Negin Binesh - MSc Student, Department of Civil Engineering, Razi University, Kermanshah, Iran
Hossein Bonakdari - Associate Professor, Department of Civil Engineering, Razi University, Kermanshah, Iran

Measuring the mean velocity profile through the channel cross section is one of basic issues in hydraulic modeling. In wide open channels, the log law and the log-wake are suitable for describing the velocity profile, but in case of narrow open channel flow, such models deviate from experimental data near the free surface. In such open hannels, the three dimensional nature of flow, and transport momentum from the side walls to the central zone, causes the maximum velocity to occur below the water surface producing the velocity-dip-phenomenon. Therefore, developing new relations which is able to show this phenomenon seems to be necessary. Numerous models for etermination of the velocity profile in narrow open channels have been proposed by many researchers. This study provides a comparison among various existing methods for computing the longitudinal velocity distribution in narrow channels. Four models of velocity distribution were selected such as Yang’s, Absi’s, Kundu & Ghoshal’s and Bonakdari’s model. To evaluate the accuracy of the models in calculating the velocity profiles, the results of the models were compared to the experimental data obtained from an actual measurement site in narrow sewer. Sewers are designed on the basis of principles of open-channel-flow. The results show that all models represent good agreement with the experimental data, however, Absi’s model is more successful in predicting the position of maximum velocity, and provides the least errors in most cases. Also it is concluded that for lower aspect ratios (which makes the condition of narrow channels to be satisfied better), the results of the models tend to coincide better withthe measured data.

sewer, compound open channel, velocity distribution, dip phenomenon