Overbank Lengths and Meandering Channels

Brett's answer to Noahs questions prompts to ask something that I've
wondered about. The EXTRAN user's manual seems to indicate that when
modeling natural channel sections with left/right overbanks, that the
user cannot specify different values for the lengths. (Actually, it
indicates that any values entered for left and right overbank lengths
are ignored). This would be different then than the approach taken in
HEC-RAS, Unet or Floodwave. My copy of the manual is old and I don't
know if enhancements are made, if I've misunderstood the manual, or
if some of the 3rd party post-processing packages have added
capabilities beyond the original code.

I'm not a direct user of EXTRAN so I don't have any experience with
it. Is this the case? If so, how do EXTRAN modelers treat meandering
natural channels. Is the issue of valley lengths shorter than
overbank lengths ignored, or does the user need to manual enter an
effective overbank length?

In some separate work I've done with UNET, I've found that on many
natural channels, the difference between specifying meander in the
overbank and note can be significant for floodwave timing, due to the
different volumes involved.

Bill,

What you have said is correct: Extran currently does not support the
ability to have different overbank and main channel lengths when the
reach is modeled with a single cross-section. If you need different
lengths, you have to break the reach up into parallel sections.
Hopefully, the ability to specify different overbank and main channel
lengths within a single cross-section will be added in one of the
upcoming releases.

Brett