Earlier this week, Engineered Efficiency announced the release of the EE ProPack Hydro add-on into the wild.  [Hopefully before this post hits, but if not it should be real close].  In Part 1 of the Hydraulic Modeling series, we discussed the theory of the River Reaches and cross-sections. 

HEC-RAS reads cross-section from left to right looking downstream.  Cross-sections should be drawn perpendicular to the flow of the water.  When you install the HEC-RAS example projects, the geometrics might be a little confusion.   Most of the files appear to be parallel.

This leads me to the topic of this post – Civil 3d Sample Line Groups.  (it is related to hydraulic modeling, just bear with me)

Sample Line Groups are design to be completely perpendicular to the alignment.  At the bottom of the create sections dialog box is the ability to select existing polylines.   This would be the recommended practice to add your HEC-RAS cross-sections to the Sample Line Group.

But, here is the gotcha.  Just watch it.  Look at the image below.  What’s wrong?

Review the grips.  The little cyan triangular grip is the station location of this section, right? Is 5+03 the correct station? 

The easiest way to remedy this conundrum – Grip it! Move the Station Grip to the correct intersection of the alignment and the polyline.  Grip the remaining Sample Line grips to the correct polyline locations.  

It appears Civil 3d pulls the station information from the Left end of the polyline and a line normal to the alignment. This will cause some very odd section numbers if your creek alignment is curvilinear.  

Then,… use the EE ProPack Hydro to export the data out to HEC-RAS.

[March, 4, 2009 update: Note this will not happen if your polylines are down at elevation zero.  Since I used my polylines to also apply the floodplain elevation, my polylines that I converted to Sample Lines are up at elevation. – Matt]