HEC-2 Program Methodology

We have recently bought a copy of BOSS-RMS.
I would like to ask you a question concerning the
HEC-2 Program Methodology in the BOSS-RMS manual.
The question is about the composite roughness
coefficient on page 8-17, equations 8.3.1 and 8.3.2.

This methodology was developed by the Army Corps of
Engineers, and fully described within the document
"Hydraulic Design of Flood Control Channels,"
Engineer Manual EM 1110-2-1601. Office of the Chief
of Engineers. 1970. This document can be retrieved
from the Army Corps of Engineers.