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.