Flood risk mapping and vulnerability assessment in the Manafwa catchment, Eastern Uganda

Abstract

Flood hazards and their consequential damages have seen a global increase, yet our comprehension of flood risk patterns, particularly in tropical catchments, remains inadequate. Recognizing this knowledge gap, this study undertook a flood risk mapping and vulnerability assessment within the Manafwa catchment was undertaken. The objectives were to: 1) Assess conditions influencing spatial and temporal patterns of flood risk from 1995 to 2017, 2) Examine dimensions and drivers of social vulnerability to flood risk, and 3) Evaluate perceived effectiveness of household coping strategies to flood risk. To fulfil these objectives, a multi-faceted methodology was employed. Firstly, conditions affecting flood risk patterns were investigated using the Soil and Water Assessment Tool model, integrated with Hydrologic Engineering Center River Analysis System. Secondly using household survey data from 210 respondents, Focused Group Discussions and Key Informant Interviews factors that influenced social vulnerability and effectiveness of coping strategies were assessed. Using Principal Component Analysis technique, a Social Vulnerability Index was developed. Multi variate Probit Models and Ordered Probit Models were used to analyse determinants and effectiveness of coping strategies respectively. Results showed decrease in forest areas and wetlands, and conversions into farmlands and built-up areas from 1995 to 2017 that resulted in increased annual surface runoff, sediment yield, and water yield. Built-up areas and commercial farmlands are highly vulnerable, subsistence farmlands are moderately vulnerable, while bushland, grassland, tropical high forest, woodland, and wetlands exhibit low vulnerability to flooding. Eight factors explained 80% variance, with social vulnerability primarily impacted by information and knowledge and demographic characteristics. Most households (88.8%) exhibited moderate to high levels of social vulnerability. Construction of temporary embankments (37%) and afforestation (44%) were the commonest short-term and long-term coping strategy respectively. Family size, number of adult males in family, location of house within floodplain, and duration of residence in affected area (P>0.05) significantly influenced choice of coping strategies. Evacuation of children and proper siting of buildings emerged as most effective short-term and long-term coping strategy respectively. This study underscores the utility of integrated modeling systems in predicting flood inundation extents, with developed flood risk maps poised to guide policy makers in incorporating flood hazard assessment into planning and development processes. Furthermore, social vulnerability assessment can be integrated into existing flood risk assessments. While initially applied to the Manafwa catchment, this framework can be adapted for assessing current and future flood risks across Uganda.