Jun 21 2011

The Big Assumption

When I graduated from Colorado State University and entered the working world I had very limited exposure to hydraulic modeling.  I had taken fluid mechanics and hydraulic structures where I was first introduced to a hydraulic model.  I also had some exposure to HEC-RAS, but didn’t even know that 2-D modeling existed nor how it was different than 1-D modeling.  Although I new how to setup a basic HEC-RAS model I did not fully understand how everything worked, or all of the assumptions that I was making when creating and running the model.

2D Model_velocity

Over the course of my career I have had the opportunity to learn from some very knowledgeable hydraulic engineers and have worked on several 2-D models for a wide variety of applications.  Having this exposure to 2-D modeling has really helped me become a better 1-D modeler because I am more aware of the assumptions I am making in the 1-D environment.

While there are several assumptions to be aware of when setting up a 1-D model, one of the biggest ones is what you are doing when drawing cross sections.  Cross sections should be cut so as to be perpendicular to the anticipated flow lines.  The more 1-D the flow actually is the easier this is to do, like in a canal.  When you cut a cross section you are essentially stating that at this location the water surface is equal across the entire section.  When confronted with a more complex hydraulic condition the anticipated flow lines are a lot more difficult to assume, which can lead to errors in your model.

Of course the best way to understand this is visually, so lets look at an example.  Below is an aerial image and USGS Topo Map of the Little River at the Highway 122 crossing east of Barney Georgia.  The floodplain is a little over a mile wide.  Flow is from north to south The skew between the floodplain and the roadway alignment is around 45°.  There is also a relief bridge east of the main span bridge.  Model limits are shown in blue.

Little River aerial

Little River topo

So, how would you position cross sections for a HEC-RAS model?  Remember cross sections represent equal water surface elevation.

In the next post I will show the 2-D model results that display the water surface elevation contours across the floodplain.

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