New Geospatial editing tools coming in HEC-RAS Version 5.1

Written by Chris Goodell
Copyright © The RAS Solution 2017.  All rights reserved

New geospatial editing tools are currently in development for the next version of HEC-RAS-Version 5.1.  While no date has been set for the release of Version 5.1, it is expected sometime within the next year or two.  But no guarantees!  This will be an exciting new addition to HEC-RAS in that users will no longer be reliant on 3rd party software like ArcGIS or AutoCAD to develop geometric features like cross sections, river lines, flow lines, storage and 2D areas, etc.  All of this will be done within the HEC-RAS program environment.  And this will effectively eliminate the need for HEC-GeoRAS.

HEC recently published it's Spring 2017 Newsletter, which has a very nice article by Cameron Ackerman ,P.E., D.WRE on some of the new tools and capabilities that will be available within HEC-RAS Version 5.1 for extracting GIS information from digital terrain models.  The newsletter can be read here: 

http://www.hec.usace.army.mil/newsletters/HEC_Newsletter_Spring2017.pdf. 

Cameron's article on new geometric editing features in Version 5.1 starts on page 20.  

The new geospatial editing toolbar will serve as the base point for adding and editing geospatial features in HEC-RAS and will be docked to the RAS Mapper window.  It includes editing tools for adding new features, selecting/editing features, undo and redo, and  plotting terrain profiles.

With these tools, you'll be able to add geospatial features and interact with them directly within HEC-RAS.  Here you see the ability to move stream centerline points in RAS Mapper:

HEC also plans to have interactive help messaging while working in the geospatial environment to help the user identify errors in the geometry setup.  The following figure shows help messages for locations where bank lines do not intersect cross sections, which would result in missing bank stations.

These new features will make it much easier to identify problems with your model construction and fix those problems without having to leave HEC-RAS.  Here we see a poor definition of study limits not allowing for proper mapping of the flood inundation zone.  By quickly adjusting the study limit polygon, a more appropriate inundation map can be drawn.

This is just a brief preview of all of the exciting geospatial editing tools HEC-RAS will include in Version 5.1.  Please read the article by Cameron Ackerman in the HEC Spring 2017 Newsletter for more information.  

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.  

Deleting Portions of HEC-RAS Cross Sections Quickly

Written by Chuck Davis, P.E., CFM  |  WEST Consultants
Copyright © The RAS Solution 2014.  All rights reserved.  


Ahead of the wide-scale implementation of the 2-dimensional (2-D) modeling module in HEC-RAS, I wanted to share a quick tip that will be useful for editing your 1-dimensional (1-D) HEC-RAS model to prepare to couple your 1-D channel with a 2-D overbank area. It’s common to have a cross section with a well-defined channel and a wide, shallow floodplain in the overbank adjacent to the channel. This can be seen in the figure below. As can be seen from this figure, flow in the channel (between the bank stations denoted by the red dots) will have to reach a stage of approximately ten feet before spilling into the left overbank.