Multi-Criteria Hydraulic Design of Roadside Ditches

Roadside ditches play a fundamental role in roadway drainage systems by conveying surface runoff, preserving pavement performance, and mitigating erosion-related deterioration. However, conventional hydraulic design procedures typically require iterative calculations and independent verification of hydraulic capacity, flow velocity, and material-related design criteria, making the design process time-consuming and potentially inconsistent. This study presents a multi-criteria hydraulic design methodology for triangular roadside ditches that integrates hydraulic capacity, minimum self-cleansing velocity, maximum allowable velocity, longitudinal slope, and material-related performance criteria within a unified design framework. The methodology combines the Rational Method, continuity equation, and Manning’s equation to generate capacity–velocity–slope relationships for lined and unlined roadside ditches and was implemented in a web-based computational environment. Analytical verification of nine lined cross sections at three representative longitudinal slopes comprised 27 geometry–slope combinations and 54 hydraulic values, showing agreement between analytical calculations and computational outputs apart from numerical rounding. For unlined ditches, seven predefined ditch-floor materials were evaluated through two Manning roughness levels; increasing n from 0.020 to 0.025 reduced both mean flow velocity and discharge capacity by 20% under identical geometry and slope conditions. The results demonstrate that maximum hydraulic capacity alone does not define the most appropriate design, because admissible solutions must simultaneously satisfy conveyance, velocity, slope, and material-resistance requirements. The proposed methodology transforms conventional sequential hydraulic verification into an integrated multi-criteria framework for faster, standardized, and reproducible roadside ditch design.