Assessing the effects of sedimentation on the flooding of River Manafwa in Mbale District, Busiu Subcounty

Abstract

Flooding is a pervasive natural hazard that causes substantial damage to infrastructure, disrupts communities, and poses risks to human life. This study aimed at investigating the impact of sedimentation on the flooding patterns of river Manafwa at Busiu sub-county, Mbale district with the coordinates of 0:56:13 N, 34:9:28 E and provided insights into the underlying processes. Through a comprehensive analysis of historical river discharge and precipitation data, this research project established the impact of sedimentation rates on flood occurrences within Busiu, Mbale district.. Using the Universal Soil Loss Equation (USLE) empirical method along with Sediment Delivery Ratio (SDR), temporal variation in the amount of sediment loads was found in Manafwa Basin and the amount of sediment loads reaching the outlet at Busiu sub-county was defined. Busiu sub-county was selected because of it has a gauging station and hence ease of data accessibility. Using historical precipitation data collected from the Ministry of Water and Environment, the sediment loads at three different years 2000, 2010 and 2020 were determined to be 453,721.4269, 3,541,565.405, 4,342,982 tons/year, respectively. Field measurements on river characteristics and sediment properties from Busiu outlet to about 1km along the river were done and the results aided in sediment transport and flood inundation simulation. HEC-RAS model was used for sediment transport analysis and flood mapping. Increased sedimentation rates led to the reduced conveyance capacity of the river channel in some of the cross-sections resulting in higher water levels during flood events. Furthermore, sediment deposition altered the river's flow dynamics, leading to changes in river flow velocities which contributed to the expansion of flood-prone areas. The flood depth increase due to sediment deposition varied from 45.3 % to 51.1 % from the 2-Year return period to the 200-Year return period respectively. The study also investigated the factors contributing to sedimentation, including upstream land use practices, erosion, and sediment transport processes. The main factor was seen in the reduction of vegetation and an increase in cultivation land, exposing the soils to erosion This phenomenon has important implications for river management and flood mitigation strategies. Understanding the relationship between sedimentation and flooding enables the development of targeted interventions, such as sediment removal techniques and channel restoration measures. These interventions can enhance the river's capacity to accommodate high flows, mitigate flood risks, and safeguard communities and infrastructure. In conclusion, this research project underscored the significant influence of sedimentation on river flooding. The findings emphasized the need for proactive sediment management strategies to reduce flood vulnerability, enhance river resilience, and promote sustainable river basin management practices in the face of evolving environmental conditions.