INFLUENCE OF BED ROUGHNESS AND CROSS SECTION GEOMETRY ON MEDIUM AND MAXIMUM VELOCITY RATIO IN OPEN CHANNEL FLOW

The paper deals with the analytic-theoretical derivation of the relationships between the entropic quantity Φ(M), representing the ratio between the mean and maximum flow velocity, and the relative submergence and aspect ratio, using classical open channel flow equations. Φ(M) is found to be highly dependent on the relative submergence when large or intermediate roughness scale occur, while it might be assumed almost constant for a small roughness scale. Furthermore, considering the hydraulic geometry relationships, it is attempted to relate the relative submergence to the aspect ratio of flow through a log-relationship whose coefficients depend on the local bed slope, with an important implication for hydrological practices. Then, a practical relation between Φ(M) and aspect ratio is proposed and validated in the operative chain for discharge assessment, showing high robustness and stability. The proposed model has been applied to a set of experimental velocity data collected in gaged river sites with different geometric and hydraulic characteristics as well as low, medium and high flows.