I have a couple cross sections (at bridges) where I am getting the following error message in the output table:
“When the Manning’s n value for the channel was composited, the computed n value was larger [smaller] than the largest [smallest] user entered n value. The n value has been set to the largest [smallest] entered value. The user may wish to examine this cross section and enter a single n value for the entire channel.”
I am using horizontally varied N-values typically ranging from .025 in the channel bottom and .05 along the vegetated banks, occasionally with walls, dirt paths, etc. included. Has anyone encountered this error and figured out how to fix it? I could enter a single N-value for the section as the message suggests, but I’d like to be able to run the model at a range of flows with significantly different wetted perimeters. I have a fairly irregular natural channel and I am wondering if there are aspects of the channel geometry that could be triggering this message…
Thanks!