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.

A considerable quantity of weeds is drafted with flow due to maintenance activities and moving vessels causing serious problem to hydraulic structures and hydropower plants along open channels. Aquatic weed trash screens are normally constructed upstream hydraulic structures to control the drafted weeds and preventing them from passing through the hydraulic structures. The effect of the presence of accumulated aquatic weed upstream trash racks on water surface profile is the main disadvantage of trash racks. In this research, the effect of the presence of trash rack and the accumulated weeds upstream it with different densities on water surface profile is studied experimentally. Ninety runs were applied in the hydraulic lab of Channel Maintenance Research Institute. Equations relating different hydraulic parameters and accumulated weed density to the water surface profile were established. It was concluded that accumulated weed depth is the most effective parameter on heading up, while the blocked area of trash rack screen and extension length of weed upstream the barrier are the most effective parameters on head loss.