Tuesday, March 3, 2009

Caution with interpolating cross sections

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2009. All rights reserved.

The interpolation feature in HEC-RAS makes it very easy to add cross sections to satisfy numerical spacing requirements. Althought the routines are very robust and easy to use, caution should be used when interpolating between cross sections with multiple blocked ineffective flow areas. While normal ineffective flow areas are interpolated quite well (provided there are ineffective flow triggers on both bounding cross sections), multiple blocked ineffective flow areas are NOT interpolated.

So if you have multiple blocked ineffective flow areas on either of the bounding cross sections, you must manually add the multiple blocked ineffective flow areas to the interpolated cross sections. I suggest using the graphical cross section editor when doing this, especially if you have a lot of cross sections to fix.


  1. In addition to IFAs, I have had a lot of trouble with Manning's n values on interpolated cross sections. When I have several n values across a cross section, the interpolation generally causes a mess on the n value placement on the interpolated cross sections. It generally takes some time to clean up the cross sections.

  2. I have had issues with the way HEC-RAS sets the n value locations when interpolating cross sections. If you have several n value changes across a cross section, the interpolation function can jumble them up. It can be a real pain to set them up correctly.

  3. I totally agree about the n-values. That is why if I have a highly dynamic model (i.e. dam breach), I try to stick with the traditional 3-n value method (1 each for lob, ch, rob). Interpolation of n values is much better that way (the interpolated sections keep the 3-n value mode).