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Meeting the challenges of flood risk assessment developing countries, with particular reference to flood risk management in Lagos, Nigeria.

Student thesis: Doctoral Thesis

  • Ugonna Chimnonyerem Nkwunonwo
Flooding poses significant threats to many urban areas globally. Realistically, flood risk assessment (FRA) which underlies solutions to these threats is often difficult to accomplish within the context of data poor developing countries (DPDCs), such as Nigeria, Mozambique and Bangladesh, and this highlight key issues, which are crucial for research. Despite the increasing vulnerabilities of people and urban assets, and the weak institutional capacity that prevails, previous research efforts relating to flood risk management (FRM) have failed to address this lingering issue of data paucity. Vulnerabilities of social systems in particular have not been assessed, although few discussions relating to vulnerability largely show that this concept is being considered, but within a limited scope and application. Although the level of awareness of urban flooding in the DPDCs is considerably poor, more scientific procedures, such as flood modelling, is lacking.

The present research is motivated by these issues, and therefore provides a possible workaround, using Lagos, Nigeria as a case in point. A critical review of flooding and current efforts to address flood risk in Lagos was undertaken. This is crucial to identify key objectives that will need to address present urban flooding challenges, and close the gaps in knowledge of FRM, between increasing urban flood risk and the means of protecting human lives and urban assets, to achieve a major sustainable urban development goal. Using the general view of vulnerability, proposed by Adger (2006) and IPCC (2007) in which vulnerability is defined on the basis of exposure, susceptibility and lack of coping capacity, social vulnerability to urban flooding in the area was evaluated. Indices of social vulnerability (SocVI) were constructed for the sixteen Local Government Areas (LGAs) that make up the Lagos city. A new flood model, GFSP-1 (Geoinformation Flood Simulation Program 1), was developed by combining two conceptual parts – Cellular Automata (CA) and Semi-Implicit Finite Difference Scheme (SIFDS).

The new model was tested and validated in Portsmouth, United Kingdom, using a severe flooding event that occurred on September 15th 2000. This event was chosen since map of hotspots of surface water flooding and social media-based information, especially photographic images of the event, were available to enable a rigorous validation of the new model. Simulation of various spatial and temporal scenarios for the July 11th 2011 flooding in Lagos was also carried out. In both of the test cases, the new model required only a 2-m horizontal resolution airborne LiDAR DEM, Manning's friction coefficient, and a rainfall intensity value to simulate urban flood hydrodynamics.

The results emerging from the research are as follows. Firstly, the SocVI construct indicates a high level of social vulnerability to urban flooding in Alimosho, Kosofe and Agege LGAs in Lagos metropolis. Secondly, in relation to the new flood model, the results show that GFSP-1 simulated flooding at locations similar to those depicted by the map of hotspots of surface water flooding in Portsmouth, and identified during the reconnaissance survey in Lagos. Simulated maximum flood water depths from ten sampled locations in Portsmouth and six in Lagos compared well with estimated maximum flood water depths. The Pearson correlation coefficient (r) between model predictions and estimated values is 0.986 for Portsmouth, and 0.968 for Lagos. This indicates optimal performance for the new model in terms of reconstructing the characteristics of urban flooding. Additionally, the plots of water depth vs. time which produce a smooth curve throughout the simulation, and the short time spent in the simulation show that the model's outputs are unconditionally stable, and inexpensive from a computational point of view. These are major issues of considerations in flood modelling research.

The challenges of flooding in the DPDCs will continue unabated unless significant improvements are made on current flood risk policy and management efforts. This will necessitate evolving new measures, by which the urgent needs to protect human lives and economic infrastructure in the DPDCs outweigh considerations for uncertainty and standardisation in FRM. These new measures will consider the critical understanding of the dynamics of urban flooding, and the factors that influence social vulnerability to the hazard in the DPDCs. While such understanding is underpinned by provision of data and mapping of urban flood hazard and risk, considering climate change scenarios, how to maximise the potentials within presently available datasets in the DPDCs should be explored as a major research opportunity. The present research explores this opportunity, and, through its objectives and findings, provides flood hazard underpinnings, as well as makes significant contributions to knowledge in the area of ameliorating the impacts of urban flooding in Lagos in particular, and data poor urban centers in general. It is fundamental to innovative FRM policy and practice within these areas, as well as existing strengthening existing flood risk adaptation efforts.
Original languageEnglish
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Award dateSep 2016

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