Page 248 - AJWEP-22-4

Asian Journal of Water, Environment and Pollution. Vol. 22, No. 4 (2025), pp. 240-248.
doi: 10.36922/AJWEP025190146

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 modeling

Abdul Mohamed and Zacharia Katambara*
Department of Civil Engineering, College of Engineering and Technology, Mbeya University of
Science and Technology, Mbeya, Tanzania
*Corresponding author: Zacharia Katambara (zacharia.katambara@must.ac.tz)

Received: May 08, 2025; 1st revised: June 21, 2025; 2nd revised: June 26, 2025; Accepted: June 27, 2025;
Published online: July 21, 2025

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.
Keywords: HEC-RAS modeling; Side drainage design; Supercritical to subcritical flow; Sediment accumulation;
Hydraulic performance evaluation

1. Introduction and functionality. Geometric design defines the physical
1
layout of the highway, including cross-sections, curves,
An efficient highway system requires an integrated sight distances, and alignment standards. In contrast,
2
design approach encompassing geometry, pavement, pavement design – whether rigid, flexible, permeable,
drainage, and traffic control elements to ensure safety or impermeable – follows mechanistic-empirical or

Volume 22 Issue 4 (2025) 240 doi: 10.36922/AJWEP025190146