Morphodynamic role of wart-type roughness in shaping flow patterns and resistance in steep channels

• Bridges Fluvial Geomorphology and Hydraulic Engineering • Reveals the Morphodynamic Role of Roughness Spacing • Novel Laboratory Analog for Natural Channel Features • High Practical Value for Model Validation and Prediction • Quantitative Framework for Roughness Parameterization Open channel flow, governed by gravity and characterized by a free surface, is critical for applications such as flood control, irrigation, and stormwater management. The Manning formula, with its roughness coefficient ( n ), is a cornerstone for estimating flow velocity and discharge, yet its determination in natural and controlled settings remains complex due to factors like channel geometry and roughness. This study investigates Manning’s roughness coefficient under controlled laboratory conditions, focusing on three bed configurations: a smooth bed, wart-type roughness with 20 cm spacing ( S = 20 cm), and wart-type roughness with 30 cm spacing ( S = 30 cm). Experiments conducted in a 7 m-long recirculating flume at Kyoto University’s Ujigawa Hydraulic Laboratory reveal that roughness significantly increases n , with mean values of 0.00985, 0.0252, and 0.0189 for smooth, S = 20 cm, and S = 30 cm beds, respectively. Dimensional analysis simplified the functional relationship highlighting the influence of Reynolds number and roughness spacing-to-height ratio. Flow visualization showed distinct patterns, including symmetric streams and recirculation zones for rough beds, with closer spacing enhancing turbulence and flow resistance. Statistical analysis via one-way ANOVA confirmed significant differences in n across conditions. These findings provide quantitative insights into roughness effects, offering practical guidance for refining hydraulic models in open-channel systems.