Multi-criteria approach for flood risk mapping to support flood management: a case of Nata river basin, Botswana

Abstract

Floods are the most frequent natural disasters, and they have negative impacts on socialeconomic livelihoods of countries, not forgetting their environment. To effectively manage flood risks and make informed decisions, it is crucial to identify and categorise flood-prone zones. This study predicts flood frequency and suggests a multi-criteria technique for mapping flood risk in Botswana's Nata River Basin. Floods are common in the Nata River Basin and can result in substantial infrastructure damage, fatalities, and financial losses. The suggested method involves integrating multiple criteria, like the soil type, slope, elevation, land use, and hydrographic conditions, to identify areas at high risk of flooding. Satellite images, weather information, and field observations were only a few of the sources used to compile the data for the different criteria. Gumbel distribution was utilised to analyse rainfall data in this study through a period of 31 years. The results of the distribution showed that annual precipitation of 63.28 mm was most likely to occur annually, while 300.59 mm precipitation has 100% probability of occurring every 32 years. Distribution graphs indicated that there is 3% chance of 301 mm rainfall occurring for every year. They also show that there are greater odds of having 63mm, as indicated by exceedance likelihood of 97%. As for objective number two which used Geographic Information System (GIS), ArcGIS Pro, to create a map that shows risk of flooding, the results indicated that an area covering 41.3% of the basin mostly in the southern parts of the study area have high flood risk, while moderate risk was the highest area covering with 54.7% and the low-risk areas covers 8%. Accuracy of the flood risk map was validated through comparison with different AHP scenarios. It was also validated by overlaying it to a flood extent map derived from flooding season satellite images. Most flooded areas from the 2020 floods, according to the flood extent map, fell into the high and moderate risk categories. This indicates

that the GIS-MCA technique serves as a reference for future, more effective flood management and risk decision-making in the basin. The findings demonstrate the effectiveness of the suggested multi-criteria method in identifying locations at high risk of flooding. The strategy is adaptable and can be tailored to the requirements of various river basins in Botswana and other nations facing comparable difficulties. Decision-makers can use the flood risk map produced using the suggested methodology to create efficient flood risk management and mitigation plans. For instance, maps can be used to pinpoint the locations where dredging, building embankments, and other flood control techniques would be most beneficial. The map can also be used to guide decisions on how to use land, such as whether to zone it for residential, commercial, or agricultural purposes.