The Best Design And Performance for Grassed open Channals

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, theflow resistance was commonly calculated by one of the famous flow formulaslike Manning’s formula. The net cross-section was used for the wetted area andthe hydraulic radius. The result is a roughness coefficient, such as n, whichrepresents mainly the roughness on the wetted perimeter . The flow formulaswere originally derived for sections with little to no obstruction elementspresent, such as concrete lining or large alluvial channels . However severeweed growth reduces the effective flow area considerably and the flow velocitywithin the vegetated part is considerably neglected . Therefore it has beenassumed that the obstructed part has no effect on the discharge capacity.According to the above assumption, analysis process has been applied oncollected field measurements along two main vegetated canals suffering fromsubmerged weeds in upper Egypt in order to achieve the best design andperformance of grassed open channels. Also to help irrigation engineers indetecting the most suitable time of maintenance program for the vegetated openchannels depending on what efficiency is needed. Through this study, it wasassumed that the flowing discharge passes only through the net area free ofweeds. So open channel dimensions were calculated for the net area of thecross-sections free of weeds. It has been calculated a net hydraulic radius, a nethydraulic mean depth, and new open channel dimensions that calculated for thenet cross-section area. Then Manning’s equation has been used to calculate thecanal dimensions in case of no vegetation then in case of vegetation growth fordifferent weed densities. Also the hydraulic efficiency has been calculated tostudy the effect of weed growth and the change in channel dimensions on theflow efficiency.