Tuesday, July 29, 2014

Tuesday Tip of the Week–Shift to Pan

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants

Copyright © The RAS Solution 2014.  All rights reserved.

 

As most HEC-RAS users quickly find out, there are some convenient tools for managing your view in an HEC-RAS graphic.  Zoom In, Zoom Previous, Zoom Out, Full Plot, and Pan.  These generally work how you think they should and are available on all of the HEC-RAS graphics including the geometry schematic, profile plots, cross section plots, xyz plots, etc.  Click on Zoom In, and you can click once on the graphic to zoom in centered on the location you clicked.  You can also click and drag a box to zoom into.   Zoom Previous, takes you to the previous view extents.  Zoom Out allows you to zoom out similar to how you zoomed in. 

When checking input data in the geometry schematic, I frequently use the pan option.  Panning allows the user to click and move the view around, while maintaining the same view magnification.  This is particularly useful in spatially large systems if you want to move up and down your reach to check on bank station locations, placement of ineffective flow triggers, and cross section alignment.  The  pan tool allows you to “pull yourself” along the stream centerline, checking data as you go.  It’s also very convenient in the graphical cross section editor. 

There are two ways to get to the pan menu item:  you can right-click on the view and you’ll see a dropdown menu with all of the view options, as well as a bunch of display options.  Or you can go to the View menu option at the top of the graphic.  Either way, you are clicking a couple of times to get the pan feature.  Furthermore, the pan feature, when selected this way, remains active (you continue to see a “hand” for the cursor).  You have to go back to the menu and uncheck the pan option to get out of pan mode. 

 

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An easier way to pan around, when you want to quickly view schematic data or output data in your graphics is to use the Shift key on your keyboard.  The Shift key is a shortcut for panning.  As long as you hold the Shift key down, your cursor will be a “hand” and you will be in pan mode.  Release the shift key and you are back to normal mode.  Try it out.  It is much easier to get around your geometry schematic to check data this way. 

Monday, July 28, 2014

Deleting Portions of HEC-RAS Cross Sections Quickly

Written by Chuck Davis, P.E., CFM  |  WEST Consultants
Copyright © The RAS Solution 2014.  All rights reserved.  


Ahead of the wide-scale implementation of the 2-dimensional (2-D) modeling module in HEC-RAS, I wanted to share a quick tip that will be useful for editing your 1-dimensional (1-D) HEC-RAS model to prepare to couple your 1-D channel with a 2-D overbank area. It’s common to have a cross section with a well-defined channel and a wide, shallow floodplain in the overbank adjacent to the channel. This can be seen in the figure below. As can be seen from this figure, flow in the channel (between the bank stations denoted by the red dots) will have to reach a stage of approximately ten feet before spilling into the left overbank.

Figure1

Tuesday, July 8, 2014

Tuesday Tip of the Week–HTAB Headwater Maximum for Bridges

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants
Copyright © The RAS Solution 2014.  All rights reserved.



Have you ever looked at your profile plot from an unsteady flow model and found “walls of water” on top of your bridge?  Or just unexplained unrealistically large amounts of energy loss over a bridge?  Take this for example:
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Notice the three large “steps” in energy grade line (and water surface elevation).  It’s hard to see in the figure, but these all take place at bridges.  The very first thing you should do in this case is review the HTAB parameters for the bridges. 
 
Notice here the Head water maximum elevation is set to 773 m.  If you look at the cross section of the plot behind it, the bridge deck itself is around 773 m in elevation.  HEC-RAS uses the Head water maximum elevation to put an upper limit on the elevation that the HTAB rating curves are computed to. 
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If RAS is limited to rating curves that end abruptly at the bridge deck elevation, then any energy elevations computed during the simulation that are higher than the bridge deck were computed by extrapolation.  By rule of thumb, any time HEC-RAS has to extrapolate, results can get strange, as we see in the first figure above.  But, you don’t want to make your Head water maximum elevation too high, because that will stretch out the rating curves and decrease your resolution-also a bad thing in HEC-RAS unsteady flow. 
The fix:  Increase the Head water maximum elevation for the bridge to an elevation that is slightly greater than the maximum energy elevation that will occur at that bridge during the simulation.  This of course requires an initial guess (we don’t know what the maximum elevation is until we run it), and generally some trial and error afterwards.  In this case, I increased the Head water maximum for this bridge to 780 m.  I also like to increase the Number of Points, Number of Submerged Curves, and Number of Points on each Submerged Curves to improve the resolution.  I did the same thing for the other bridges in the data set.  The result is an expanded range of HTAB rating curves for the bridge and a nice smooth water surface profile.
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And you can check the cross section plot for the bridge to make sure your Head water maximum elevation is high enough (but not too high).  Notice here the EG Max WS elevation is 773.99 m.  We could probably lower the Head water maximum elevation to 775 m, but since it’s working fine at 780 m, I’ll leave it here. 
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