Negin Bines - 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 channels, the three dimensional nature of flow, and transport momentum from the side walls to the central region, causes the maximum velocity to occur below the water surface producing the velocity-dip-phenomenon. Therefore, developing new relations to be able to show this phenomenon appears to be necessary. Numerous models for determining 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 profile distribution in narrow sewers. Sewers are designed on the basis of open-channel flow principles. Five models for computing velocity profiles were selected such as Chiu’s, Yang’s Absi’s, Kundu & Ghoshal’s and Bonakdari’s model. To evaluate the accuracy of the models in describing the velocity profile, the calculated results of the models were compared with theexperimental data obtained from an actual measurement site in narrow sewer. The results show that all models except for Konduand Ghoshal’s model represent good agreement whith the experimental dat, but Yang model and both Absi’s models are moresuccessful in predicting the position of maximum velocity, and provides the least errors in most cases

sewer, open channel, velocity distribution, dip phenomenon