Assessment of Urban Flood hazard in Lagos Mega CityOlusegun Adeaga Floods of great magnitude that exceed the carrying capacity of rivers and other conveyance structures and available water system platforms are not uncommon in recent years. This has being attributed to climate change and variability and its effect on hydrological processes residence and turnover time and anthropogenic activities. Such activities include excessive modification of the catchment characteristics most especially in the emerging metropolises and mega cities, without adequate consideration for the entire hydrological environment. Lagos remains the industrial and commercial hub in Nigeria and West Africa sub-region, and is projected to be one of the world’s five largest cities by 2015. The city has expanded to nearby suburbs and state with massive developmental activities, which include encroachment of the urban facilities on floodplain and water ways, unprecedented land reclamation and impervious surfaces, without strict adhesiveness to effective land use and natural waterways planning. Hence, the combined effect of heavy rainfall and storm surges and ill-defined planning and her coastal location always result in worrisome flood incidence with severe human, environmental, socio- economic and psychological consequences. The need to resolve the severity of flood incidence within the Lagos Mega city and reduce populace vulnerability, call for a well defined decision planning and warning tool with a detailed preparation and planning network. In this study the landuse/lndcover pattern was derived from the satellite imageries of the region and runoff coefficient estimation of the different landuses was also carried-out to estimate probable peak discharge of the different landuse/landcover classes, using the rational formula while the extreme –value analysis of the partial annual rainfall analysis was carried-out using Weibul model and the distribution assumption is based on Gumbel type 1 extreme distribution. A dimensional Unit Hydrograph was also generated for different rain storm scenario based on the relations between geomorphometric attributes and hydrologic variables in the establishment of the transfer response function. This information was spatially integrated within the geographical information system (GIS) decision support system framework towards the provision of a detailed flood pre-disaster and lead time geo-information services within the city through appropriate estimation of the Hazards associated with flooding within Lagos city. The service to be provided through the system include information on flood risk analysis, flood extent, acceptable risk level modelling and mapping and potential damages based on recent, historical and simulated data. |
