Tuesday Tip of the Week–Computing Critical Depth Everywhere for Unsteady Flow

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants Copyright © The RAS Solution 2014.  All rights reserved.   As a follow up to the just-posted Critical Depth Primer, I want to share a trick for getting HEC-RAS to compute and display critical depth at every cross section for unsteady flow.  As I mentioned in the previous post, if you are running a steady flow model, there is an option to have RAS compute critical depth everywhere.  It is located in the steady flow analysis window, under Options…Critical Depth Output Option. This will tell RAS to compute critical depth everywhere, and if you have the critical depth elevation variable turned on in your output plot, you’ll see critical depth plotted at every cross section. 
However…what if we want to have critical depth computed at every cross section for an UNSTEADY FLOW model?  There is no such option in the unsteady flow analysis window.  To make RAS compute critical depth everywhere in unsteady flow, you have to open up the steady flow analysis window, and select Options…Critical Depth Output Option (just like the figure above).  Once you check the box, just close the steady flow analysis window without saving that steady flow plan.  Then go to File…Save on the main RAS window.  There you go.  RAS will now compute critical depth everywhere for that unsteady flow plan.

Critical Depth Primer

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants Copyright © The RAS Solution 2014.  All rights reserved.   Critical depth is an important hydraulic parameter when evaluating hydraulic modeling results.  As we know from our college hydraulics 101 class, water flowing at depths less than critical depth is supercritical and water flowing at depths greater than critical depth is subcritical.  Supercritical flow is characterized by relatively shallow depths and high velocities.  Subcritical flow is characterized by relatively deep depths and slower velocities.  The forces dominating the movement of supercritical flow are inertial, while the forces dominating the movement of subcritical flow are gravitational.  The flow regime (subcritical or supercritical) a particular cross section, or series of cross sections is in, can be determined by the Froude Number, , where F = Froude Number, V = Velocity, g = gravitational constant, and d = depth. A Froude Number greater than 1 indicates supercritical flow, a Froude Number less than 1 indicates subcritical.  A Froude Number = 1 is considered “critical” and possesses the minimum amount of specific energy (potential energy plus kinematic energy, per unit mass).  This is considered an unstable condition in nature and is very rare.  If you ever get a Froude Number = 1 in your results, most likely, there is a problem with the computations (i.e. in steady flow, RAS could not come up with a valid solution, so it defaults to critical depth).  For those of you who have run steady flow HEC-RAS models, you know that there are three flow regime options for computing a steady flow run:  subcritical, supercritical, and mixed flow (both sub and supercritical).  If you select RAS to run in subcritical, and somewhere in your system RAS is not able to compute a valid subcritical answer, then it defaults to critical depth and moves on.  If you select RAS to run in supercritical, and somewhere in your system RAS is not able to compute a valid supercritical answer, then it also defaults to critical depth and moves on.  If you select mixed flow, RAS will compute both a subcritical and supercritical profile and anywhere there is a valid solution for both regimes, RAS will select the one that has the higher specific force value.  If, in mixed flow, there are any cross sections that “default to critical depth”, that means there was a problem with RAS obtaining a solution.  Usually, this means your cross section spacing is too far apart, you are in an area of rapidly varied flow (with not enough cross sections), or just bad input data.  In unsteady flow, contrary to intuition, checking the “Mixed Flow” box in the unsteady flow analysis window does NOT tell RAS to evaluate both sub and supercritical solutions.  This is done anyway in unsteady flow-whether “Mixed Flow Regime” is checked or not.  Let me repeat:  HEC-RAS can compute solutions in both subcritical and supercritical in unsteady flow regardless of whether “Mixed Flow Regime” is checked or not.  Checking the “Mixed Flow Regime” box in the unsteady flow analysis window simply uses a stabilizing scheme for situations near critical depth and with large changes in velocity with respect to time (the local acceleration term in the St. Venant Equation).  This is described further in http://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.html.   It’s very helpful to understand the solution you’ve obtained after running RAS by viewing the water surface profile plot with the “critical depth” variable turned on.  This allows you to gage how close you are to critical depth, and more importantly, allows you to quickly evaluate if your solution has defaulted to critical depth anywhere, indicating a problem with the solution.  The critical depth variable can be turned on in any of the graphical plots by selecting Options…Variables.  Then check the box next to Critical Depth Elevation.    When you do this, RAS will plot critical depth, but ONLY at certain locations.  Notice the plot from the Single Bridge HEC-RAS example data set.  Critical depth (in red) is only plotted downstream of the bridge, and at a couple of cross sections upstream of the bridge.  That is because RAS will only compute critical depth if your answer is supercritical, close to critical depth, or RAS is not able to come up with a valid solution (defaulting to critical depth) and at the boundaries.  If you would like HEC-RAS to compute critical depth everywhere for you, go to the steady flow analysis window, and select Options…Critical Depth Output Option. 
Then check the box next to “Critical Always Calculated.

HEC-RAS Version 5.0 Comments, Suggestions, and Bug Reports

Hi HEC-RAS users.  HEC is extremely busy dealing with bugs and questions at the moment.  They’re currently working mostly to fulfill their obligations for technical support to the Corps of Engineers only.  If you are trying out the new version 5.0 beta, and you discover a bug, or if you simply have a comment or suggestion, please post them to The RAS Solution Forum.  I’ve agreed to collect, filter, and screen bug reports and comments and periodically send to HEC.  You can get to the forum and two new sub-forums: “HEC-RAS Version 5.0 Bugs”, and “HEC-RAS Version 5.0 Suggestions/Comments”, by following this link or by clinking the “The RAS Solution Forum” button up above. 
http://hecrasmodel.blogspot.com/p/hec-ras-bloggery-forum.html

Please post in the appropriate folder and keep the comments civil and constructive.  If you have a bug report, please post in the “bugs” folder and include as much information as is practical, but still keep it as brief as possible.  If you discover a “work-around” in the meantime, please included that as well. 

Also, don’t forget to get the latest version of HEC-RAS 5.0 beta here:
http://hecrasmodel.blogspot.com/2014/10/updated-hec-ras-version-50-beta-now.html

Thanks-
Chris G.
@RASModel

Update to HEC-RAS 5.0 Beta

Hey 5.0 Beta users.  Please get a new copy of HEC-RAS 5.0 Beta.  It has some of the early problems already fixed.  See below for instructions and have fun.  Please read everything below before you download.

NOTE!!! Latest October 2014 beta version is available here:  http://hecrasmodel.blogspot.com/2014/10/updated-hec-ras-version-50-beta-now.html


Dear HEC-RAS 5.0 Beta (with 2D) tester,      You are receiving the this email, and a link to an updated version of "HEC-RAS 5.0.0 June 6, 2014 Beta (with 2D Flow Areas)", because you are either a previous Beta tester, or you have expressed interest in the new 2-Dimensional modeling capabilities being added to HEC-RAS.  A few problems were found over the last two weeks with the first 5.0 Beta release (May 23, 2014), so we fixed those problems and created a new Setup package.  If you already installed the previous 5.0 Beta, please Install this version and use it instead.  Here is the link to the latest version: http://www.hec.usace.army.mil/misc/files/ras/HEC-RAS_5.0_Beta_2014-06-06.exe     

This link lets you download a self extracting archive file called "HEC-RAS_5.0_Beta_2014-06-06.exe"  Download the file to a separate directory and run the self extracting archive file.  The self extracting archive file contains:
         
1. The installation setup package called: " HEC-RAS_5.0_Beta_2014-06-06_Setup.exe"                
2. An updated PDF version of the User's Manual called "Combine 1D and 2D Modeling with HEC-RAS.pdf"  Here is a separate Link to the new 1D/2D User’s manual, if you want to download it separately: http://www.hec.usace.army.mil/misc/files/ras/Combined_1D_and_2D_Modeling_with_HEC-RAS.pdf           
3. Two 2D Test Data sets in a directory called "RAS_50 Test Data"     

This installation setup package is password protected.  The password to run the setup package is: "ras4you!"     

This version of HEC-RAS has many new features.  I have enclosed a PDF File called "What’s New in HEC-RAS 50 Beta.pdf", that describes most of the new features. The main new features are the following:

1.         2D Only Computations (no 1D elements) and Multiple 2D areas in the same model
2.         Several New Ways to Link 2D Flow Areas to 1D Elements
3.         Hydraulic Structures Inside of 2D Flow Areas
4.         Improved Computational Speed for 2D and 1D
5.         New HEC-RAS Mapper Features and Output Capabilities           
6.         New Computational Options for 2D and 1D           
7.         Improved Speed and Memory Usage of the 2D Pre-Processor
8.         Additional 2D Flow Area Initial Conditions Options
9.         New Sediment bank erosion capabilities (USDA-ARS Bank Stability and Toe Erosion Model             (BSTEM))
10.       Unsteady Flow Sediment Transport Modeling (please Contact Stanford Gibson at HEC for                   updated Sediment documentation)

IMPORTANT NOTE:  We have changed the file format for how we store Terrain data in HEC-RAS.  To use this version of HEC-RAS, you will need to redo the development of your Terrain model for use in RAS Mapper and 2D modeling.  You will also have to rerun your model to see mapping output.  Please See Section II of the enclosed User’s Manual for building Terrain models in HEC-RAS.  Previously we were using our own file format called the “Tiled Mapping System” (*tms).  We have switched to using the GeoTiff file format (*.tif).  The GeoTiff files are tiled, pyramided, and compressed, just like our own format was.  So it has all the same benefits of the TMS files.  However, GeoTiff is a standard file format, which you can drag and drop into ArcGIS, or other software.  So for compatibility with other industry standard software, we have changed how we make and store terrain data.  We have also switched all of our output grids to the GeoTiff format to make it easier for user’s to get the results into a GIS or other piece of software.     

I provided a link to the updated version of the User's Manual on how to use the 2D modeling capabilities within HEC-RAS 5.0 Beta ("Combined 1D and 2D Modeling with HEC-RAS.pdf".  This document has been dramatically expanded.  If you really want to learn how to use the 2D modeling capabilities in HEC-RAS, Please Read This Manual.  I have spent a lot of time working on this document, and I think anyone who currently knows how to use HEC-RAS for 1D unsteady flow modeling will be able to learn how to use the new 2D unsteady flow modeling capabilities with just the software and this document.  Please give me some feedback on this User’s manual if you find mistakes or there is information you feel is missing, or could be explained better.           

There are two example data sets that come with the self extracting zip file: “Muncie.prj” and “BaldEagleDamBrk.prj”.  The Muncie data set is a 1D river with a 2D Flow Area inside of a Levee system.  The levee system is breached and flow goes into the protected area.  The BaldEagleDamBrk data set has seven different Plans in it.  This data set has examples of all the different ways to use 2D areas.  The purpose of this data set is just to demonstrate all the different Ways 2D areas can be linked to 1D elements, as well as modified computational mesh examples, and hydraulic structures inside of a 2D Flow Area.           

Our Plan is to have approximately a four month Beta test period, then release the Final HEC-RAS 5.0 version this fall at the end of the calendar year.  During this time we hope to get feedback from Users on bugs, interface issues, usability, missing features, documentation, etc…  So, we need you to test this version of the software and provide us feedback.  Please send all comments to hec.ras@usace.army.mil      Thank you for testing HEC-RAS, we look forward to your comments and feedback. Gary W. Brunner, P.E., D.WRE, M.ASCE Senior Technical Hydraulic Engineer Hydrologic Engineering Center. USACE  www.hec.usace.army.mil

International Settings

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

 

To my international HEC-RAS user friends.  It’s time again to remind everyone that you need to have your Region and Language Windows settings set to English (United States), particularly if you are running unsteady HEC-RAS models.  If you live somewhere other than the USA, and you get an error message relating to the “time” data, or “date” inputs, it’s a good bet that you need to change your region and language settings.