Cross Section Points Filter

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants

Copyright © The RAS Solution 2014.  All rights reserved.

As RAS model development is done more and more through GIS, using the HEC-GeoRAS extension, cross section point filtering is becoming a standard part of the HEC-RAS model development exercise.  It’s not too uncommon for a model created in GIS to have multiple hundreds of cross section station elevation points.  This is normally not a problem, as long as you don’t exceed 500 points (the maximum allowed for any cross section in HEC-RAS.  However, do an interpolation with bounding cross sections that have more than 255 points each, and you will have interpolated sections with more than 500 points.  Try to run that model and HEC-RAS will give you a message like this:

Fortunately, this is easily remedied.  Simply use the Cross Section Points Filter tool located in the Tools menu item in the Geometry schematic:

In this tool, you can filter a single cross section, or do multiple cross sections together.  For a single cross section, either go to the cross section indicated in the error message, or scroll through the river station dropdown box until you find one with more than 500 points.

Once you’ve selected your cross section(s), you have a number of different ways to filter.  Near and Colinear filtering will remove points that are either very close to another point, or that are in line with other points.  You just provide the tolerances (tell RAS how close is too close, etc.) . Once you enter the tolerance values, press the “Filter Points on Selected XS”, and check to see if you removed enough points to get below 500.  If not, you’ll have to tighten your tolerances.  This can take a little bit of trial and error as you figure out what tolerance levels will remove enough points. 

My preferred method for filtering is to use the “Minimum Area Change” method.

Here, all you do is enter in the desired number of cross section points you wish to have in the cross section (500 or less), and then HEC-RAS will remove points until it gets to that number.  No trial and error necessary.  BUT, you do not have control on which points are removed.  HEC-RAS uses a built-in scheme to remove points while minimizing the change in cross section area.  At first I was a bit reluctant to use this feature, thinking that it might drastically alter the shape of my cross section.  However, when comparing the original and filtered cross sections, I’ve never noticed a visual difference.  The “minimum area change” scheme that is used by HEC-RAS does a very good job at preserving the shape and hydraulic characteristics.  Give it a try next time you need to filter. 

Filtering Station Elevation Data in HEC-RAS

Written by Daniela Todesco | WEST Consultants
Copyright © RASModel.com. 2013.  All rights reserved.

I am working on a dam breach project where I’m trying to simplify a complicated model with dozens of tributaries and storage areas. In the process, I’m converting a few tributaries into storage areas (which are surrounded by other storage areas). For example, I want to convert the reach below (from 5.43 to 0.55) to a storage area. We happen to have the topographic data used to generate the original model (although we don’t have the original geodatabase), so we’re good to go. In order to do that, we need to delineate the storage area polygon, define new storage area connections, and generate lateral structures. The new “reach” (now a storage area) will look like the one below. Now we need station-elevation data for storage area connections and lateral structures. Easy, right? Well, not so fast. HEC-GeoRAS seems to have some issues exporting data when you take an existing project and try to modify shapefiles and extract data for only a portion of the entire model. It is especially difficult if you’re not the one who built the model in the first place. The shortcut is to think outside GeoRAS and obtain simple 3D station-elevation points directly from a TIN or grid for the storage area or structures you need. The bad part about that is that extracting tools in ArcGIS doesn’t really have an option to limit the number of points that are generated, and you end up with thousands of points that you can’t really use in HEC-RAS (the limit for cross section points, storage area connection weir/embankment points, and lateral structure weir points is 500). For example, in the connection called 83Coal in the figure above, I end up with over 2300 points. I’m stuck. Or so I thought… It turns out that RAS automatically “expands” its Station Elevation table to store however many points you need. In my case, 2339… HEC-RAS will still give you an error if you try to run the model with this data (the limit of 500 points still stands for computations!), but a simple click of the “Filter” button you see in the figure above solves the problem. Voila! Hope you liked this new (for me) discovery.

Extending your Cross Sections to High Ground?

Copyright © RASModel.com. 2013.  All rights reserved.

What are the implications of having a cross section that is too short and doesn't extend all the way out to the highest computed water surface elevation?  Does it affect the results?  Take this cross section for example. It is missing much of the left overbank (presumably).

Image courtesy of Adam Bohnoff

First of all, when RAS encounters this situation, it will automatically extend the last station elevation point vertically to the height of the computed water surface.  This adds a so-called "vertical wall" to the end of the cross section.  Additional wetted perimeter will be included for water that comes into contact with the "vertical wall". 

So what does this mean?  Well, you will be missing out on wetted area-possibly a LOT of wetted area.  Maybe it's negligible.  It's up to you to decide.  For typical rivers, the added wetted perimeter associated with the "vertical wall" will not make much of a difference in the results.  If you plan on mapping the computed flood plain in RAS Mapper, or in GIS using the GeoRAS extension, you'll miss out on some areas that should be shown as inundated. 

I see a few possible scenarios that you would need to consider.  Your course of action will depend on whether your model is steady or unsteady, and how much error you're willing to accept at this location:

1.  The missing wetted area is actually very small.  Either the maximum water surface elevation just exceeds the end point or perhaps there is a bluff just to the left of the first station elevation point that would contain all of the water.  In this case, you probably don't waste time getting additional survey data and leave the cross section as is, or you manually approximate in a station elevation point to capture the bluff. 

2.  There is considerable flow area that is missing, but it is so far out in the overbank or it's in a flow separation area and it can all be considered ineffective.  In a steady flow model, you can probably leave this as is.  Ineffective flow area is ignored in steady flow computations.  The answer will be slightly different if you extended the cross section and put in an ineffective flow trigger.  This is strictly due to the difference in quantified wetted perimeter.  For typical rivers, where the width is much greater than the depth, this will make little difference in your results.  For unsteady flow, there could potentially be a huge error in the results if you leave the cross section as is.  In unsteady flow modeling, ineffective flow areas are accounted for as hydraulic storage in HEC-RAS.  Hydraulic storage will attenuate the flood wave as it progresses through a system.  Omitting available storage can significantly affect both the propagation and attenuation of your flood wave.  I strongly recommend extending the cross section to high ground in this case. 


For steady flow, the differences in RAS will be very slight between these two options, limited to the wetted perimeter computed added at the vertical wall (ineffective flow assumes a frictionless boundary). In unsteady flow, these two options could produce VERY different results. 

3.  There is considerable flow area that is missing, and it is actively conveying flow.  In this case, steady, or unsteady, you'll want to extend the cross section to high ground.  Omitting this portion of your cross section will have a direct impact on the computed water surface elevation.  The degree to which depends on how much of the cross section area you are omitting, but it could be quite significant. 

So...how do we extend the cross sections?  In a perfect world, you'd have your survey crew go out and get you more points.  Unfortunately this cost money and takes time, frequently both of which you don't have an excess of when doing a hydraulic model study.  If your RAS geometry is already set up in GIS and your terrain model extends far enough laterally, you could simply extend the cross section cut line to the high ground and reimport into RAS.  Easy! 

However, if you do not have a georeferenced model and you can't get your survey crew out to the field in a timely (and cost-effective) manner, you can always approximate the extension of your cross sections using a USGS topo map. 

These "Quad" maps can be found for free on-line for any location in the US.  In fact, there are similar topography data sets for just about the entire world-available on-line for free.  The downside is that their resolution is quite inadequte for typical river modeling, and they don't include bathymetry (underwater topography).  However, for the purposes of extending your cross section to high ground, this can be an acceptable alternative to a physical survey. 

Simply find and download a terrain map that covers your area of concern.  Locate your existing cross section line on that map.  Then extend it to high ground, marking the locations where your cross section line crosses contour lines. Note the elevations, and the relative distances between contours, then manually enter that data as new station elevation points.