As a side issue regarding the variability of roughness coefficients, the
Hydrologic Engineering Center did a study regarding the variance in
roughness coefficient estimates with Corps of Engineers employees taking
training courses at HEC in the mid-1980's. The compared the variation
in roughness coefficients estimated by inexperienced engineers taking
the introductory HEC-2 class to the roughness coefficients estimated by
the more experienced engineers taking the advanced HEC-2 and unsteady
flow classes. They provided the students with a USGS publication showing
roughness coefficients for various channels. They showed slides of
various streams studied by HEC around the country. They found that the
variation in roughness coefficient estimates was greater for the more
experienced engineers. They attributed this to the more experienced
engineers relying less on the USGS publication and using their
experience in modeling streams to estimate the roughness coefficient. I
am not aware of a publication documenting the results of the study.

John G. Morgan
john.morgan@cte-eng.com