Assessing the effect of land use changes on runoff and sediment yield in Lubigi catchment in Kampala.
Nafuna, A. Soniah
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Sediment deposition in drainage channels has hampered the performance of many urban drainage systems in Uganda. These sediments reduce the channel carrying capacity, resulting into reduced velocity and discharge of the run off, stagnation of the water which in turn causes flooding that destroys property and lowers the living standards of the population around these channels. Worldwide, models have been used to predict sediment yield from larger river catchments, however little modelling has been done for sediment yield in urban catchments more so with the Soil and Water Assessment Tool. The main objective of this study was to assess the effect of land use change on the run off, sediment yield and sediment characteristics in the Lubigi channel. Using the Lubigi Catchment as our case study, the flow and sediment yield from the catchment was predicted using a physically based model; Soil and Water Assessment Tool (SWAT). This study was done by comparing two secondary drainage channels that drain into the Lubigi channel which is the primary channel. These two channels are Nakamiro channel and Katanga channel. The Katanga channel flows from Wandegeya through a slum area and finally drains into Lubigi channel, Nakamiro channel flows through Kawempe division and finally drains into Lubigi channel. According to the model, Nakamiro drainage channel contributes an average annual sediment yield of 880.613 tonnes to Lubigi drainage channel while Katanga drainage channel contributes an average annual sediment yield of 540 tonnes. Therefore, Nakamiro drainage channel contributes more sediments to Lubigi drainage channel than the Katanga drainage. Water and sediment samples from the two channels were collected in 2023, three times a month and taken to the laboratory for analysis of sediment concentration, the sediment load was calculated by multiplying the sediment concentration by the measured discharge. The measured sediment yield was 277.94 ton/month in the month of April and 314.60 ton/month in the month of May for Nakamiro channel. For Katanga channel, the measured sediment yield was 205.23 ton/month in the month of April and 250.65 ton/month in the month of May. This study was conducted using the Soil and Water Assessment Tool (SWAT). The model was calibrated and validated using sediment data that was collected by (Malingha-CEDAT-CIV, n.d.). Soil data, rainfall data and temperature data were collected and used during this study; as shown in section 4.5. The results show that to manage the runoff and sedimentation build up in Lubigi and its secondary channels, management of land use, land cover activities in the catchment should be prioritized. Therefore, the results will be helpful to decision and policy makers and relevant stakeholders responsible for urban planning.