Exploring HEC-RAS: XS Interpolation Within a Reach-Part 2

Written by Chris Goodell | August 4, 2014

The last post on XS interpolation gave an overview for reasons cross section interpolation may be necessary and a detailed description of Interpolation Option A:  “Within a Reach”.   http://hecrasmodel.blogspot.com/2014/05/exploring-hec-ras-xs-interpolation.html
This post will cover Option B:  “Between 2 XS’s”.  Between 2 XS’s simply means HEC-RAS will interpolate cross sections ONLY between two selected “non-interpolated” cross sections.  It will not interpolate over a reach.  Obviously the disadvantage here is that you have to interpolate between pairs of cross sections one-at-a-time, which could prove to be  a long exercise for a large model.  However, Option B has a huge advantage over Option A, in that you are able to control how interpolation is done, by drawing your own connecting Master Chords.  More on that in a bit.  First, to initiate interpolation with Option B, go to the Geometric Data window and select Tools…XS Interpolation…Between 2 XS’s…

When Option B is selected, the following window will open:


Here we can see two non-interpolated cross sections.  *Caution-in HEC-RAS, the cross sections displayed in the XS Interpolation Editor are drawn backwards to normal convention.  This is the only place in HEC-RAS where you are looking in the upstream direction when viewing cross sections.  The following plots show the same two cross sections in the cross section plotter, oriented correctly (left to right looking downstream).

Now, let’s say we wish to add an interpolated cross section between these two.  I usually put in a value for Maximum Distance, but in Option B, you can also interpolate by Constant Distance or Set Location.  Since the original distance between the two cross sections is 833, if I put in a value less than 833 for the Maximum Distance, RAS will interpolate by evenly spacing the interpolated cross section(s), and updating the box “Distance Between XS’s” with the resulting spacing.

In this example I simply added the Maximum Distance value and pressed the Interpolate New XS’s button.  Quick and easy, but this essentially interpolates the same way as Option A.  However, if you look closely, you can see a few discrepancies that should be addressed.  First of all, it appears that the levee (the pink boxes) doesn’t fall on the high ground feature in the interpolated section.  Also, the side channel is not treated as a common feature between the two bounding cross sections, so instead of preserving that feature in the interpolated section, RAS linearly diminishes it.  The result is two smaller side channel features in the interpolated section.

In Option B, we have the advantage of preserving common features by connecting them with user defined master chords: 
Click this button and you can snap lines from one bounding cross section to the other, to connect common features like levees, dikes, side channels, etc.  The other button,  will allow you to “cut” master chords (remove them).  By default, RAS automatically draws master chords connecting the end points, the bank stations, and the channel invert (the green lines-the invert is a red line, since even though it is automatically added by RAS, it is removable by the user).  The next figure shows the user defined master chords (red lines) that I drew to connect the side channel feature and the high ground that the levee sits on.

And the result illustrates how the side channel and levee are both preserved in the interpolated cross section.