Conceptos Basicos Hec Ras

8/14/2019 Conceptos Basicos Hec Ras

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CONCEPTOS BASICOS HEC RAS

I am working on a dam breach project where Im trying to simplify a complicated model with dozensof tributaries and storage areas. In the process, Im converting a few tributaries into storage areas(which are surrounded by other storage areas). For example, I want to convert the reach below(from 5.43 to 0.55) to a storage area. We happen to have the topographic data used to generate theoriginal model (although we dont have the original geodatabase), so were good to go.
http://lh3.ggpht.com/-KNJiSfcrXqM/UnpbZF00CAI/AAAAAAAAA_U/lAIBmgHTMqE/s1600-h/clip_image002%255B4%255D.jpghttp://lh3.ggpht.com/-KNJiSfcrXqM/UnpbZF00CAI/AAAAAAAAA_U/lAIBmgHTMqE/s1600-h/clip_image002%255B4%255D.jpg


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In order to do that, we need to delineate the storage area polygon, define new storage areaconnections, and generate lateral structures. The new reach (now a storage area) will look like theone below.

Now we need station-elevation data for storage area connections and lateral structures. Easy,right? Well, not so fast. HEC-GeoRAS seems to have some issues exporting data when you take anexisting project and try to modify shapefiles and extract data for only a portion of the entire model. Itis especi ally difficult if youre not the one who built the model in the first place. The shortcut is tothink outside GeoRAS and obtain simple 3D station-elevation points directly from a TIN or grid forthe storage area or structures you need. The bad part about that is that extracting tools in ArcGISdoesnt really have an option to limit the number of points that are generated, and you end up withthousands of points that you cant really use in HEC -RAS (the limit for cross section points, storagearea connection weir/embankment points, and lateral structure weir points is 500). For example, inthe connection called 83Coal in the figure above, I end up with over 2300 points. Im stuck. Or so Ithought

It turns out that RAS automatically expands its Station Elev ation table to store however manypoints you need. In my case, 2339
http://lh3.ggpht.com/-04VR749KvLM/UnpbaKlkEmI/AAAAAAAAA_g/GA8sJaOTEH8/s1600-h/clip_image003%255B5%255D.jpg


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HEC-RAS will still give you an error if you try to run the model with this data (the limit of 500 pointsstill stands for computations!), but a simple click of the Filter button you s ee in the figure abovesolves the problem. Voila! Hope you liked this new (for me) discovery.

Posted by Chris G. at 7:06 AM 1 comments

Email ThisBlogThis !Share to TwitterShare to Facebook Labels: Connection , Filter , GeoRAS. , GIS , HEC-RAS , Lateral Structures , Station-Elevation Points , Storage Areas

Reactions:TUESDAY, OCTOBER 8, 2013

Stabilizing a Dynamic Unsteady HEC-RAS Model Written by Chris Goodell | WEST ConsultantsOne of the most frustrating aspects of unsteady HEC-RAS modeling can be the model stabilizationprocess. You know, youve gone to great lengths collecting the best survey/topo data and solidhydrology. Then youve painstakingly spent hourspossibly days entered all of that data only tofind that once you press the Compute button, the model crashes. The dreaded Red Bar!
http://www.blogger.com/profile/00354834185663924786http://www.blogger.com/profile/00354834185663924786http://hecrasmodel.blogspot.com/2013/11/filtering-station-elevation-data-in-hec.htmlhttp://hecrasmodel.blogspot.com/2013/11/filtering-station-elevation-data-in-hec.htmlhttp://hecrasmodel.blogspot.com/2013/11/filtering-station-elevation-data-in-hec.html#comment-formhttp://hecrasmodel.blogspot.com/2013/11/filtering-station-elevation-data-in-hec.html#comment-formhttp://hecrasmodel.blogspot.com/2013/11/filtering-station-elevation-data-in-hec.html#comment-formhttp://www.blogger.com/share-post.g?blogID=1675184707067447729&postID=2081402977766950204&target=emailhttp://www.blogger.com/share-post.g?blogID=1675184707067447729&postID=2081402977766950204&target=twitterhttp://www.blogger.com/share-post.g?blogID=1675184707067447729&postID=2081402977766950204&target=twitterhttp://hecrasmodel.blogspot.com/search/label/Connectionhttp://hecrasmodel.blogspot.com/search/label/Connectionhttp://hecrasmodel.blogspot.com/search/label/Connectionhttp://hecrasmodel.blogspot.com/search/label/Filterhttp://hecrasmodel.blogspot.com/search/label/Filterhttp://hecrasmodel.blogspot.com/search/label/Filterhttp://hecrasmodel.blogspot.com/search/label/GeoRAS.http://hecrasmodel.blogspot.com/search/label/GeoRAS.http://hecrasmodel.blogspot.com/search/label/GeoRAS.http://hecrasmodel.blogspot.com/search/label/GIShttp://hecrasmodel.blogspot.com/search/label/GIShttp://hecrasmodel.blogspot.com/search/label/GIShttp://hecrasmodel.blogspot.com/search/label/HEC-RAShttp://hecrasmodel.blogspot.com/search/label/HEC-RAShttp://hecrasmodel.blogspot.com/search/label/HEC-RAShttp://hecrasmodel.blogspot.com/search/label/Lateral%20Structureshttp://hecrasmodel.blogspot.com/search/label/Lateral%20Structureshttp://hecrasmodel.blogspot.com/search/label/Lateral%20Structureshttp://hecrasmodel.blogspot.com/search/label/Station-Elevation%20Pointshttp://hecrasmodel.blogspot.com/search/label/Station-Elevation%20Pointshttp://hecrasmodel.blogspot.com/search/label/Station-Elevation%20Pointshttp://hecrasmodel.blogspot.com/search/label/Storage%20Areashttp://hecrasmodel.blogspot.com/search/label/Storage%20Areashttp://hecrasmodel.blogspot.com/search/label/Storage%20Areashttp://hecrasmodel.blogspot.com/2013/10/stabilizing-dynamic-unsteady-hec-ras.htmlhttp://hecrasmodel.blogspot.com/2013/10/stabilizing-dynamic-unsteady-hec-ras.htmlhttp://www.blogger.com/email-post.g?blogID=1675184707067447729&postID=2081402977766950204http://lh5.ggpht.com/-tFGOEnWDJhk/Unpba2zjouI/AAAAAAAAA_w/A9dNULtCXMo/s1600-h/image%255B3%255D.pnghttp://www.blogger.com/email-post.g?blogID=1675184707067447729&postID=2081402977766950204http://lh5.ggpht.com/-tFGOEnWDJhk/Unpba2zjouI/AAAAAAAAA_w/A9dNULtCXMo/s1600-h/image%255B3%255D.pnghttp://hecrasmodel.blogspot.com/2013/10/stabilizing-dynamic-unsteady-hec-ras.htmlhttp://hecrasmodel.blogspot.com/search/label/Storage%20Areashttp://hecrasmodel.blogspot.com/search/label/Station-Elevation%20Pointshttp://hecrasmodel.blogspot.com/search/label/Lateral%20Structureshttp://hecrasmodel.blogspot.com/search/label/HEC-RAShttp://hecrasmodel.blogspot.com/search/label/GIShttp://hecrasmodel.blogspot.com/search/label/GeoRAS.http://hecrasmodel.blogspot.com/search/label/Filterhttp://hecrasmodel.blogspot.com/search/label/Connectionhttp://www.blogger.com/share-post.g?blogID=1675184707067447729&postID=2081402977766950204&target=twitterhttp://www.blogger.com/share-post.g?blogID=1675184707067447729&postID=2081402977766950204&target=twitterhttp://www.blogger.com/share-post.g?blogID=1675184707067447729&postID=2081402977766950204&target=emailhttp://www.blogger.com/share-post.g?blogID=1675184707067447729&postID=2081402977766950204&target=emailhttp://hecrasmodel.blogspot.com/2013/11/filtering-station-elevation-data-in-hec.html#comment-formhttp://hecrasmodel.blogspot.com/2013/11/filtering-station-elevation-data-in-hec.htmlhttp://www.blogger.com/profile/00354834185663924786


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Sometimes you canget your simulation to complete without crashing, but the listed numerical errors are so high that youcant with good conscience submit that as your final simulation. Either way, approaching anunsteady HEC-RAS model (especially a dynamic one) as a beginner with little experience andunderstanding of how to stabilize it can cause significant delays in your project and worse,completely blow up your budget. Ive uploaded two HEC -RAS projects to the following DropBoxSite :https://www.dropbox.com/sh/ebbaz8es6d4ncdf/flYbMC16bP

RawleyResUnstable.prj is an unsteady flow dam breach HEC-RAS project recently sent to me forhelp. Although the model ran to completion without crashing, it had unacceptably high errors.RawleyResStable.prj is the fully stabilized version of the model with no numerical errors.

The following lists out the courses of action taken to stabilize the model. Feel free to download theUnstable and Stable models and try these techniques on your own. The links following some ofthese items will take you to more information about that particular technique.1. Cross Section Spacing. The initial spacing was way too coarse. A visual check alone of thegeometry schematic and profile plot should encourage you to investigate a finer cross sectionspacing.
https://www.dropbox.com/sh/ebbaz8es6d4ncdf/flYbMC16bPhttps://www.dropbox.com/sh/ebbaz8es6d4ncdf/flYbMC16bPhttps://www.dropbox.com/sh/ebbaz8es6d4ncdf/flYbMC16bPhttp://lh4.ggpht.com/-KpE2NKuTvIQ/UlRNE5yAIvI/AAAAAAAAA6E/B_iqpIdzCWg/s1600-h/clip_image002%255B5%255D.jpghttps://www.dropbox.com/sh/ebbaz8es6d4ncdf/flYbMC16bP


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Geometry Schematic

Profile Plot Samuels equation suggests anywhere from 15 ft to 50 ft spacing (depending on whatbed slope you use). I interpolated to 50 ft for the entirereach .http://hecrasmodel.blogspot.com/2008/12/samuels-equation-for-cross-section.html
http://hecrasmodel.blogspot.com/2008/12/samuels-equation-for-cross-section.htmlhttp://hecrasmodel.blogspot.com/2008/12/samuels-equation-for-cross-section.htmlhttp://hecrasmodel.blogspot.com/2008/12/samuels-equation-for-cross-section.htmlhttp://lh6.ggpht.com/-YVrL_yU1-C0/UlRNGrwXtEI/AAAAAAAAA6g/EmNkqL46NPc/s1600-h/clip_image006%255B4%255D.jpghttp://lh3.ggpht.com/-A7kIaJ_I8sY/UlRNFzhX--I/AAAAAAAAA6Q/WZ0LdgAtTx4/s1600-h/clip_image004%255B4%255D.jpghttp://lh6.ggpht.com/-YVrL_yU1-C0/UlRNGrwXtEI/AAAAAAAAA6g/EmNkqL46NPc/s1600-h/clip_image006%255B4%255D.jpghttp://lh3.ggpht.com/-A7kIaJ_I8sY/UlRNFzhX--I/AAAAAAAAA6Q/WZ0LdgAtTx4/s1600-h/clip_image004%255B4%255D.jpghttp://hecrasmodel.blogspot.com/2008/12/samuels-equation-for-cross-section.html


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up a very low depth at the downstream boundary (which was causinginstabilities) .http://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.html

Notice in the following profile plot of the downstream end of the reach how the water surface at theboundary cross section is below critical depth (the red dot). This creates an overestimation of thewater depth at the next upstream cross section, which in turns creates some instability over the next

several timesteps.

The proper way to handle this would be to find out what is downstream of your model and select aboundary condition that best represents those conditions. In absence of downstream conditions, the0.001 slope for normal depth provides a reasonable solution. Either way, this underscores theimportance of moving your downstream boundary far away from your area of interest in your studyreach. That way the errors that do originate from your downstream boundary assumption will havediminished to negligible levels before impacting your area of interest.
http://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.htmlhttp://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.htmlhttp://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.htmlhttp://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.htmlhttp://lh4.ggpht.com/-Gur0Vr51Shc/UlRNI6sQyVI/AAAAAAAAA7U/sHrnmq1pMPk/s1600-h/clip_image012%255B4%255D.jpghttp://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.htmlhttp://hecrasmodel.blogspot.com/2010/01/downstream-boundary-normal-depth.html


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4. In the unsteady flow editor, the initial flow and the first time step flowshould always be equal. In the original model, the initial flow is left blank (which is actuallyokay because RAS will use the first timestep flow if left blank). However, the first time stepflow is very low at 0.14 cfs. Peak discharge of your inflow hydrograph is around 600 cfs. Iput in a baseflow (minimum flow) of 5% of this, which is 30 cfs. Just to be safe, I put 30 cfsin the initial flow input box as well. Its important to make sure that the 30 cfs baseflow doesnot have an impact on the peak of the breach outflowhydrograph. http://hecrasmodel.blogspot.com/2009/02/minimum-flow-requirements.html 5. The upstream end of your reservoir is very steep (14%). This one was easy to spotin the profile plot because of the very nice smooth profile plot followed by a sudden spike inthe energy level (green dashed line) at the upstream end. Notice to that there is acorresponding supercritical solution at the upstream most cross section. These are bothindications that more cross sections are neededhere. http://hecrasmodel.blogspot.com/2010/01/another-reason-for-interpolated-cross.html
http://hecrasmodel.blogspot.com/2009/02/minimum-flow-requirements.htmlhttp://hecrasmodel.blogspot.com/2009/02/minimum-flow-requirements.htmlhttp://hecrasmodel.blogspot.com/2009/02/minimum-flow-requirements.htmlhttp://hecrasmodel.blogspot.com/2010/01/another-reason-for-interpolated-cross.htmlhttp://hecrasmodel.blogspot.com/2010/01/another-reason-for-interpolated-cross.htmlhttp://hecrasmodel.blogspot.com/2010/01/another-reason-for-interpolated-cross.htmlhttp://lh3.ggpht.com/-KYQpLLP1Npo/UlRNJo7otHI/AAAAAAAAA7g/hGEApJJTEdk/s1600-h/clip_image014%255B4%255D.jpghttp://hecrasmodel.blogspot.com/2010/01/another-reason-for-interpolated-cross.htmlhttp://hecrasmodel.blogspot.com/2009/02/minimum-flow-requirements.html


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I added more cross sections by interpolating the steep slope at the upstream end of the reservoir(10 ft spacing).
http://lh4.ggpht.com/-iXcvvwbAxyo/UlRNKmDNkOI/AAAAAAAAA7w/NkgzmcEnFtA/s1600-h/clip_image016%255B4%255D.jpg


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Notice the energy spike is not completely gone, but it is much better. Refining the HTAB parameters(http://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.html ), turning on mixed flow andbumping up n values will collectively take care of the rest of this (see numbers 8, 9, and 10 below).

7. From RS 4577 to 3791 you have your main channel defined in a small elevatedside channel. The right overbank is lower in elevation than the channel.
http://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.htmlhttp://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.htmlhttp://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.htmlhttp://lh4.ggpht.com/-mac1l-Ae_R8/UlRNLQ2W3kI/AAAAAAAAA8A/KvteZyRvjPc/s1600-h/clip_image018%255B4%255D.jpghttp://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.html


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This is problematic in RAS. Using the graphical cross section editor, I redefined the bank stations toget rid of this problem.
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I readjusted the cross section HTAB parameters to reflect the new bank station definitions (I usedthe Copy Invert button for al l cross sections). While in there, I maximized the resolution of theHTAB parameters by changing number of points to 100 and minimizing the increment as much aspossible while still having full coverage of each crosssection. http://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-

line.html http://hecrasmodel.blogspot.com/2011/03/more-on-htab-parameters.html
http://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.htmlhttp://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.htmlhttp://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.htmlhttp://hecrasmodel.blogspot.com/2011/03/more-on-htab-parameters.htmlhttp://hecrasmodel.blogspot.com/2011/03/more-on-htab-parameters.htmlhttp://lh6.ggpht.com/-wzJB_xhfSFw/UlRNNhXL35I/AAAAAAAAA8w/EGKJLZ6nkes/s1600-h/clip_image024%255B4%255D.jpghttp://hecrasmodel.blogspot.com/2011/03/more-on-htab-parameters.htmlhttp://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.htmlhttp://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.html


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9. In the figure below, notice all of the red areas in this zoomed in section of the upstream end of the reach.
http://lh3.ggpht.com/-joydLl1Uy4U/UlRNPOn-SgI/AAAAAAAAA9E/Y37RS3TbYy0/s1600-h/clip_image026%255B4%255D.jpg


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These are areas that are near critical depth. I turned on the Mixed Flow option in the UnsteadyFlow Analysis window, which helps stabilize the near critical depths at the upstream end of thereservoir. http://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.html Ifyouve been following along correcting your own copy of the model, youll notice that the model now

finally runs to completion without crashing. There are still a few minor errors as shown in theComputation Messages.
http://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.htmlhttp://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.htmlhttp://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.htmlhttp://lh5.ggpht.com/-_eWxXRvQpyQ/UlRNQH6AscI/AAAAAAAAA9U/5H5jvf2esE0/s1600-h/clip_image028%255B4%255D.jpghttp://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.html


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Its debatable w hether you need to take care of these relatively small errors. Particularly for a dambreach model, where the presumed errors of the input data probably far outweigh these smallnumerical errors. Nevertheless, theres a sense of pride in putting out a very robust model,completely free of numerical errors. Solets continue.

10. The errors in the computation window above suggest a problem around river station6200. That happens to be the upstream end of the reservoir and the source of the error canclearly be seen with the energy grade line and critical depth turned on.
http://lh3.ggpht.com/-9Pg5rhmeTy4/UlRNRCHCFkI/AAAAAAAAA9k/zKrCYvLtZLo/s1600-h/clip_image029%255B4%255D.png


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The section at the upstream end of the reservoir is very steep at about 13.5%. In fact, thats greaterthan the suggested maximum slope of 10%, as stated in the HEC-RAS manuals. You can see asthe pool starts to lower, the steep reach is exposed and because of the low n values, the watersurface is calculated to be supercritical. Lets assume that the geometry is correct, and the

upstream end of the reservoir really is that steep. Jarretts Equa tion suggests very high n valuesshould be used here (~0.2), based on the bed slope and hydraulic radius (after the breach when thereservoir has drained). The original model had 0.07 and 0.035 for the overbanks and main channel,respectively. I changed all the n values in this steep reach to 0.15, because I think 0.2 might be alittle overkill. http://hecrasmodel.blogspot.com/2009/12/n-values-in-steep-streams.html
http://hecrasmodel.blogspot.com/2009/12/n-values-in-steep-streams.htmlhttp://hecrasmodel.blogspot.com/2009/12/n-values-in-steep-streams.htmlhttp://hecrasmodel.blogspot.com/2009/12/n-values-in-steep-streams.htmlhttp://lh6.ggpht.com/-BpKhJ9t-WjY/UlRNR97q7KI/AAAAAAAAA9w/YUX92So3hx0/s1600-h/clip_image031%255B4%255D.jpghttp://hecrasmodel.blogspot.com/2009/12/n-values-in-steep-streams.html


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There no errors! However, there is still a Warning about extrapolating above/beyond the ratingcurve at a bridge (R.S. 277).


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http://lh5.ggpht.com/-19sEHCUikyc/UlRNSph2ErI/AAAAAAAAA-E/0aYVjGGIJ-s/s1600-h/clip_image032%255B4%255D.png


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11. To get rid of that warning, I increased the HTAB headwater maximum elevation atRiver Station 277 (the bridge) from 278 ft to 280 ft so that RAS doesnt have to extrapolate.

There we have it. A clean solution. No errors, no warnings.
http://lh3.ggpht.com/-HvZd1JZG4Ac/UlRNUMt2bGI/AAAAAAAAA-Q/y4kmdUwsWbk/s1600-h/clip_image033%255B4%255D.png

Conceptos Basicos Hec Ras

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