AccScience Publishing / AJWEP / Online First / DOI: 10.36922/AJWEP025190146
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SHORT COMMUNICATION

Evaluating the hydraulic performance and sustainability of the Simike–Nzovwe roadside drainage system in Mbeya City, Tanzania, using the hydrologic engineering centre’s river analysis system modelling

Abdul Mohamed1 ,  Zacharia Katambara1*
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1 Department of Civil Engineering, College of Engineering and Technology, Mbeya University of Science and Technology, Mbeya, Tanzania
AJWEP, 025190146 https://doi.org/10.36922/AJWEP025190146
Received: 8 May 2025 | Revised: 26 June 2025 | Accepted: 27 June 2025 | Published online: 21 July 2025
© 2025 by the Author(s). This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution 4.0 International License ( https://creativecommons.org/licenses/by/4.0/ )
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Abstract

This study addresses the hydraulic inefficiencies and maintenance challenges associated with the roadside drainage system along a 1.85 km stretch of the TANZAM Highway between Simike and the Nzovwe River, which includes five circular culverts. The objective was to evaluate the system’s hydraulic performance under rainfall events using the Hydrologic Engineering Centre’s River Analysis System (HEC-RAS) one-dimensional hydraulic model. Specifically, the study focused on analyzing flow regimes, specific energy transitions, and sediment transport dynamics to identify critical points of inefficiency. The methodology involved simulating steady flow conditions, assessing the influence of channel and culvert geometry, and performing a sensitivity analysis on key hydraulic parameters, including Manning’s roughness coefficient, channel slope, and culvert dimensions. The model results revealed that subcritical flow conditions (Froude number, Fr <1) upstream of culverts lead to sediment accumulation, while steeper channel sections with supercritical flow (Fr >1) pose erosion risks. Pronounced hydraulic jumps were observed near culvert outlets, resulting in significant turbulence, abrupt energy dissipation, and localized erosion. Flow velocities decreased sharply from over 7 m/s to below 1 m/s across these transition zones. This study provides an integrated evaluation of hydraulic and sediment transport interactions in a real-world drainage system using HEC-RAS, supported by targeted design optimization strategies. Key recommendations include modifying side slope geometry, increasing longitudinal gradients, and enlarging culvert dimensions to enhance flow capacity and reduce sediment deposition. In addition, the application of riprap in high-velocity zones, vegetative lining in low-velocity areas, and the inclusion of sediment traps are proposed to control erosion and minimize maintenance.

Graphical abstract
Keywords
HEC-RAS modeling
Side drainage design
Supercritical to subcritical flow
Sediment accumulation
Hydraulic performance evaluation
Funding
None.
Conflict of interest
The authors declare no competing interests.
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Asian Journal of Water, Environment and Pollution, Electronic ISSN: 1875-8568 Print ISSN: 0972-9860, Published by AccScience Publishing
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