opposite bank in the Lagos Lagoon and the Five Cowrie creek were conducted using a Raytheon echo sounder.
A Media and Public forum was held on June 6th to orally present the draft report to government, media and stakeholders.
Results of the data collected show that flooding of the Victoria and Ikoyi Islands is predominantly caused by excessive rains, which are very prevalent during the months of June to July when rainfall usually exceeds 100mm per day. The second maxima of rainfall are experienced during the months of September to October. Analysis of historical tidal data show that tides are predominantly semi-diurnal with two inequalities with tidal ranges of 1 to 1.5m above the zero of the tide gauge. Mean sea level between 1992 and 1996 range from 1.7m to 1.9m above the zero of the tide gauge. However, the months of October to September are characterised by mean sea level of about 2m above the zero of the tide gauge. Storm surges are experienced during the months of April to May and August to September. During these storm surges, high waters exceed 4m above low low water resulting in flooding.
Analysis of drainage channel leveling data revealed that many channels are usually blocked by refuse and sediment, while some are obstructed by buildings. Some channels also have reverse gradients, which could cause backflow of storm waters. Many of these drainage channels also lack enough drainage heads to make them efficient to drain off storm waters. Also, when heavy rains or storm surges coincide with high tides, tidal waters flow back into the channels through the outlets causing excessive flooding.
Bathymetric survey of the relevant parts of the Lagoon off the outfalls of the surveyed channels showed that most channel outfalls are less than 1m from the mean sea level. At astronomical high tides therefore, tidal waters usually backup far into the drainage channels causing flooding especially when excessive rainfall occurs.
Analysis of answers to questionnaires received from residents on the flooding issue show that most residents are not well prepared to tackle the problem. Drainage channels are not efficient enough to drain off storm waters around their location. Also, refuse containers are inadequate with the result that residents discharge refuse into the drainage channels.
About 200 participants attended the media and public forum, which was held at the conference room of the Nigerian Institute for Oceanography and marine Research (NIOMR), Victoria Island Lagos on 6th June 2000.
From the Media Seminar:
There was a general consensus on the inefficiency of the present drainage system resulting in flooding with concomitant adverse effects on the socio economic activities of the State.
The public authorities represented by some of the high level officers, namely, The Lagos State Commissioner of Environment and Physical Planning and the Commissioner of Public Works agreed that immediate actions are needed to be taken to address the situation.
The media representatives accepted that they have a key and active role to play in developing public awareness to address the problem.
Based on the results of this study it is recommended that:
Short-term actions consisting of periodic clearing of drainage channels, widening of some channels, are required .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 activities in Lagos, which ultimately affects national socio-economic activities of the nation.