Hydraulic Modeling (Part 1)

Ever since I started as a young engineer, I have been tasked with the Hydraulic Modeling for many projects.  That is, I have used nearly everything and anything to pull cross-section data to model in WPS2, LISLE, HEC-2, and HEC-RAS.    Too many hours spent reading poor copies of dot-matrix-microfished data has left my wrist sore and my eyes a little tired.   At least people left me alone as I punched away at the number keys as a skillful accountant might during the middle of tax season. 

Back in the days of DCA / Softdesk – we drew polylines,  cut and labeled sections, and manually entered data.  When the tools became available, we exported the HEC-2 output.   The polylines pretty well stayed the same with some of the more automated tools. 

One thing remained constant – we always need to demonstrate, in plan, where this cross-section information is located.

Enough sappy history, Matt.   What does this have to do with Civil 3d ?

Let’s cover some theory first.


Let me refer you to the HEC-RAS Reference Manual – Chapter 3 (v 4 March 2008). Quoting from the HEC-RAS Manual, cross-sections should be drawn or queried in the following manner:

The general approach to layout out cross sections is to ensure that the cross sections are perpendicular to the flow lines.  This requires an estimation of what the flow lines will look like in the overbank areas away from the main channel.  One option is to draw the stream center line down the main channel along what is perceived to be the center of mass of flow.  The same thing should be done for the left overbank and the right overbank.  The assumed flow paths for the channel and overbank areas are shown as dashed lines in Figure 3-4.  These lines will not only help in drawing the cross section perpendicular to the flow lines, but they also represent the path for measuring the reach lengths between the cross-sections.


A little later on, the HEC-RAS Reference Manual also states:

Each cross section in a HEC-RAS data set is identified by a River, Reach, and River Station label.  The cross section is described by entering the station and elevation (X-Y data) from left to right, with respect to looking in the downstream direction. The River Station identifier may correspond to stationing along the channel, mile points or any fictitious numbering system.  The numbering system must be consistent, in that the program assumes that higher numbers are upstream and lower numbers are downstream.

(Emphasis added by me)

“Okay, Why are you pointing this out?” you ask…

If you think about AutoCAD Civil 3d’s alignments, and cross-sections – they are formatted the other way.  The Alignment direction is from Low Station to High Station.  Cross-Sections are viewed left-to-right looking UP station.

Now, if we just had some tools to do this with…


  1. Charles Barnes says:

    After being involved with HEC2 and HEC-RAS for the past 15 years, I’ve come to one conclusion, these programs are GARBAGE. Just like many other programs written by or for the Government, they are clumsy, counterintuitive, and just plain hard to work with. Yet in order to get our reurired approvals we are forced to work with them. If any private company would have produced such programs they would have been bankrupt because nobody would buy them. I hope, no pray that you or sombody else can produce a quality product. Sorry for the rant, but I could have been much more harsh with my criticism than I was.
    Charles Barnes, PE

  2. Why could you not just run your alignment downstream?

  3. Mark –

    I run River Stations Low as the downstream end of the Reach.

    If I ran my C3d alignment backwards, my low alignment station is now the upstream.

  4. Mark,
    It is an interesting catch-22.

    If you run your alignment from upstream to downstream then your sections may be cut the right way but your alignment is running the wrong way.

    If you run your alignment from downstream to upstream then your alignment is running the right way but your sections are right to left when looking downstream.

    Thus, the reason we spent a lot of time when creating EE ProPack Hydro to try to look at every scenario and correct this. For EE ProPack Hydro, create your alignment correctly (downstream to upstream) and we will handle the cross sections for you.