Manning in vegetated floodplains 1

[macBrown]

Hi,
I'm searching for a way to extimate manning coeff. for floodplains with both large trees and grass on the ground.
I've got 2 problems:
- how changes the manning coeff. with the depth of flow? I know that after full submergence the manning number becomes smaller but I can't find values...
- When i choose manning for the grass and the trees, how can I put them togeter?
Thank you very much...

Riccardo

 

[TerryScan]

I recommend looking into the document "Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains - United States Geological Survey Water-supply Paper 2339

http://preview.tinyurl.com/37rtjm

It has pretty comprehensive coverage of your question.