A COMPREHENSIVE STUDY ON FLOOD RISK ASSESSMENT CONSIDERING THE IMPACTS OF CLIMATE CHANGE AND LAND USE: CASE STUDY OF FREETOWN, SIERRA LEONE.

Abstract

This study presents a comprehensive assessment of flood risk in Freetown, Sierra Leone, incorporating the impacts of climate change and land use changes over a 30-year period from 1991 to 2020. Utilizing the Mann-Kendall trend analysis, it evaluates historical climate data, including rainfall and temperature, to find out the climatic influences on flooding. The study leverages the Rainfall-Runoff Inundation (RRI) model to simulate flood hazard inundation depths in two key basins within Freetown. Additionally, it integrates the Analytic Hierarchy Process (AHP) with Multi-Criteria Decision Making (MCDM) and Geographic Information System (GIS) technologies to create an intricate flood risk map of the city. A detailed analysis of land use and land cover (LULC) changes over the two decades, based on MODIS classification, illustrates how rapid urbanization and the reduction in vegetation cover elevate flood risks. The research utilizes 11 indicators for flood risk assessment using AHP_MCDM method, Rainfall, Slope, Elevation, Distance from River, Slope, LULC, Drainage Density, Population Density, Distance from Road, NDVI and TWI. The findings indicate a steady increase in rainfall and temperature throughout the study period, with notable seasonal variations that influence flooding patterns. The RRI model's performance was validated by correlating simulated depth against observed flood depths from twelve (12) data points, showing coefficients of determination (R2) of 0.6582, Nash-Sutcliffe Efficiency (NSE) of 0.621, and Percent Bias (PBIAS) of -4.73%, indicating a generally reliable model for simulating flood hazards in Freetown. The results from the AHP_MCDM reveal that a significant portion of Freetown is subjected to high to very high flood risks, highlighting the urgent need for effective flood management, adaptation and mitigation strategies. By combining climate data analysis, hydrological modelling, and spatial analysis within a multi-criteria decision framework, this thesis not only delineates the current flood risk landscape but also lays a robust foundation for future urban planning and environmental management initiatives in Freetown