Written by Christopher Goodell, P.E., D.WRE |
WEST Consultants
Copyright © The RAS Solution 2017. All
rights reserved.
Lately, I’ve seen a lot of
basic bank station issues for models I have reviewed. Some real basic stuff. So I thought it would be good to go back to
the basics a bit here and review proper placement of bank stations for cross
sections in HEC-RAS.
What do bank stations do for
us? First of all, they separate your
channel into three distinct conveyance zones.
One for the left overbank, one for the main channel and one for the
right overbank. Not every application
has multiple conveyance zones (i.e. canals), but most natural systems do. By segregating out the different conveyance
zones, we are using Manning’s equation to more appropriately determine energy
loss through the system. Here’s an
example of a simple cross section with properly placed bank stations:
Notice the bank stations (the red dots on the plot) also
reside at the grade break between the physical channel and the flatter
overbanks. While this is typically what
is done, remember the correct placement should always be made based on the
location of the change in conveyance.
For example, if you have a lot of thick vegetation down the banks of the
channel, you might conclude that the excessive roughness there pushes the
boundary between conveyance zones down closer to the toe of the banks like so:
Sometimes locating the bank stations are not as obvious as
these examples. For example, where
should the bank stations be placed for a cross section like this?
One might initially conclude that the deeper channel should
get the bank stations in which case you may place them like this:
However, it is important to know what is happening upstream
and downstream of this location before you can make this decision. Perhaps the smaller channel is actually the
main conveyance and there just happens to be a large low-lying area in the left
overbank.
You would only know this by studying the reach above and
below this spot. Having nice aerial
imagery behind the geometry schematic can help to make this decision for
you.
Notice in the figure above, the main channel is very obvious. Even though there may be some low spots in
the right overbank, we can clearly see where the main channel is and the bank
stations have been placed accordingly.
It’s also important to point out that as you move through your reach,
the placement of bank stations should be fairly consistent from cross section
to cross section. Changes in main
channel width should generally be gradual from one cross section to the
next.
One of the most basic steps in constructing your HEC-RAS
model is to go through every cross section and properly place bank
stations. If you are importing your
cross sections from GIS (e.g. via GeoRAS), make sure that your bank line
delineation placed the bank stations properly.
While your bank lines may look like they follow the conveyance
boundaries well, you may see a very different picture once you’ve imported your
cross sections and look at them in cross section view. It’s always important to fine-tune your bank
station placement in HEC-RAS after importing cross sections.
As with most things in HEC-RAS, there are always exceptions
to the rule. The key thing to remember
is that you want to place bank stations so that they capture the change in
conveyance between the main channel and the overbanks and that the resulting
main channel width doesn’t change too drastically from one cross section to the
next.
