Back to the Basics: Bank Station Placement - Part 2

Written by Martin J. Teal, P.E., P.H., D.WRE  |  Vice President, WEST Consultants 
Copyright © The RAS Solution 2017.  All rights reserved. 

Expanding upon Chris’ discussion of where to place bank stations, what should you do about high terrain somewhere in the middle of your cross section?  Here is an example:

How should we treat the left overbank?  It’s hard to tell if the high area next the left bank is isolated (i.e., it would be an island if the water surface were to get to elevation 370 or so) or if it is a continuous feature (such as a levee) that would prevent flow from accessing the left overbank until it is overtopped.  Looking at this another way, is the lower ground of the left overbank a continuous flow path or is it an isolated low spot (for example, a mining pit)? Aerial photography can often help determine the situation; here is the overhead view for our example:

The area in question is vegetated (the terrain goes up steeply when it gets to the storage yard on the bottom of the photograph) but it is hard to tell if the high point in the terrain would be constraining flow or if the low area is a potential flow path.  Looking at the cross sections upstream and downstream of the one in question will often provide answers, but does not help in this particular example.  In this case, the best course of action would be to go out to the river and see for yourself, then imagine how the water would behave.  Depending on your conclusion, there are several ways that this can be modeled.

1.  Isolated high spot.  If flow can simply go around the high spot in this particular cross section then we probably don’t need any further adjustments. You may get a “divided flow” warning in the output that signifies that the program detected dry ground with water on either side, but no action is needed to address the warning in this case. Assuming that the computed water surface elevation is high enough, this solution will also allow flow in the left overbank.

2.  Isolated low spot in overbank.  You could model this as per #1 above but in that case you should check flow distribution between the channel and overbanks up and downstream of this cross section for reasonable transitions (see earlier blog post from May 20, 2009).  Or, if you think that the low area should only store but not convey water you could set an ineffective flow limit as shown below.

3.  Continuous high ground.  If the high ground is really a ridge that would prevent the water from accessing the lower ground in the left overbank, it should be modeled as a levee. However, in this case another decision needs to be made depending on what happens after the levee is overtopped. Will the water be conveyed on the land side, or will it just pond?  If the latter you may need to add an ineffective flow limit at or just to the left of the levee.

4.  Something in between.  Regardless of whether the high or low features of the cross section are continuous, water is able to access the left overbank.  Natural streams often have “backswamp” areas behind either human-made or natural levees that flood and store water but do not really convey much flow downstream. If the left overbank in our example is like this, we could model it by using the ineffective flow limit as per #2 above.  However, ineffective flow means zero conveyance.  If we expect some water to move in the overbank, albeit very slowly, you may want to allow a small non-zero conveyance.  A few sharp-eyed readers may have noticed that we are using a Manning’s roughness coefficient of 0.3 in the left overbank. Using this value allows a small amount of conveyance in that overbank without zeroing it out completely.

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.  

Vertical Walls-What are the Implications?

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants Copyright © The RAS Solution 2014.  All rights reserved.

Many water surface profile software programs prior to HEC-RAS had a quirky rule against having perfect vertical sides.  In other words, you could not have two or more elevations at the same stationing.  A perfect example of this is a rectangular flume.  The workaround for the older software was to define the tops of the flume to be 0.01 ft or meters just outside of the bottom edge of the flume, like this:

In HEC-RAS, you don’t need to do this.  In fact, you shouldn’t do this as you’ll get the wrong answer.  Let me explain why...