Flood risk modeling and analysis in suburban Freetown, Sierra Leone

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Insinööritieteiden korkeakoulu | Master's thesis
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Nordic Master Programme in Environmental Engineering
53 + 54
A flood risk analysis for the Pottor community in Freetown, Sierra Leone was completed in collaboration with Engineers Without Borders – Denmark for the ongoing Project 119 which promotes climate change resiliency. ERA5 global reanalysis data was retrieved from Copernicus to source hourly historical rain rate data. Design storms were calculated for various return periods (1-, 2-, 5-, 10-, and 20-year) and storm durations (1-, 2-, 6-, 12-, and 24-hour) using frequency analysis. IDF curves were derived and utilised to calculate hyetographs for each storm event. Identified historical storms were classified as reported in the ERA5 data. A 2D MIKE21 flood model of the watershed was completed for a 2-year, 12-hour storm event. Additional models were run for each return period, lower surface roughness, and climate change simulations. Results were validated against reported flooding from community survey data. The resulting maximum water depth speed within the study area was analysed for each model scenario to determine parameter influence. The influence of rainfall intensity on flooding extent for extreme events was limited by the local geography and little difference was seen for water speed. Similarly, the impact of climate change on flooding due to increased intensity is limited. The influence of roughness was greater within observed flow paths. An exposure analysis identified impacted structures and major flow paths using open-source GIS data retrieved from OpenStreetMap. Key factors of vulnerability were identified as residence structure type, low physical mobility, and level of income. Recommendations for future project objectives included gathering local rainfall data, improving mapping of the community, identifying and protecting evacuation routes, formalizing and increasing the capacity of major flow paths, and identifying vulnerable members of the community. Flood risk maps and a guide for downloading ERA5 rain rate data were provided for use by stakeholders.
Varis, Olli
Thesis advisor
Skov, Marianne
Arnbjerg-Nielsen, Karsten
climate change resiliency, flood modeling, risk analysis, hazard assessment, ERA5
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