[01] Kolade Victor Otokiti, Department of Urban and Regional Planning, Federal University of Technology, Akure, Nigeria. [02] Opemipo Akinola, Institute of Ecology and Environmental Studies, Obafemi Awolowo University, Ile-Ife, Nigeria. [03] Kayode Nelson Adeniji, Department of Urban and Regional Planning, Osun State University, Osogbo, Nigeria.

Flood risk in coastal megacities of developing nations is higher than those in developed nations due to inadequate fiscal resources, urbanization, and poor flood maintenance infrastructure, amongst other factors. Lagos is one of the coastal megacities of the developing nations characterized by increasing urbanization and population growth. This study presents geospatial mapping of flood risk areas in the coastal megacity of Nigeria. The flood prone areas were derived by overlaying seven flood factors in ArcGIS environment, which include: elevation, curvature, slope, flow accumulation, normalized difference water index (NDWI), land cover and drainage density. Results showed that of the 2507.2 km² land area covered by this study area, 0.006% (0.15km²) falls in very high risk, 30.9% (774.7 km²) falls in high risk, 68.8% (1725km²) falls in moderate risk and only 0.31% (7.8km²) falls in low risk areas. Furthermore, highly urbanized Local Governments in relatively low elevations with low slope angles such as Eti-Osa, Apapa, Lagos Island, Lagos Mainland and Ojo have high risk of getting flooded while most Local Governments with low level of urbanization and high elevations have moderate to low risk of getting flooded. These findings have implications on sustainable decision making and planning for flood risk prevention and management, prioritizing flood risk control mechanisms where necessary, and the development and implementation of potent flood control policies with appropriate infrastructure in areas that fall in both very high risk and high risk.

Coastal Megacity, Flood Risk, Geospatial, Nigeria, Ikeja, Lagos State

[01] Fernandez, S. and Lutz, A., “Urban flood hazard zoning in Tucumán Province, Argentina, using GIS and multicriteria decision analysis.,” Eng. Geol., vol. 111, p. 90–98, 2010. [02] Abdel Karim, A., Gaber, D., Youssef, M. and Pradhan, B., “Flood Hazard Assessment of the Urban Area of Tabuk City, Kingdom of Saudi Arabia by Integrating Spatial-Based Hydrologic and Hydrodynamic Modeling,” Sensors, vol. 19, p. 1024, 2019. [03] Danumah, J. H., Odai, S. N., Saley, B. M., Szarzynski, J., Thiel, M., Kwaku, A., Kouame, F. K., and Akpa, L. Y., “Flood risk assessment and mapping in Abidjan district using multi-criteria analysis (AHP) model and geoinformation techniques, (cote d’ivoire).,” Geoenviron. Disasters, vol. 3, p. 10, 2016. [04] I. Elkhrachy, “Flash Flood Hazard Mapping Using Satellite Images and GIS Tools: A case study of Najran City, Kingdom of Saudi Arabia (KSA),” Egyp. J. Remote Sens. Space Sci., vol. 18, p. 261–278, 2015. [05] E. Forkuo, “Flood hazard mapping using aster image data with GIS,” Int. J. Geomat. Geosci., vol. 1, p. 932–950, 2010. [06] Papaioannou, G., Vasiliades, L., and Loukas, A., “Multi-Criteria Analysis Framework for Potential Flood Prone Areas Mapping,” Water Resour. Manag., vol. 29, p. 399–418, 2015. [07] Ezemonye, M. N. and Emeribe, C. N., “Flood characteristics and management adaptations in partsof the Imo river system,” Ethiop J Environ Studies Manage., vol. 4, no. 3, p. 56–64, 2011. [08] D. Guha-Sapir, “The Emergency Events Database-Universit! (EM-DAT) e Catholique de Louvain (UCL) CRED,,” Brussels, Belgium, 2017. [09] The United Nations Office for Disaster Risk Reduction (UNISDR), “The human cost of weather-related disaters 1995–2015,” UNISDR, 2015. [10] Klein, R. J., R. J. Nicholls, and F. Thomalla, “The Resilience of Coastal Megacities to Weather-Related Hazards,” in Building Safer Cities: The Future of Disaster Risk. Disaster Risk Management Series No. 3, M. A. A. C. A. Kreimer, Ed., Washington, DC, World Bank, 2003, p. 101–120. [11] H. Li, “Management of Coastal Mega-Cities: A New Challenge in the 21st Century,” Marine Policy, vol. 27, no. 4, p. 333–337, 2003. [12] Pedersen, A., Mikkelsen, P., and Arnbjerg-Nielsen, K., “Climate change-induced impacts on urban flood risk influenced by concurrent hazards.,” J. Flood Risk Manag., vol. 5, p. 203–214, 2002. [13] P. Vino, “Flood hazard assessment of Vamanapuram River Basin, Kerala, India: An approach using Remote Sensing & GIS techniques,” Adv. Appl. Sci. Res., vol. 4, p. 263–274, 2013. [14] Salami, H. S. and Otokiti, K. V., “Ineffective development control and flood susceptibility in Lokoja, Nigeria,” Discovery, vol. 55, no. 285, pp. 468-476, 2019. [15] Didovets, I., Krysanova, V., Bürger, G., Snizhko, S., Balabukh, V., and Bronstert, A., “Climate change impact on regional floods in the Carpathian region,” J. Hydrol. Reg. Stud., vol. 22, p. 100590, 2019. [16] Lee, J., Kang, E., and Jeon, S., “Application of frequency ratio model and validation for predictive flooded area susceptibility mapping using GIS,” in Geoscience and Remote Sensing Symposium (IGARSS), Munich, Germany, 2012. [17] Tehrany, S., Pradhan, B., Mansor, S., and Ahmad, N., “Flood susceptibility assessment using GIS-based support vector machine model with different kernel types,” Catena, vol. 125, p. 91–101, 2015. [18] Stefanidis, S. and Stathis, D., “Assessment of flood hazard based on natural and anthropogenic factors using analytic hierarchy process (AHP),” Nat. Hazards, vol. 68, p. 569–585, 2013. [19] B. Pradhan, “Use of GIS-based fuzzy logic relations and its cross application to produce landslide susceptibility maps in three test areas in Malaysia,” Environ. Earth Sci., vol. 63, p. 329–349, 2011. [20] B. Pradhan, “Flood susceptible mapping and risk area delineation using logistic regression, GIS and remote sensing,” J. Spat. Hydrol., vol. 9, p. 1–18, 2010. [21] Lohani, K., Goel, K., and Bhatia, S., “Improving real time flood forecasting using fuzzy inference system,” J. Hydrol., vol. 509, p. 25–41, 2014. [22] Abdelkarim, A., Gaber A. F. D., Alkadi, I. I. and Alogayell, H. M., “Integrating Remote Sensing and Hydrologic Modeling to Assess the Impact of Land-Use Changes on the Increase of Flood Risk: A Case Study of the Riyadh–Dammam Train Track, Saudi Arabia,” Sustainability, vol. 11, no. 6003, pp. 1-32, 2019. [23] Huang, X., Tan, H., Zhou, J., Yang, T., Benjamin, A., Wen, S. W., Li, S., Liu, A., Li, X., Fen, S., et al., “Flood hazard in Hunan province of China: an economic loss analysis,” Nat Hazards, vol. 47, no. 1, p. 65–73, 2008. [24] United Nations Office for Disaster Risk Reduction (UNDRR), “Global Assessment Report on Disaster Risk Reduction,” Geneva, Switzerland., 2019. [25] Bubeck, P., Botzen, W., and Aerts, J., “A review of risk perceptions and other factors that influence flood mitigation behavior,” Risk. Anal., vol. 32, p. 1481–1495, 2012. [26] Mason, D. C., Speck, R., Devereux, B., Schumann, G. J. P. Neal, J. C., and Bates, P. D., “Flood Detection in Urban Areas Using TerraSAR-X, IEEE T,” Geosci. Remote, vol. 48, p. 882–894, 2010. [27] Zhou, Z., Liu, S., Zhong, G., and Cai, Y., “Flood Disaster and Flood Control Measurements in Shanghai,” Nat. Hazards Rev., vol. 18, 2016. [28] United Nations (UN), “The World’s Cities in 2016,” 2016. [29] Lagos State Ministry of Economic Planning and Budget (LMEPB), “Lagos State Development Plan 2012–2025,” Lagos, 2013. [30] I. Adelekan, “Flood risk management in the coastal city of Lagos, Nigeria,” J Flood Risk Management, vol. 9, p. 255–264, 2016. [31] LASG (2012) Lagos state government (LASG), “Lagos state government official website on population,” 2012. [32] International Human Dimension Programme (IHDP), “International Human Dimension Programme Global Environmental Change. Background on coastal zones,” 2012. [33] Maplecroft, “World’s fastest growing populations increasingly vulnerable to the impacts of climate change – 4th global atlas reports,” http://maplecroft.com/about/news/ccvi_2012.html, 2012. [34] I. Adelekan, “Vulnerability of poor urban coastal communities to flooding in Lagos, Nigeria,” Environ and Urban, vol. 22, p. 443–450, 2010. [35] A. Young, “Coastal megacities, environmental hazards and global environmental change,” in Megacities and the coast: risk, resilience and transformation, Oxon: Routledge, 2013, p. 70–99. [36] J. B. Marco, “Flood Risk Mapping, Coping with Floods. NATO ASI series,” Applied Sciences, vol. 257, 1994. [37] Krouska, Z. and Parcharidis, I., “Hazard maps for flash-floods in the Thriassion Plain,” in The 10th International Congress of Hellenic Geographical Society, Greece, 2014. [38] Raji, S. A., Adeniyi, P. O. and Odunuga, S., “Multicriteria Flood Risk Analysis of Lower Ogun River Basin,” Journal of Environmental and Earth Science, vol. 14, no. 13, pp. 112-125, 2014. [39] Mmom, C. and Ayakpo, A., “Spatial Analysis of Flood Vulnerability Levels in Sagbama Local Government Area Using Geographic Information System (GIS),” International Journal of Research in Environmental Studies, vol. 1, pp. 1-8, 2014. [40] Cao, C., Xu, P., Wang, Y., Chen, Y., Zheng, L. and Niu, C., “Flash Flood Hazard Susceptibility Mapping Using Frequency Ratio and Statistical Index Methods in Coalmine Subsidence Areas,” Sustainability, vol. 8, pp. 948-960, 2016. [41] S. Das, “Geospatial Mapping of Flood Susceptibility and Hydro-Geomorphic Response to The Floods in Ulhas Basin, India,” Remote Sensing Applications: Society and Environment, vol. 14, pp. 60-74, 2019. [42] S. K. McFeeters, “The Use of the Normalized Difference Water Index (NDWI) in the Delineation of Open Water Features,” International Journal of Remote Sensing, vol. 17, no. 7, pp. 1425-1432, 1996. [43] M. P. Mwangi, “The Role of Land Use and Land Cover Changes and GIS in Flood Risk Mapping in Kilifi County, Kenya.,” 2016. [44] Ouma, Y. O. and Tateishi, R., “Urban Flood Vulnerability and Risk Mapping Using Integrated Multi-Parametric AHP and GIS,” Water, vol. 6, pp. 1515-1545., 2014. [45] Kazakis, N., Kougias, I. and Patsialis, T., “Assessment of Flood Areas at a Regional Scale Using an Index-based Approach and Analytical Hierarchy Process: Application in Rhodope-Evros Region, Greece,” Science of the Total Environment, vol. 538, pp. 555-563, 2015. [46] Eguaroje, O. E., Alaga, T. A., Ogbole, J. O., Omolere, S. and Alwadood, J., “Flood Vulnerability Assessment of Ibadan City, Oyo State, Nigeria,” World Environment, vol. 5, no. 4, pp. 149-159, 2015. [47] Rimba, A. B., Setiawati, M. D., Sambah A. B. and Miura, F., “Physical Flood Vulnerability Mapping Applying Geospatial Techniques in Okazaki City, Aichi Prefecture, Japan.,” Urban Science, vol. 1, no. 7, pp. 1-22, 2017. [48] Korah, P. I. and Lopez, F. M. J., “Mapping Flood Vulnerable Areas in Quetzaltenango, Guatemala using GIS,” Journal of Environment and Earth Science, vol. 5, no. 6, pp. 132-145, 2015. [49] Gregory, K. J. and Walling, D. E, “The Variation of Drainage Density Within a Catchment,” International Association of Scientific Hydrology Bulletin, vol. 13, no. 2, pp. 61-68., 1968.