The floods that bedevilled most river basins in Nigeria this year were predicted by NIMET and have been particularly devastating. No wonder, discussions about water in the media have been dominated by news of flood disasters, but mostly concentrated on issues of politics of floods. At the end of the rainy season, we will have respite, but the trend in occurrence of flood disasters shows no signs of abetting in years to come. Freshwater resources is source of life but only if it is managed well. As Kofi Annan puts it: "The task is not just to preserve water resources to sustain life, but also to reduce the capacity of water to take life away. ....we can and must reduce the number and impact of disasters by building sustainable communities that have the long-term capacity to live with risk."
The nation's response to the flood has been anything but articulate and comprehensive. All we have done is to react after the disastrous events to provide relief to the unfortunate victims, and then we wait for another deluge.
We must however understand that routine smaller floods that moderately inundate floodplains commonly called the 'fadama' are desirable because they play a vital role in replenishing freshwater resources, recharging wetlands and groundwater, support agriculture and fishery as well as maintain ecosystems and floodplain biodiversity in the river corridors that are essential to the well-being of many communities. It is the destructive floods which are occasional but more recently, very devastating, that are worrisome. It is not unreasonable to project that an increasingly populated floodplains coupled with the climate change would further escalate the toll in lives and livelihoods of these floods in future, unless we comprehensively managed them.
Floods are as a result of flow of water exceeding the capacity of the river channel, particularly at bends or meanders, or the unexpected drainage obstructions, or from combination of tidal sea surges in coastal areas or water in the lake or reservoir overflowing or breaking of levees or catastrophic event such as dam breakage. However, the most common flooding in Nigeria are caused by intense rain, particularly at the peak of rains, especially where rain has previously fallen and the ground are saturated; such that the additional rain runs off over the surface and accumulates in streams and rivers faster than their drainage capacity. Most often, such intense rainfalls are accompanied by strong winds. Other causes might be due to outbursts of natural water retention dams created occasionally by floating debris accumulating at a natural or man-made obstruction such as bridges thus restricting the flow of water and causing flooding upstream. Contrary to the widely held believe, only very rarely have the flash floods in Nigeria been caused by failure of dams, or other hydraulic infrastructure.
Other factors that decisively influence the occurrence of flash floods - apart from the intensity and duration of the rainfall - are the topography, soil conditions, and coverage of the terrain. Steeply sloping high terrains, narrow valleys or ravines hasten the runoff and increase the likelihood of flood occurrence. Saturated soil or shallow watertight geological formation would significantly increase surface runoff. Urbanization processes, especially those associated with construction with watertight materials such as concrete and asphalts would result in the runoff becoming 2 to 6 fold greater in comparison to terrains with natural coverage (fields, meadows, forests).
The projected effects of global warming are that more heavy and intense rainfall in overall fewer numbers of events would be experienced in future, implying greater incidence of extreme floods and droughts. Furthermore, global sea levels are rising with attendant inundation of the coastal lowland; altered tidal range in rivers and bays; more severe storm surge flooding; and increased saltwater intrusion into estuaries and freshwater aquifers. It also poses a major conceptual challenge to the long held assumption that the long-term historical hydrological conditions will continue into the future. Our selection of appropriate design flood should therefore seek to balance risks and benefits.
Other major risk factors are; low flow capacity of most of our river beds and bottlenecks arising from heavy vegetation, consolidated debris and sedimentation in the river bed, poorly developed river channel, and frequent meandering; low drainage capacity of the clayey soil found in some basins; low institutional capacity and lack of integrity in human capacity decisions; insufficient and ill-advised infrastructure for flood defences characterised by difficulty in securing investment funds for middle and small-sized streams flood embankment; insufficient flood control facilities in urban areas; poorly designed and executed engineering works deterioration of stream functions; lack of social systems, cooperation between sectors and poor public participation.
The contribution to flood risks of some of the factors enumerated are influenced by human intervention; thus flood is one of the most difficult natural hazards to predict accurately, but we can indicate areas that are more susceptible to flooding. So how do we mitigate floods?
Although, the most logical means of virtually eliminating the hazardous floods may be by moving away settlements from near rivers and other bodies of water, yet we also know that since early civilization, mankind has lived and worked by the rivers to seek sustenance and capitalize on the gains of most affordable means of transport and commerce. In an agrarian economy like ours, food security is synonymous with securing livelihood and floodplains popularly called fadama provide in many cases excellent, technically easy means for food production and livelihood opportunities.
Accordingly, flood intervention measures should refrain from tampering with the smaller flooding because it would not only damage the ecosystems but would affect livelihoods that may have developed around the wetland. Furthermore, increased competition for access to limited land resources in the densely populated regions of Nigeria is jeopardizing the weaker sections of the population who largely resort to living in the floodplains. This has increased their vulnerability to flooding. These urban ghettos also suffer from a lack of health and sanitation facilities and are thus most vulnerable to disasters and post-disaster consequences.