Aug 18 2011

The Water Does Not Flow There

Another assumption modelers have to make when setting up a 1-D model is the location of ineffective flow areas.  Again, this is not that difficult where the flow is primarily 1-D.  It can get complicated really quick the more hydraulically complex a reach becomes.  Before I show some examples I have a question to ask.


How do you know where the ineffective flow areas are?  There are some guidelines you can follow like from NHI courses, as shown below, but I am interested in what experience you pull from to know where flow will be ineffective.

Ineffective flow areas

I have benefited tremendously from my summer “’internship” during college as a whitewater rafting guide where I also learned how to kayak, which I still love to do.  Just spending time on rivers has helped me understand how water flows around obstructions, through bridges, the changes that occur due to high or low flow, and where eddies form.

One of the other things that have improved my 1-D modeling is the exposure I have had to 2-D modeling.  Seeing the flow fields in complex areas helps me better understand how water actually flows through a specific reach.

Let’s look at some results from the Lynches River model one more time to better understand areas of ineffective flow.  I am just going to be focusing on upstream of the RR embankment and downstream of Hwy 1.  Below is a plot of the 100-yr velocity contours and vectors.  The areas of purple color indicate velocities below 0.25 feet.


As a reminder, ineffective flow areas, as designated in HEC-RAS, are areas of a cross section that are not actively conveying flow downstream.  The location of ineffective flow areas in HEC-RAS is dependent on where the cross sections are located.  I included two cross sections just for this example.  I also included the same plot with water surface elevations.   Notice that in this case the areas of ineffective flow are not in the ‘standard’ shape and size.  In this case, the multiple openings, skew of the RR embankment, and floodplain topography affect the direction and distribution of flow.

100-yr_Inective flow _Velocity

100-yr_Ineffective flow _wse

Now let’s look downstream of the Hwy 1 crossing.

100-yr_Ineffective flow _Velocity_ds

100-yr_Ineffective flow _wse_ds

The cross sections are just for example purposes, but the point is that not all ineffective flow areas fit with the standard 1:1 contraction and 2:1 expansion, before and after a bridge.  Next time you are creating a HEC-RAS model, make sure your ineffective flow areas make sense for your specific reach of river.

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