Flood risk assessment for pro-active response to flood disasters in Gaborone, Botswana.

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Gorata, Samuel
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The University of Zambia
Floods have been one of the recurring hazards with increased magnitude in Botswana due to heavy convective rainfall and tropical cyclones effects which eventually cause flood disasters. Gaborone is one of the areas hardest hit by floods in Botswana, yet the country is always caught off guard by floods due to inadequate preparedness to natural disasters. This study sought to conduct flood frequency analysis and integrate geospatial methods to enhance an operational and proactive practice of preparedness in Botswana focusing on Gaborone. The objectives of the study were to (i) determine the magnitude and frequency of flood, (ii) develop flood disaster risk assessment tool using geospatial techniques, (iii) model potential flood risk for proactive flood disaster management response for Gaborone. Data collection included secondary data supplemented by field observations, key informant interviews, satellite imagery for three-dimension (3D) surface modeling. Precipitation data was collected from the Department of Meteorological Services for two stations located in the Gaborone catchment area for the period 1972 to 2017. Satellite imagery (Sentinel-2) and digital elevation model (Shuttle Radar Topography Mission) of the catchment area were obtained. The analysis of data collected involved determining the occurrence and frequency of floods, preparing flood disaster risk maps using geospatial techniques. Arc Scene 10.5 was used for three-dimension (3D) mapping whereas field observation and ground truthing were used for satellite imagery validation. Findings of the study were that low magnitude floods were more frequent but with shorter recurrence intervals (less than one year, while high magnitude floods had low frequency of occurrence (two to three percent) with much longer recurrence intervals up to 45 years. The probability of flood occurrence also revealed higher frequency of occurrence for low rainfall amounts than high amounts that produce high flood magnitudes. The Digital Elevation Model showed that most of Gaborone area is generally flat (highest 1311m; lowest 925m) hence experiences regular flooding due to low gradient in the downstream reaches of the Notwane River where Gaborone is located. The DEMs showed that about 126.1 km2 of the area (52.4%) is the most high-risk flood zone because it is largely characterized by urban land uses (concrete / tarmac pavement, roof tops, artificial drainage, etc.) and promote flow accumulation on the surface. The area above 1072m measuring about 9.3 km2 is not liable to flooding on account of its high slopes, while area lying between 1004m-1072m is moderately affected by flooding. It is concluded that most of Gaborone city is prone to flooding owing to its flat topography and the high frequency of flood occurrence from heavy rain storms especially those related to cyclonic activities. This calls for the need by planners and local authorities to adopt flood risk assessment in decision making. An early warning flood forecasting and response to flood disaster system is recommended for city planning to come up with flood proof structure standards for buildings and bridges within the Gaborone City. Keywords: Flood Risk Areas, Flood Preparedness, Flood Frequency Analysis, Geospatial Technology, Gaborone
Floods--Botswana , Floods control , Floods--Risk management , Floods damage prevention