Hydraulic modelling of the Bwaise drainage system for stormwater management

Abstract

The Bwaise drainage system in Kampala, Uganda, faces significant challenges in managing stormwater, leading to frequent flooding during extreme rainfall events. The floods indicate that the drainage system lacks sufficient capacity to manage stormwater effectively. This project utilized hydraulic modelling to assess the system's performance and identify improvements for effective stormwater management. A frequency analysis of 21 years of rainfall data (2000-2021) using the Gumbel distribution estimated precipitation depths for extreme events. The Gumbel and rational methods determined the drainage system's capacity, revealing a total contributing drainage area of 1.98 km², validating the rational method's applicability. The hydraulic model was developed with geometric data and evaluated for various scenarios, across a total of ten stations. Flow velocities ranged from 1 to 2.5 m/s, with fluctuations indicating changing flow regimes, particularly at Station 6, where hydraulic jumps were observed due to abrupt slope reductions. Calibration against Manning’s roughness coefficient revealed a Root Mean Square Error of 0.7602 between measured and simulated velocities. Further observations indicated that the channels in Bwaise III had inadequate slopes and were obstructed by debris, reducing their conveyance capacity. In conclusion, the hydraulic jumps were identified as a primary contributor to flooding, necessitating structural modifications to enhance the drainage system's effectiveness. Detailed discussions of these findings and proposed solutions will follow in subsequent chapters.