Manning roughness coefficient in vegetated open channels

ABSTRACT Manning equation is one of the most common equations used in estimating the mean velocity and discharge in open channels. The equation depends on some measured hydraulic parameters as water surface slope and hydraulic radius of open channel cross-section in addition to the Manning roughness coefficient. Estimating the value of Manning`s roughness coefficient is dependent on the nature of the channel, the number of obstacles that resist the channel flow and the field engineers experience. In vegetated channels, the percentage of infestation seems to be the most effective parameter on the roughness coefficient value. Through this study, based on laboratory investigation and physical model several trials were conducted to deduce an equation to predict the value of the Manning`s roughness coefficient in the vegetated channel based on measured parameters as weed infestation percentage, water surface slope. Two simple equations were deduced based on these parameters to practically estimate the value of the Manning roughness coefficient in vegetated open channels. The results of these equations were successfully verified using field measurements.