A systematic flood risk assessment of Bloemfontein Watershed, South Africa

Abstract

Understanding the elements contributing to flood risk can create effective plans and policies for decision-makers to lessen vulnerability and boost resilience. Over the years, Bloemfontein has experienced seasonal flooding, despite the low amount of rainfall it receives compared to other towns in South Africa. This study systematically assessed flood risk in the most vulnerable watershed of Bloemfontein. The analytical hierarchy process (AHP) and GIS-based multi-criteria decision-making were utilized to map and identify potential locations that are vulnerable to floods. The relative weight of each criterion taken into account for the susceptibility mapping was determined using AHP. Geospatial techniques were used to assess the vegetation cover, moisture content, surface water, and land surface temperature (LST) from 1991 to 2023, as well as ten physical environmental factors linked to the occurrence of floods. Findings from this study revealed a decrease in vegetation cover and moisture content and an increase in the LST from 1990 to 2023. The susceptibility to flood map shows that 9.63% of the watershed is low susceptible to flooding, 79.26% is moderate, 11.07% is high, and 0.03% is very high susceptible to flooding. The built areas and major farmlands fall within the high susceptibility zone of 11%.

Keywords

• Analytical hierarchy process
• Bloemfontein
• flood risk
• GIS
• vulnerability