Optimizing Hydraulic Performance and Sustainability of the Simike-Nzovwe Roadside Drainage System in Mbeya City, Tanzania Using HEC-RAS Modeling

The side drainage system along the TANZAM Highway section from Simike to Nzovwe River, extending approximately 1.85 km and featuring five circular culverts, was evaluated to assess its hydraulic performance and behaviour during rainfall events. Using the HEC-RAS model, the study analysed flow regimes, specific energy transitions, and sediment transport to identify design inefficiencies and propose optimization strategies. The model effectively captured key processes, revealing significant silt accumulation upstream of culverts due to subcritical flow (Fr < 1) and erosion risks in steeper sections with supercritical flow (Fr > 1). Hydraulic jumps observed near culverts caused turbulence, localized erosion, and debris deposition, with flow velocities decreasing from approximately 7 m/s to less than 1 m/s during transitions. Sensitivity analysis demonstrated that Manning’s roughness coefficient, channel slope, and culvert dimensions significantly influence flow behaviour, sediment accumulation, and erosion patterns. Based on these findings, the study recommends optimizing channel geometry by reducing side slopes, steepening longitudinal slopes, and enlarging culvert dimensions to enhance flow efficiency and minimize sediment deposition. Material selection, such as riprap for high-velocity zones and vegetative linings for low-velocity sections, is proposed to improve erosion resistance and promote ecological stability. Additionally, sediment traps and sustainable drainage practices are suggested to mitigate sedimentation and reduce maintenance demands. This study highlights the value of one-dimensional flow models like HEC-RAS in designing and maintaining efficient roadside drainage systems, providing actionable recommendations to improve hydraulic performance, reduce maintenance costs, and ensure environmental sustainability in similar contexts..