Assessing Urban Flood Risks under Changing Climate

Climate change has become one of the global environmental issues more visible in recent decades. Flooding is one of the natural disasters that combine with climate change to create effects and cause some of the most destructive damages in Cote d’Ivoire. The District of Abidjan located in the south of Cote d’Ivoire which is subjected to heavy rainfall, rapid population growth and uncontrolled urbanisation is not an exception to the problem of flood risk. The aim of this study was to identify, map and model areas of flood risk to facilitate decision making for better land use planning under changing climate in this District. The maximum likelihood classification algorithm and post-classification change detection procedures were used in this study.

The spatio-temporal land use/ land cover change in relation to urbanisation sprawl was assessed based on a series of Landsat images for 1990, 2002 and 2014. Statistical methods were also used to characterise and determine hydro-climatic drivers of flood. LARS-WG model was used to generate future scenario based on HadCM3 model and SRA1B emissions and in rclimdex model, ten (10) rainfall indices were calculated. LARS-WG and rclimdex were used to determine and analyse trends of rainfall and temperature under present and future climatic conditions. Analytic Hierarchy Process (AHP) method as a multi-criteria analysis allows the integration of several elements under two criteria: hazard and vulnerability for flood risk assessment and mapping. Flood Hazard Index (FHI) model was carried out to determine flood prone areas by combining rational and statistical methods. The results revealed urban area expansion (15%) as a major land use change for the period 1990-2014. However, there was an important increase in urban area between 2002 and 2014, compared to 1990-2002. Regarding future weather, LARS-WG statistical results showed that, temperature will increase from 0.32°C to 2.54°C for the period 2011-2100. Also, rainfall in the same period will increase from 4 %, to 10 %. Results of rclimdex through ten (10) rainfall indices and comparison

between observed and future indices indicated an increased trend of some rainfall indices: Consecutive wet Days (CWD) and number of heavy rainfall days above 10mm (R10) from 2011 to 2100. AHP flood risk map showed that areas under high and very high flood risk covers 34% of the study area while Flood Hazard Index (FHI) model revealed that about 25.09 % of the study area were within the high FHI areas. Both results showed that eight out of thirteen (8/13) municipalities of Abidjan District are within high and very high flood risk areas.

Therefore, there is a need for decision makers to call for optimal design of technical solutions and an effective preparedness strategy to be developed for future flood occurrence within Abidjan District (South of Cote d’Ivoire).