This will probably will require separating the bed shear stress and side-wall shear stress. Probably the attached paper by Guo and Julien (2005) entitled "Shear Stress in Smooth Rectangular Open-Channel Flows" could help.

Dear Dr Bong. Thanks for the article. I've seen it before. The problem is, among variety methods presented, which one is working well or better or more reliable. I wanted to know if someone has an experience about this.

If your purpose is a standard calculation on alluvial channel for real world application, bed shear in the classic way of uniform regime should give you good rough numbers suitable for fairly good predictions.

Dear Dr. Moñino, I meant local shear stress not mean shear stress. There are geometrical methods which are used to predict shear stress distribution, but they are not so accurate.

Do you have (or can you obtain) velocity measurements?

I meant theoretically calculation of shear stress distribution. tow=gamma*R*S.

If my answer is yes what's next dear Dr. Fernandes?

Dear Mr Engineer Farhadi,

Please follow my papers on Boundary Shear Stress in Open Channels.

regards.