Background: The accurate determination of Manning's roughness coefficient (n) is critical for open-channel flow modeling and hydraulic design. However, existing methods are often subjective, labor-intensive, or fail to account for the dynamic complexities of sediment transport. This study addresses this gap by proposing a simplified, yet robust, alternative method for determining n using a limited number of velocity measurements.

Methods: Laboratory experiments were conducted in a rectangular flume under both equilibrium and non-equilibrium sediment transport conditions. Using an Acoustic Doppler Velocimeter (ADV), detailed velocity profiles were measured to establish the relationship between flow characteristics and bed roughness. A novel theoretical framework was then derived to calculate Manning's n based on velocity measurements at just two specific points (e.g., 0.2 and 0.8 of the flow depth). The results were validated by comparing the calculated n values with those derived from traditional methods and full velocity profiles.

Results: The two-point velocity method successfully determined Manning's n with high accuracy across all experimental conditions. The results revealed a clear influence of both equilibrium and non-equilibrium sediment transport on the roughness coefficient, with distinct variations observed in each regime. Statistical analysis showed a strong correlation and low error between the n values obtained from the proposed method and those from traditional approaches, confirming the method's reliability.

Conclusion: The developed two-point velocity method offers a practical and accurate alternative for determining Manning's roughness coefficient. It overcomes the limitations of traditional methods by providing a rapid, objective, and data-driven approach that is particularly valuable in dynamic, sediment-laden open channels. This research significantly advances the field by providing engineers and hydrologists with an improved tool for hydraulic analysis and modeling.