A Preview of RAS2D, two-dimensional modeling in HEC-RAS

Written by Aaron A. Lee | WEST Consultants

I’ve gotten a chance to play around with the alpha version of HEC-RAS 4.2 and check out the program’s new two-dimensional (2D) modeling capabilities. From what I’ve seen this will be a really useful feature! Being the alpha version of RAS2D there are still features in development, and is bound to be a bit “buggy”, but it is definitely worth looking forward to.

What is it?
The new build of RAS will allow users to connect 2D flow elements to a 1D river system. You will now be able to model overland areas as dynamic, 2 dimensional grids, rather than level pool storage areas. The figure below shows the 1D and 2D features together in the geometry window.

• The 2D area can be drawn in as a polygon (much like a storage area)
• The grid cells are automatically generated at a user-specified size within the 2D area
• Cells can be added, removed, or edited manually

How does it work?

The 2D mesh and the 1D system are tightly coupled during an unsteady simulation. This means that water surface elevation is calculated at each XS and each grid cell for every timestep, allowing direct feedback at the connections. 2D flow areas can be linked to the 1D system the same way storage areas are. The Figure shown below is a schematic of how the 2D mesh is built, and how RAS routes flow from cell to cell.

Upon creating the 2D mesh, you need to load a digital terrain file (.flt format). Both the pre- and post-processing steps for the 2D flow area are done through RAS Mapper.

• RAS pre-processes the 2D mesh separately from the 1D system. During this process RAS creates an elevation-storage curve for each cell, and calculates hydraulic properties for each cell face. These hydraulic properties are similar to the cross section hydraulic properties (HTAB curves).
• The Cell Center is where water surface elevation is computed for the whole cell.
• Cell Faces control flow between cells by acting as a detailed XS. Station/elevation data is captured directly from the underlying terrain file.
• Cell Face Points are used for stationing to connect to a lateral structure. They also represent the ends of cell faces.
• Manning’s n-values for each cell will be assigned by a spatially varied polygon or manually entered.

Computationally, RAS will allow the user to choose between using 2D Diffusion Wave equations (default), or the full 2D Dynamic Wave equations. Most flood applications should be adequately modeled using the Diffusion Wave equations.

What are the advantages?
Besides computing in 2-dimensions, the main advantage is the program’s ability to maintain computational robustness while preserving the details of the underlying terrain. Smaller features (i.e. drainage ditches) that run through large cells will be captured in the hydraulic properties of the cell faces. Therefore, these features will be preserved and accounted for both computationally and visually, even though they are smaller than the grid cell size, as demonstrated in the RAS2D output displayed in the figure below. Traditionally, many 2D models require cell size to be consistent with the size of the features to be included. Not the case with RAS2D.

Additional advantages include:

• Cells can be any size and shape. This allows the user to model odd-shaped features within the 2D flow area as well as provide more computational detail around areas of interest.
• Faster computation times. RAS uses an implicit scheme to calculate water surface and flow at each XS and cell simultaneously. The implicit scheme is also more stable, which allows for larger cell sizes.
• Detailed mapping. RAS Mapper will be able to post-process results to map depth and velocity grids on detailed terrain.
• RAS2D will be able to utilize multiple processors (if available).

New Forum Up!

First, let me apologize for the downtime with the previous HEC-RAS Bloggery Forum.  There were some issues with the forum hosting service.  Unfortunately, I lost all of the previous posts and comments.  The upside is that now I have much more control over content/storage. 

Please have a look at the new forum and post comments/questions/replies.  I encourage you all to register to the forum and don't worry, no spam!  You can get to the forum by clicking on the page link above titled "Forum" or by pasting this web address in your internet browser:  http://hecrasmodel.blogspot.com/p/hec-ras-bloggery-forum.html.  There is also a link to the right in the "Welcome" section. 

Since undoubtedly, many of the previous forum commentors will be wondering where the old forum went, please help me spread the word about this new forum.

Thanks-
Chris

 

Dam Breach Class Boston, MA May 1-3, 2013

Written by Chris Goodell | WEST Consultants

Hi everyone. For those of you who are HEC-RAS dam breach enthusiasts, I will be teaching a 3-day course on dam breach modeling with HEC-RAS in Boston in a few weeks. The course will be held next to the historic MIT campus overlooking the Charles River and downtown Boston, May 1-3. In this course you'll learn all the standard and some of the not-so-well-known techniques for stabilizing and improving the accuracy of very dynamic and stubborn unsteady flow HEC-RAS models (dam breach models are notorious for this). You'll also learn state-of-the-art approaches to dam breach modeling, including level-pool versus dynamic reservoir drawdown and probabilistic techniques for dam breach modeling. Hope to see you there.

Check here for more information:
http://mylearning.asce.org/diweb/catalog/item/id/78019/q/c=79&t=2116&t=2122

If you can't make this one, the next one is in Denver September 11 - 13, 2013. 

Chris

HEC-RAS File Types

Written by Aaron A. Lee   | WEST Consultants

Behind the scenes, HEC-RAS automatically creates a series of input and output files when working with a model. It is important to know what each of these files does and how they fit into the overall scheme of your project. Keeping track of these files in an organized manner is good practice, especially as your models grow in size and complexity. This post will feature a steady flow example project, and will list common HEC-RAS files that you’ll see for Unsteady flow, Sediment Analysis, Water Quality, and Hydraulic Design projects.  Although these examples all use the number "01" in the extension, RAS can have multiple instances of each of these files for a given project (except the .prj-only one of those).  Numbers can go as high as "99" and are assigned in the order in which the files were created.   This screenshot is a folder containing the input files generated by RAS after opening and saving one of the installed example projects. At the very least, you need these input files to run the model. If someone asks you to send them your model, these files must be sent, at a minimum. Differences between Steady and Unsteady files are listed when relevant.

• .prj is the Project file. Contains current plan files, units and project description.
• .g01 is the Geometry file. Cross-sectional data, hydraulic structures and modeling approach data are stored here.
• .f01 is the Steady Flow file. Profile information, flow data and boundary conditions written in this file.
• For Unsteady Flow, .u01 is the flow file extension. This is where hydrographs and initial conditions are stored, as well as any user-defined flow options.
• For Quasi-Unsteady Flow (for a sediment analysis), .q01 is the flow file extension.
• .p01 is the Plan file. Contains a list of the associated input files, and all simulation options.

These are all text files and can be directly read and edited in a text editor. The following screenshot shows the input and output files after the steady flow model has been run. Note that some of these are only used by RAS as intermediate files during computations.

• .O01 is the Output file. Contains all of the computed results from the associated plan. This file is written in binary format and can only be read from the user interface.
• For Unsteady Flow, a .dss file is automatically generated as an output. This file contains time series data that is viewable by any program that can read dss files (typically HEC software).
• If your model links to a dss file for use as input data (e.g. inflow hydrographs, stage hydrographs, observed data, etc.), then that .dss file will be necessary to run the model and should be included in your group of files you send to a reviewer.
• .r01 is the Run file for steady flow analysis. Contains all of the necessary input data required for the RAS computational engine. The run file is created during the model simulation, and is not required to view final results.
• For Unsteady Flow, .x01 is the extension.
• .comp_msgs.txt is the Computational Message text file. Records the computational messages that pop up in the computation window. The messages file is not required to view final results, but can be useful in troubleshooting errors identified by RAS.
• .hyd01 is the Detailed Computational Level output file. This can be switched on in the Unsteady Flow Analysis window.
• .p01.rst is a Restart File (also called a Hot Start File, or Initial Conditions File). This option can be switched on by the user in the Output Control Options window. See the Hot Start post for more guidance.

For Unsteady Flow analysis, these files are categorized as “intermediate,” which means that they are not essential for running a model or viewing results, since they are recreated by RAS during run-time.

• .c01 is the Geometric Pre-Processor output file. Contains the hydraulic properties tables, rating curves, and family of rating curves for each cross-section, bridge, culvert, storage area, inline and lateral structure. This file is rewritten each time you change your geometry file.
• .b01 is the Boundary Condition file.
• .bco01 is the Unsteady Flow Log output file.
• .p01.blf is the Binary Log file.
• .IC.O01 is the Initial Conditions file.

If submitting your final model to a client or a reviewer, you will likely only send the necessary input files. Sending output files are optional, but will allow the reviewer to avoid rerunning the model on their end. Including the .c## files might be a good idea for larger models so that RAS can skip the pre-processing step. SEDIMENT ANALYSIS

• .S01 is the file extension for Sediment Data. This file contains flow data, boundary conditions, and sediment data.
• .SedCap01 is the extension for Sediment Transport Capacity data. When sediment transport computations are performed, RAS creates a set of intermediate files:
• .sed is the detailed sediment output file.
• .SedHeadXS01 is the header file for the cross section output.
• .SedXS01 is the cross section output file.
• .H01 is the Hydraulic Design data file.
• .H01.SiamInput is the SIAM Input Data file.
• .H01.SiamOutput is the SIAM Output Data file.

WATER QUALITY ANALYSIS

• .W01 is the file extension for Water Quality data. This file contains temperature boundary conditions, initial conditions, advection dispersion parameters and meteorological data. When water quality computations are performed, RAS creates a set of intermediate files;
• .bco is the water quality log file.
• .p01.wqrst01 is the water quality restart file.
• .color_scales is the file that contains the water quality color scale.

Remember that file extensions can be numbered from 01 to 99, and are assigned in the order that they are created.

Two-Dimensional modeling in HEC-RAS

Here's a quick sneak preview of what's coming in 2-D HEC-RAS.  Click either figure below to see an animation of the levee breach simulation.  More to come soon...

 

Extending your Cross Sections to High Ground?

What are the implications of having a cross section that is too short and doesn't extend all the way out to the highest computed water surface elevation?  Does it affect the results?  Take this cross section for example. It is missing much of the left overbank (presumably).

Image courtesy of Adam Bohnoff

First of all, when RAS encounters this situation, it will automatically extend the last station elevation point vertically to the height of the computed water surface.  This adds a so-called "vertical wall" to the end of the cross section.  Additional wetted perimeter will be included for water that comes into contact with the "vertical wall". 

So what does this mean?  Well, you will be missing out on wetted area-possibly a LOT of wetted area.  Maybe it's negligible.  It's up to you to decide.  For typical rivers, the added wetted perimeter associated with the "vertical wall" will not make much of a difference in the results.  If you plan on mapping the computed flood plain in RAS Mapper, or in GIS using the GeoRAS extension, you'll miss out on some areas that should be shown as inundated. 

I see a few possible scenarios that you would need to consider.  Your course of action will depend on whether your model is steady or unsteady, and how much error you're willing to accept at this location:

1.  The missing wetted area is actually very small.  Either the maximum water surface elevation just exceeds the end point or perhaps there is a bluff just to the left of the first station elevation point that would contain all of the water.  In this case, you probably don't waste time getting additional survey data and leave the cross section as is, or you manually approximate in a station elevation point to capture the bluff. 

2.  There is considerable flow area that is missing, but it is so far out in the overbank or it's in a flow separation area and it can all be considered ineffective.  In a steady flow model, you can probably leave this as is.  Ineffective flow area is ignored in steady flow computations.  The answer will be slightly different if you extended the cross section and put in an ineffective flow trigger.  This is strictly due to the difference in quantified wetted perimeter.  For typical rivers, where the width is much greater than the depth, this will make little difference in your results.  For unsteady flow, there could potentially be a huge error in the results if you leave the cross section as is.  In unsteady flow modeling, ineffective flow areas are accounted for as hydraulic storage in HEC-RAS.  Hydraulic storage will attenuate the flood wave as it progresses through a system.  Omitting available storage can significantly affect both the propagation and attenuation of your flood wave.  I strongly recommend extending the cross section to high ground in this case. 


For steady flow, the differences in RAS will be very slight between these two options, limited to the wetted perimeter computed added at the vertical wall (ineffective flow assumes a frictionless boundary). In unsteady flow, these two options could produce VERY different results. 

3.  There is considerable flow area that is missing, and it is actively conveying flow.  In this case, steady, or unsteady, you'll want to extend the cross section to high ground.  Omitting this portion of your cross section will have a direct impact on the computed water surface elevation.  The degree to which depends on how much of the cross section area you are omitting, but it could be quite significant. 

So...how do we extend the cross sections?  In a perfect world, you'd have your survey crew go out and get you more points.  Unfortunately this cost money and takes time, frequently both of which you don't have an excess of when doing a hydraulic model study.  If your RAS geometry is already set up in GIS and your terrain model extends far enough laterally, you could simply extend the cross section cut line to the high ground and reimport into RAS.  Easy! 

However, if you do not have a georeferenced model and you can't get your survey crew out to the field in a timely (and cost-effective) manner, you can always approximate the extension of your cross sections using a USGS topo map. 

These "Quad" maps can be found for free on-line for any location in the US.  In fact, there are similar topography data sets for just about the entire world-available on-line for free.  The downside is that their resolution is quite inadequte for typical river modeling, and they don't include bathymetry (underwater topography).  However, for the purposes of extending your cross section to high ground, this can be an acceptable alternative to a physical survey. 

Simply find and download a terrain map that covers your area of concern.  Locate your existing cross section line on that map.  Then extend it to high ground, marking the locations where your cross section line crosses contour lines. Note the elevations, and the relative distances between contours, then manually enter that data as new station elevation points. 

Quasi Two-Dimensional Modeling in HEC-RAS

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

One of the limitations of HEC-RAS is that it is a one-dimensional model. Simply put, RAS assumes all flow moves along a singular dimension. For a given cross section, all of the flow is assumed to move either downstream, or all of it moves upstream, along the singular dimension (which can be defined as a polyline-does not have to be a straight line). The consequence of this is that there is only one water surface elevation (stage), and one total flow for a given time step at a given cross section. All of the other variables for a given cross section that you see in the profile output table, detailed output table, DSS, etc. are derived from the stage and flow values. This includes the velocity and shear stress distributions over a cross section, which can provide the appearance of a 2-dimensional analysis. But that is all based on a conveyance distribution over geometric segments of the cross section using that single water surface elevation and single total flow.

So why do I bring this up? First, it's always good to know ALL of the limitations of whatever model you're using to predict future outcomes. But I also want to demonstrate the "quasi-2-dimensional" capabilities of HEC-RAS. While planning a hydraulic study in an estuarine environment, you may immediately start thinking about which 2-dimensional model you want to use. But I've seen many great (and creative) applications of HEC-RAS in these 2-dimensional environments that produce very reasonable, if not accurate results.  In short, a quasi-2-d analysis in RAS requires you, the user, to understand up front the likely flow patterns in your study area. This is best accomplished by going out to the field and looking at your site, studying topographic and bathymetric maps, looking at aerial photographs, and simple common sense and experience. Once you've determined your perceived flow paths, all water outside of these flow paths should either be ineffective flow areas, storage areas, or even separate reaches.
  

Here’s an example of an estuarine environment on the Oregon Coast (Yaquina Bay). I haven’t modeled this yet, but if I were, here’s how I would approach my model setup:










1. Draw a stream centerline (blue in the figure) that represents the singular dimension of flow movement-i.e. flow will either move downstream or upstream along in the direction of this line. Cut cross sections at an appropriate spacing, making sure to cover all areas that could get wet during the simulation. Yes, the trib channel south of the main reach is not covered, but I’ll get to that in a second.

2. Define ineffective flow areas. These are areas that you will expect WON’T have flow actively moving along the singular dimension. Be sure to appropriately define expansion and contraction of flow as you draw in the ineffective polygons. All portions of your cross sections that fall within these areas should be set to be ineffective in your RAS model.    


3. Areas that could possibly have a different water surface elevation than the nearest cross section should be split out and modeled as an off-line storage area. Connect that Storage Area to the main reach using a Lateral Structure. You’ll have to come up with a stage-storage curve for the storage area, to be able to model it in RAS. This is a very easy and straight-forward exercise in GIS, as long as you have sufficient topographic coverage. Keep in mind, RAS uses the simplified level pool routing method for Storage Areas. Lateral Structures used for this application will not have an actual “structure” associated with it, so the discharge coefficient you use is very subjective. Typically values on the order of 0.5 to 1.5 are used. Calibrate this if you can.   


4. Alternatively, you can model the tributary as its own reach, connected to the main channel with a junction. This will allow you to model it using the full dynamic St. Venant equations, giving a more physically representative answer in the trib. However, if movement of water through this reach is relatively slow (i.e. typical ebb and flood tides), a storage area will be fine-and easier!  You can get as complex as you want. There are no limitations within RAS to the number of storage areas, ineffective flow areas, lateral structures, and tributary reaches you use. Just keep in mind, the more complex you make it, the more difficult it will be to troubleshoot any instabilities.


The following video is a great example of a quasi-2-d application of HEC-RAS. This very complex model and the video were created by Gary Brunner at the Hydrologic Engineering Center.

HEC-RAS model of the Lower Columbia River Estuary-Courtesy Gary Brunner