Investigating the performance of Lubigi drainage channel under unexpected loading conditions

Abstract

In this study, emphasis was put on assessing the performance of the Lubigi drainage channel under unexpected loading conditions that could be as result of factors like extreme rainfall events and increased levels of impervious surfaces in the catchment area. Our study was conducted based on a previous study by Kigemuzi and Kimera (2018) about the hydraulic performance of the Lubigi channel, Bwaise section where the conclusion was that the channel should be fit to convey the runoff without flooding but sedimentation mainly affects its performance. Bwaise area is prone to floods and this triggered our research to test the performance of the channel section through this area under severe loading scenarios assuming that sedimentation of the channel has been addressed. The objectives of our study were to develop a model in SWMM to assess the performance of the Lubigi channel, assess the impact of the change in climate characterized by extreme rainfall events on performance of the channel, and investigate the impact of the land use changes characterized by high imperviousness on the performance of the channel The hydrological analysis of the channel watershed was carried out using the ArcGIS. IDF curves for rainfall data of 21 years were generated and the 10-year return-period design storm was used to generate the block rainfall events that represented the extreme loading scenarios that can be imposed onto the channel. The time of concentration was calculated as 60 minutes. The hydraulic properties of the Lubigi channel were extracted from (KDMP, 2016) and the plan drawings obtained from KCCA ,designed by SMEC international. These included widths of different sections, length of the section under study and channel depth of the different sections and chainages. The channel performance was assessed basing on the discharge attained at the outfall from running different simulations in the model with the maximum discharge at the outfall of 118.5 m3/s as specified in the KDMP, 2016.We run a simulation in the model under the normal loading conditions for 10 years design storm and obtained a maximum discharge of 106.033 m3/s which indicated proper channel performance. We then run the extreme loading scenarios due to which gave maximum discharges of 299.956 m3/s for the 60 minute block rainfall events, 206 m3/s for the 120 minute block rainfall event. These scenarios indicated severe loss of the hydraulic and operational performance of the channel. The simulations for the different imperviousness percentages produced discharges of 106.033 m3/s for 25%, 150.057 m3/s for 50% and 167.383 m3/s for 75% .The Change in land use scenarios represented by an increase in PIMP also indicated loss of system functionality.