Friday, April 21, 2017

Back to the Basics: Bank Station Placement

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.