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HEC-RAS study of Simike–Nzovwe drainage

standard methodologies. Recent studies emphasize This study aims to evaluate the hydraulic performance
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reliability-based design to enhance highway safety, of side drains along a 1.85 km stretch of the TANZAM
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and the strategic use of traffic control devices has been Highway between Simike and the Nzovwe River. It
widely documented. Equally important is drainage addresses a gap in existing research by incorporating
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design, which plays a critical role in maintaining specific energy variations into the design assessment,
road safety and infrastructure integrity during adverse thereby capturing transitions between subcritical and
weather events. Side drainage systems, comprising supercritical flows and their implications for sediment
side drains, culverts, and other hydraulic structures, are transport. By applying the HEC-RAS model, the study
essential for managing surface runoff, sediment, and provides practical recommendations for optimizing
debris. Typically configured as trapezoidal channels for side drain design to reduce sedimentation, enhance
cost-effectiveness and stability, these systems must be flow efficiency, and ensure system sustainability under
designed to meet hydrologic and hydraulic demands. extreme hydrologic conditions.
However, inadequate design and maintenance can lead
to inefficiencies, such as siltation and localized flooding. 2. Methods
Poorly performing systems exacerbate flood risks,
particularly in rapidly urbanizing areas. Wakjira and 2.1. Study area
Negasa, for example, used ArcGIS to assess drainage The study was conducted along the TANZAM Highway,
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issues in Bale Robe Town, Ethiopia, and attributed specifically within the 1.85 km Simike–Nzovwe
failures to structural design flaws. Similarly, Mkhandi section, which features a trapezoidal-lined drainage
and Mbwete evaluated drainage structures in Dar es channel (Figure 1). This section includes five circular
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Salaam and linked their inadequacy to urban expansion culverts that connect feeder roads to the main highway.
and poor hydraulic planning. The slope along the side drain varies from steep to
Extreme precipitation events and increased surface gentle, impacting the flow dynamics throughout the
runoff, driven by urbanization and climate change, have channel (Figure 2). Over time, this section has been
amplified drainage challenges. Effective mitigation prone to issues, such as sedimentation (silting) and
requires integrating nature-based solutions, intelligent surface runoff, which frequently spills onto the road
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flow control systems, and engineered interventions, carriageway. Figure 3 illustrates the physical condition
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such as forced retention basins. 11,12 The relevance of of the culverts at both the inlet and outlet following a
hydrodynamic modeling in addressing these challenges rainfall event, highlighting the accumulation of debris
is well-established, particularly for assessing flood and sediment that exacerbates drainage inefficiency.
risks, optimizing drainage performance, and guiding
sustainable design strategies. 13-15 Hydrodynamic and 2.2. HEC-RAS software
sediment transport models are increasingly utilized to HEC-RAS is a widely utilized tool for the design
support the design of drainage systems. These models and analysis of roadside drainage systems due to its
simulate runoff, flow transitions, and sediment movement advanced capabilities in simulating complex hydraulic
under various hydraulic conditions. Among the widely conditions. One of its key strengths lies in its ability
used tools are the Hydrologic Engineering Centre’s to model varied water surface profiles under different
River Analysis System (HEC-RAS), Modular Integrated flow regimes – subcritical, supercritical, and critical
Kinematic Emulator 11 (MIKE 11), and Storm Water – making it particularly effective in understanding
Management Model (SWMM). HEC-RAS, developed transitional flow dynamics that influence channel
by the U.S. Army Corps of Engineers, is particularly performance, water velocity, and depth. 26,27 In roadside
well-suited for analyzing one-dimensional open channel drainage applications, flow often transitions from
flows and culvert hydraulics. Its strengths include a subcritical (slow and deep) to supercritical (fast and
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user-friendly interface, extensive documentation, and shallow) due to channel slope, directly affecting
the ability to model sediment transport and variations the system’s runoff management capacity. Accurate
in specific energy. 17-19 In contrast, MIKE 11 offers more representation of these transitions is crucial for ensuring
sophisticated capabilities, including two-dimensional the resilience of drainage infrastructure, particularly
modeling and advanced sediment modules, but it during extreme weather events, when sediment
requires licensing and technical expertise. 20-22 SWMM deposition can significantly reduce flow capacity. HEC-
excels in urban stormwater analysis but is limited in RAS provides tools to simulate such sediment buildup
modeling sediment dynamics in open channels. 23-25 and long-term debris accumulation, enabling proactive

Volume 22 Issue 4 (2025) 241 doi: 10.36922/AJWEP025190146