Just in case you did not see it:

>2. Arcement G.J. and Schneider V.R. 1989. Guide for selecting
Manning's n roughness coefficients for natural channels with flood
plains. USGS Water Supply Paper #2339, USGS, Denver CO, 38 p.
As the 1 reference is this article is available online as well under
www.fhwa.dot.gov/bridge/wsp2339.pdf.

Florian pappenberger