الفهرس 
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Abstract
An appropriate flow resistance value is obtained from field measurements.
For a specific flow rate, wetted area and fall in head over a certain distance, the
flow resistance was commonly calculated by one of the famous flow formulas
like Manning’s formula. The net cross-section was used for the wetted area and
the hydraulic radius. The result is a roughness coefficient, such as n, which
represents mainly the roughness on the wetted perimeter . The flow formulas
were originally derived for sections with little to no obstruction elements
present, such as concrete lining or large alluvial channels . However severe
weed growth reduces the effective flow area considerably and the flow velocity
within the vegetated part is considerably neglected . Therefore it has been
assumed that the obstructed part has no effect on the discharge capacity.
According to the above assumption, analysis process has been applied on
collected field measurements along two main vegetated canals suffering from
submerged weeds in upper Egypt in order to achieve the best design and
performance of grassed open channels. Also to help irrigation engineers in
detecting the most suitable time of maintenance program for the vegetated open
channels depending on what efficiency is needed. Through this study, it was
assumed that the flowing discharge passes only through the net area free of
weeds. So open channel dimensions were calculated for the net area of the
cross-sections free of weeds. It has been calculated a net hydraulic radius, a net
hydraulic mean depth, and new open channel dimensions that calculated for the
net cross-section area. Then Manning’s equation has been used to calculate the
canal dimensions in case of no vegetation then in case of vegetation growth for
different weed densities. Also the hydraulic efficiency has been calculated to
study the effect of weed growth and the change in channel dimensions on the
flow efficiency.