Exploring HEC-RAS: XS Interpolation Within a Reach

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

Copyright © The RAS Solution 2014.  All rights reserved.

I’m starting a new series called “Exploring HEC-RAS”.  Each post will discuss a feature in the menu items, starting with the Tools menu item in the Geometry Schematic.  Today’s topic is “XS Interpolation Within a Reach”.

There are two options for interpolating cross sections in HEC-RAS:  “Within a Reach…”, let’s call this Option A, and “Between 2 XS’s…”, Option B.  First, let’s briefly review why you may need to interpolate cross sections in HEC-RAS.  Cross Sections should be selected early on in your project to adequately define changes in geometry, roughness, bed slope, and discharge.  Also, cross sections are needed to properly define the expansion and contraction zones around bridges, culverts, and inline structures.  Additional cross sections are needed in areas of rapidly varying flow and/or significant grade breaks to avoid overestimation of energy loss (http://hecrasmodel.blogspot.com/2010/01/another-reason-for-interpolated-cross.html).  There’s also that pesky warning message that seems to always pop up:    

 

Each of these warning messages suggest that more cross sections will provide a better solution.  Finally, in unsteady flow modeling, there’s a relationship between cross section spacing, wave speed, and computational time step that minimizes errors and numerical instabilities (Courant Condition).  Interpolating cross sections is a quick and convenient way to satisfy this relationship. 

The first interpolation method, Option A, is called Interpolation “Within a Reach…”.  This is also called “blind” interpolation, because you cannot see nor control how HEC-RAS interpolates new cross sections.  They are interpolated based on the internal interpolation scheme, with no user interaction, except for entering a maximum spacing value.   This method is good for testing whether the spacing of cross sections will help produce a more numerically stable simulation.  You can choose to interpolate over an entire river/reach, or a segment of a reach, based on your selection of the Upstream River Station and Downstream River Station. 

Here you can see there’s a single input box for the Maximum Distance between XS’s.  RAS will then determine the number of interpolated cross sections necessary between each set of original cross sections so that the main channel reach lengths never exceed that maximum distance.  There are a couple of other options to be aware of in this window:  Cut Line GIS Coordinates allows you to control whether HEC-RAS interpolates the cut line coordinates of the original cross sections, or simply displays the interpolated cross sections as perpendicular segments along the reach centerline.  Whatever you select here will have no bearing on the computations, but will only change how the cross sections are displayed in the geometry schematic.  If you have a georeferenced project with adequate initial spacing, the first option should display the interpolated cross sections just fine.  However, if you have a very sinuous river and the original cross sections are spaced too coarsely, the interpolated cross sections may not follow the stream centerline very well, sometimes resulting in interpolated cross sections laying off center or completely away from the stream centerline (see below). 

In this case, to better display the layout of the interpolated cross sections, select the “Generate for display as perpendicular segments…” option.  This will lay the interpolated cross sections centered on their bank stations and perpendicular to the stream centerline.

Notice how the interpolated cross sections now follow the stream centerline.  However, be aware that these new interpolated cross sections are NOT georeferenced.  If you will be mapping your results on inundation maps, it would be better to cut new original cross sections around the bend section of the reach and use the “Linearly Interpolate cut lines…” option. 

The dropdown box “Decimal Places in interpolated Sta/Elev” simply directs HEC-RAS to how many decimal places to carry out the station elevation points for the interpolated cross sections.  The default is 2 and is generally left alone.  One case where you may want more precision in the decimal places is with extremely shallow reaches, where the resulting bed profile may look “stair stepped” due to rounding off of the channel invert point.  Adding more precision will help to smooth this out. 

A word of warning about Interpolation Option A, “Within a Reach…”.  The interpolated cross sections under this method are generated by built-in interpolation routines, that simply linearly interpolate station elevation points based on their proportional stationing within a subsection.  You have no control over the interpolation.  As such, it is very important that before you use these interpolated cross sections for your final RAS model, you verify that they accurately describe the geometry of the reach.  If important geometric features are not captured or are improperly defined, you’ll want to get real cross sections, or interpolate using Option B, “Between 2 XS’s”.  It is advised that blind interpolation (Option A) only be used to test the numerical adequacy of the cross section spacing.  More on Option B in the next post.  Stay tuned. 

Stabilizing a Dynamic Unsteady HEC-RAS Model

Written by Chris Goodell
Copyright © RASModel.com. 2013.  All rights reserved.

One of the most frustrating aspects of unsteady HEC-RAS modeling can be the model stabilization process. You know, you’ve gone to great lengths collecting the best survey/topo data and solid hydrology. Then you’ve painstakingly spent hours…possibly days entered all of that data only to find that once you press the “Compute” button, the model crashes. The dreaded “Red Bar”! Sometimes you can get your simulation to complete without crashing, but the listed numerical errors are so high that you can’t with good conscience submit that as your final simulation. Either way, approaching an unsteady HEC-RAS model (especially a dynamic one) as a beginner with little experience and understanding of how to stabilize it can cause significant delays in your project and worse, completely blow up your budget. I’ve uploaded two HEC-RAS projects to the following Google Drive Site: https://drive.google.com/folderview?id=0B_s8OLJOgOi0YU92SGZQZm9raWM&usp=sharing



RawleyResUnstable.prj is an unsteady flow dam breach HEC-RAS project recently sent to me for help. 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 the “Unstable” and “Stable” models and try these techniques on your own. The links following some of these 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 the geometry schematic and profile plot should encourage you to investigate a finer cross section spacing.

Geometry Schematic

Profile Plot Samuels equation suggests anywhere from 15 ft to 50 ft spacing (depending on what bed slope you use). I interpolated to 50 ft for the entire reach. http://hecrasmodel.blogspot.com/2008/12/samuels-equation-for-cross-section.html

2. Dam Breach models typically have time steps on the order of a minute or less. This model was initially set with a computation interval of 10 minutes which is high even for the largest and “slowest” of dam breach models. I changed the time step to 10 seconds. The selection of 10 seconds was based on “gut” feel and lots of experience doing dam breach models. There are some methods for approximating good timesteps, notably the Courant Condition and Fread’s equation. Also, setting the cross section spacing/timestep ratio equal to a representative stream velocity will get you close. In this case, the cross section spacing is 50 ft, and I was able to pull some velocities (prior to the model crashing) at about 6 ft/s. That suggests a time step of 8.3 seconds. Close enough to 10 seconds, so we’ll stick with that.

3. I changed the downstream boundary normal depth slope from 0.01 to 0.001. Not sure what is downstream of the first cross section, but 0.01 is awfully steep and was setting up a very low depth at the downstream boundary (which was causing instabilities). 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 the boundary cross section is below critical depth (the red dot). This creates an overestimation of the water 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 a boundary condition that best represents those conditions. In absence of downstream conditions, the 0.001 slope for normal depth provides a reasonable solution.  Either way, this underscores the importance of moving your downstream boundary far away from your area of interest in your study reach.  That way the errors that do originate from your downstream boundary assumption will have diminished to negligible levels before impacting your area of interest. 

4. In the unsteady flow editor, the initial flow and the first time step flow should always be equal. In the original model, the initial flow is left blank (which is actually okay because RAS will use the first timestep flow if left blank). However, the first time step flow is very low at 0.14 cfs. Peak discharge of your inflow hydrograph is around 600 cfs. I put in a baseflow (minimum flow) of 5% of this, which is 30 cfs. Just to be safe, I put 30 cfs in the initial flow input box as well. It’s important to make sure that the 30 cfs baseflow does not have an impact on the peak of the breach outflow hydrograph. 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 spot in the profile plot because of the very nice smooth profile plot followed by a sudden spike in the energy level (green dashed line) at the upstream end. Notice to that there is a corresponding supercritical solution at the upstream most cross section. These are both indications that more cross sections are needed here. http://hecrasmodel.blogspot.com/2010/01/another-reason-for-interpolated-cross.html

I added more cross sections by interpolating the steep slope at the upstream end of the reservoir (10 ft spacing).

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 and bumping 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 elevated side channel. The right overbank is lower in elevation than the channel.

This is problematic in RAS. Using the graphical cross section editor, I redefined the bank stations to get rid of this problem.

8. Notice now in the HTAB parameters property table, the HTAB definition begins at the elevation of the invert of the side channel that was previously defined as the main channel. There is a big gap in the computation points in the newly defined main channel. This poses problems with HEC-RAS, particularly at low discharges/stages, since RAS will have to extrapolate to obtain a solution over that range.

I readjusted the cross section HTAB parameters to reflect the new bank station definitions (I used the “Copy Invert” button for all cross sections). While in there, I maximized the resolution of the HTAB parameters by changing number of points to 100 and minimizing the increment as much as possible while still having full coverage of each cross section. http://hecrasmodel.blogspot.com/2009/02/crazy-energy-grade-line.html http://hecrasmodel.blogspot.com/2011/03/more-on-htab-parameters.html

9. In the figure below, notice all of the “red” areas in this zoomed in section of the  upstream end of the reach.

These are areas that are near critical depth. I turned on the “Mixed Flow” option in the Unsteady Flow Analysis window, which helps stabilize the near critical depths at the upstream end of the reservoir. http://hecrasmodel.blogspot.com/2011/04/mixed-flow-regime-options-lpi-method.html If you’ve been following along correcting your own copy of the model, you’ll notice that the model now finally runs to completion without crashing. There are still a few minor errors as shown in the Computation Messages.

It’s debatable whether you need to take care of these relatively small errors. Particularly for a dam breach model, where the presumed errors of the input data probably far outweigh these small numerical errors. Nevertheless, there’s a sense of pride in putting out a very robust model, completely free of numerical errors. So…let’s continue.

10. The errors in the computation window above suggest a problem around river station 6200. That happens to be the upstream end of the reservoir and the source of the error can clearly be seen with the energy grade line and critical depth turned on.

The section at the upstream end of the reservoir is very steep at about 13.5%. In fact, that’s greater than the suggested maximum slope of 10%, as stated in the HEC-RAS manuals. You can see as the pool starts to lower, the steep reach is exposed and because of the low n values, the water surface is calculated to be supercritical. Let’s assume that the geometry is correct, and the upstream end of the reservoir really is that steep. Jarrett’s Equation suggests very high n values should be used here (~0.2), based on the bed slope and hydraulic radius (after the breach when the reservoir 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 a little overkill. http://hecrasmodel.blogspot.com/2009/12/n-values-in-steep-streams.html

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

11. To get rid of that warning, I increased the HTAB headwater maximum elevation at River Station 277 (the bridge) from 278 ft to 280 ft so that RAS doesn’t have to extrapolate.

There we have it. A clean solution. No errors, no warnings.

HTAB Problems with using the Drawdown Scheme for troubleshooting.

Written by Chris Goodell, P.E., D. WRE   |   WEST Consultants
Copyright © RASModel.com. 2012.  All rights reserved.

A very convenient way to troubleshoot instability problems with very complex models is the use of a hotstart run with a stepdown scheme.  Creating a stepdown scheme hotstart plan is covered in detail here.  In a previous post, I explained some problems with running the stepdown scheme when you have bridges.  Here I want to highlight problems you may run into with cross sections while running the stepdown scheme.
While drowning out the entire model to provide a high degree of numerical stability, you will be exceeding the maximum HTAB computation points, sometimes by several hundred feet/meters or more.  Technically, this is okay, since HEC-RAS will extrapolate the HTAB curves as needed.  However, extrapolation is done linearly off the last two HTAB points on the computed curve.  If the last two points of the curve happen to be at a location of discontinuity on the curve, bad things can happen.  Take for instance this computed HTAB curve of Conveyance vs. Stage:

It looks pretty good.  However, if you zoom in on the end point, you can see that there is a discontinuity in the curve. 

Not a big deal, since normally we are well below the last point on the HTAB plots.  However if you are running a step-down scheme, RAS will have to extrapolate off of the last two points when the model is “drowned” and you can see that there will be an overestimation of water surface elevation at the subject cross section during the stepdown process as shown below in the profile plot.  If the overestimation of stage is severe enough, the resulting “stairstep” could lead to numerical instability, causing your model to crash.
A quick fix to this problem is to just slightly change how the HTAB curves are constructed for the problem cross sections.  Simply remove the last point in the curve by reducing the number of points in the Cross Section Table Properties by 1 (in the example below, change 100 to 99). 

This will provide a much better linear extrapolation and get rid of the “stairstep” problem in the stepdown scheme. 

Of course a “seasoned” RAS modeler will recognize what is causing the discontinuity in the HTAB curve in the first place and could eliminate that problem by fixing the geometry, whether it be better definition of ineffective flow areas, levee markers, n-value breakpoints, etc.  However, the savvy modeler would recognize that the discontinuity exists at the end points of the cross section, well above the normal water surface elevation range.  Once the model is stable, there’s no need to spend time fixing this.  

Inline Structure Flow Stability Factor

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2012. All rights reserved.

So many of us know that if you experience instabilities at or near your inline structure, you should bump up the Inline Structure Flow Stability Factor from the default of 1 to the most stable value of 3.  But what is really going on when you do this?











Here’s an example of a RAS model where the computations are going unstable around an inline structure and the model ultimately crashes.  Notice the energy spike just upstream of the structure.  Simply changing the inline structure flow stability factor from 1 to 3 got rid of this problem and the model ran without any errors at all. 

When RAS encounters an inline structure, it uses an iterative approach of guessing an energy slope projection upstream of the inline structure, then solving the weir equation.  If the guess is close enough (within the predefined tolerance), RAS calls the solution good and moves on.  Herein lies the problem.  Sometimes RAS will guess a slope projection that produces an upstream energy level that is too high.  Normally not a big deal, as the next iterative guess will try something lower and ideally the true solution will be converged upon. 
However, if that first guess is so erroneous that its error oscillates and grows (instead of decays) during the iterative procedure, the model can eventually become completely unstable and crash.  The Inline Structure Flow Stability Factor seeks to dampen out or eliminate those oscillations by reducing the energy slope projection for the first iterative “guess” at inline structures. 
The result is much more stability at inline structures in your model.  Contrary to other stability factors in HEC-RAS, bumping up this one from the default of 1 to the most stable value of 3 does NOT decrease accuracy.  Theoretically, you should arrive at the same answer with 2 stable models whether you use “1” or “3”.  The difference is how RAS gets to that answer in the iterative procedure.  The default of 1 uses a slope projection guess that should arrive at the answer fastest (fewest number of iterations), assuming it remains stable.  A value of 3 will arrive at the solution using more iterations, but will avoid “guesses” that may cause instability along the way.  I’ll wager that if bumping up to “3” stabilizes your model, you won’t notice the extra iterations…

Theta Implicit Weighting Factor and its Effect on Sample Datasets

Written by Aaron A. Lee   | WEST Consultants
Copyright © RASModel.com. 2011. All rights reserved.
Adding to the previous topic on the Theta Implicit Weighting Factor (Theta), this post takes an objective look at how the unsteady-flow option affects model output. Theta is a weighting factor for the spatial derivative used in solving the finite difference forms of the St. Venant equations. Adjusting Theta can improve model stability or increase the accuracy of the output. In a practical sense, how much is Theta really changing the solution? This post observes the influence of Theta by running the 21 installed (sample) projects in HEC-RAS version 4.1.

Theta can be found by navigating to Calculation Options and Tolerances under the Unsteady Flow Analysis Options menu. The default value is 1.0, but the user can define a value of Theta anywhere between 1.0 and 0.6. A value of 0.5 represents a half weighting explicit to the previous time step’s known solution, and a half weighting implicit to the current time step’s unknown. A value of 1.0 gives a fully implicit formula that is highly diffusive. In theory, a higher value will improve model stability but is less accurate in the solution. The opposite is true for lower values of Theta, which can make the model more sensitive to errors and lead to oscillations.

The table below summarizes the results for the sample projects included in the experiment. Water surface elevations (WSEL) are compared at each river station between the current plan and the default plan, Theta = 1.0. The values in the table are the largest maximum differences in WSEL for the entire reach. The cells in red are the plans that failed.



Apart from the three crashed runs, the difference in the solutions is very small. The results demonstrate that for these simulations, the model is not highly sensitive to changes in Theta. Keep in mind that these models are relatively simple (shorter reach lengths, plain structures, uniform geometry) when compared to other unique project situations.

Changing Theta has a direct effect on how the solution is solved, but other factors may have more of an effect on stability and accuracy. The Hydraulic Reference Manual notes that factors such as cross-sectional properties, abrupt slope changes, flood wave characteristics, and complex hydraulic structures often overwhelm any stability considerations associated with Theta. When testing a model, pay special attention to the stability considerations listed above before laboring over Theta. While lowering Theta will yield (technically) more accurate results, it can also propagate errors where other factors may be causing problems. The User’s Manual suggests making sure that the computation interval is accurately defined, and that the maximum number of iterations is reasonable.

The HEC-RAS User’s Manual (page 8-32) suggests starting out with a Theta value of 1.0. Paraphrasing from the User’s Manual, page 8-32: “Once the model is up and running, the user should experiment with changing Theta towards a value of 0.6. If the model remains stable, then a value of 0.6 should be used. In many cases, there may not be an appreciable difference in the results when changing Theta from 1.0 to 0.6. However, every simulation is different, so you must experiment with your model to find the most appropriate value.”

The results of adjusting Theta for the 21 sample projects validates the approach suggested in page 8-32 of the HEC-RAS User’s Manual.

More on HTab Parameters

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2011. All rights reserved.
I’ve mentioned this a lot in this blog, but I’m finding more and more that a good, solid definition of your HTab parameters up front will go a long way in helping construct a nice stable unsteady flow model in HEC-RAS.

First, let me recap how they work. In unsteady flow, HEC-RAS will convert the geometry of cross sections into a set of curves defining relationships between hydraulic parameters and stage (it does the same thing for bridges and culverts, but we’ll save that for another post). These hydraulic parameters include conveyance, flow area, storage area, and top width. This is done for the main channel, overbanks, and for the total cross section. Storage area in this case represents any ineffective flow areas in the given cross section. The figure below shows the conveyance HTab curve for a cross section.



These curves (actually the paired data that creates them) are used by HEC-RAS during the unsteady flow computations. Rather than re-computing the hydraulic parameters at every time step, RAS can simply pick the value off the chart. Because these curves are defined by a number of discrete points, RAS usually must interpolate to grab a value in between points. RAS does this linearly. Herein lies the problem that can lead your model to errors and instabilities.

Notice how at the lower stages, there is a significant curvature to the conveyance relationship in the above figure? A linear interpolation in this range can be quite wrong if the resolution of points is too coarse. Notice how when I zoom in to a rather coarse Conveyance HTab curve, it becomes obvious that in between points, linear interpolation is going to give me a bad answer. This is why a good definition of HTab points is particularly advantageous when running at low stages.



Start by maximizing the number of HTab points for you cross sections. HEC-RAS allows up to 100. The grey horizontal lines in the figure below represent computation points at different stages. I know what you’re thinking. “Won’t that many points on every cross section really slow down the computations?” Well, that may have been the case years ago, but computers are so fast now, that you’ll probably never notice the difference. Furthermore, the development of these curves is only done once, during the preprocessing part of the computations. As long as you don’t change the geometry, RAS won’t have to recompute the HTab curves.



Then make the computation increment as small as possible to squeeze all the points together. You only need to extend your HTab curves to contain the maximum computed water surface.





Finally, go to the Stating El. column and click the button “Copy Invert”. Notice in the figure below there is a gap between the invert of the channel and the first computation point which is set 1 ft above the invert of the main channel. This is the default starting computation point in RAS (well, not completely true). If you check the 2nd figure up above, you’ll see that there actually was a computation point at the 0 depth point (the invert). That’s because RAS will still compute the 0 depth point, but then the next computation point, by default, is 1 ft (0.3 meters in SI Units) above the 0 depth point. From there on up, RAS will space the computation points based on the increment you define. To get more points between the invert of the main channel and 1 ft (0.3 m) above the invert, you have to set your Starting Elevation to the invert. That’s why it’s always a good idea to click “Copy Invert” and make sure that your Starting Elevation is the same as your “Chan Min” value. That way, the small computation increment is started from the channel invert, not 1 ft (0.3 m) above the channel invert.





I was recently informed that in the next release of HEC-RAS (version 4.2), the default starting point for HTab computations will be 0.5 ft above the invert (0.15 m?). This will improve things somewhat, but it still may be necessary to “copy the invert”, particularly if you have very low stages in your simulation.

How to Create a Hotstart File in HEC-RAS for Dam Breach Analysis

Written by Aaron A. Lee   | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.
While running unsteady flow simulations in HEC-RAS instabilities may occur when transitioning from the automatically created initial condition file to the first computed time step. These instabilities can be caused by mixed flow conditions, flow splits, or poorly defined initial conditions. A hotstart is another option available for defining initial conditions for the project model. This article presents one technique for setting up a hotstart run to help with initial conditions problems and to troubleshoot problem areas in your project model.

This is done by creating a new plan, using a flow file with a constant discharge as the upstream boundary, and a stage hydrograph as the downstream boundary over a 24 hour period. Typically 24 hours is long enough, but you may find that a longer hotstart period is required. The downstream boundary water surface elevation is defined artificially high, and over the simulation it is gradually reduced until it reaches the true starting depth for your project model. At this point the hotstart file will be written by HEC-RAS. In HEC-RAS lingo, the term “hotstart file” is used interchangeably with “Restart File” and “Initial Conditions file”.

STEP 1. Create a new flow file by opening the current unsteady flow file and navigate to File, Save As, and name it “hotstart”. Once saved, change the upstream boundary condition by selecting Flow Hydrograph under the Boundary Conditions tab. Change the discharge to a constant flow equal to that of the first timestep for the 24-hr period. In this example, shown in Figure 1, the entire Flow column should be modified to contain 155 for the full 24-hr simulation.



Select Stage Hydrograph as the downstream boundary condition. The beginning stage should start at an artificially high elevation - somewhere near the invert of the upstream-most cross section. The final elevation at the end of the 24-hr simulation should be equal to the starting downstream water surface elevation of the project plan. Once these values are added into the Stage column, use the Interpolate Missing Values button to add the missing elevations.

The elevation at each time interval should gradually decrease. This makes it easier for the modeler to observe problems as they occur, and where they happen in the model. For this example, the starting elevation is 820 ft and the final elevation is 637.25 ft. This is shown in Figure 2. Save the “hotstart” flow file.

STEP 2. Navigate to the Unsteady Flow Analysis window and save as a new plan named “hotstart plan”. Name the short ID as “hotstart”. This will create a new plan that will be used to define the initial conditions for the project plan. Under the Unsteady Flow Analysis window select the Ending Date and Time to 24 hours after the Starting Date and Time (or whatever time frame you want to use-just be sure it is consistent with the hotstart flow hydrograph and stage hydrograph you created in Step 1).

STEP 3. The next step is to set up the model so that the hotstart file will be written. Under the Unsteady Flow Analysis window navigate to Options, then Output Options. Figure 3 shows this window.



Check the boxes that write the initial conditions file at the Fixed Reference of the hotstart simulation ending date and time. At the end of the specified simulation time (24 hours) HEC-RAS will automatically write the initial condition file. The final step is to ensure that the hotstart plan is using the correct geometry file, and the created “hotstart” flow file. Once the plan is completed and saved, compute the Hotstart simulation.

STEP 4. The profile plot should be reviewed for problems with the hotstart model. This will indicate areas that may cause problems in your project model. Over the course of the hotstart simulation, as the water surface drops into place along your bed profile, look for hints of instabilities. If you hotstart simulation crashes, you’ll know exactly where to investigate-the intersection between the artificially high horizontal pool and the bed profile at the time of the crash. To use the hotstart file as your initial conditions, go to the Unsteady Flow editor of your project plan. Click on the “Use a Restart File” box and browse for the initial conditions file. This file will have an extension that indicates that hotstart plan number that created it, the simulation time (day-month-year) when it was created, and .rst. In this example, it should look like:

Workshop4.p03.10NOV2006.rst

*Warning-If you make a change to the geometry in your project model, you’ll have to re-run your hotstart simulation. However, once everything is set up, this is very easy to do. Simply open the hotstart plan and run it. Then open the project plan and run it.

Stability Issues with Storage Areas

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.
Storage Areas in unsteady RAS are notoriously stable. That’s why we like to use them. Get the water out of the 1-d St. Venant unstable environment, into the 0-d stable continuity environment. However, I recently discovered a problem with storage areas that could cause your model to go unstable, or at least chug along slowly at max iterations.

Storage Areas in RAS are defined solely by a storage-elevation curve. That’s another reason we like them…they’re easy to code in. A typical storage-elevation curve looks like this:



Notice how is rises fairly quickly in stage from its minimum elevation then starts to level off as the added volume per ft of stage becomes larger and larger.

Now, if you have a storage volume curve that rises too quickly, it could pose problems.



Because the storage area is handled with the continuity equation, this is typically not an issue by itself. However, when these storage areas are connected with a reach (and they typically are) in the “quickly rising” range of elevations (here in the example between elevations 4388 and 4393), then we might violate my number one rule of unsteady flow RAS modeling-Changes should happen gradually-changes in discharge, changes in stage, changes in flow area, etc. etc ,whatever. In this case we are changing the stage in the storage area which is causing a quick change in flow over the connecting lateral structure and a quick change in stage in the adjacent cross section(s).

Solution: Looking at this steeply rising storage area curve , we can guess that the quickly rising portion of the curve might represent some small ditches or creeks, or even some small pits within the storage area. Is it critical to represent these features in the model, especially if we’re most interested in the high flow portion of the simulation? Also, will it make that much difference to remove these features? Probably not. We should keep the invert or minimum elevation point, incase any connecting cross sections have inverts at that elevation, but if we remove the 2nd and 3rd points from the curve, it doesn’t drastically change the look of the curve, and it just might stabilize this area.



How to spot this problem: While you’re running your model, if it gets caught on maximum iterations at a storage area, or if it bounces between a storage area and an adjacent part of a reach, then this could be the problem. Also, if you’re maxing on iterations at a cross section or range of cross sections that is adjacent to a lateral structure, and the elevation of the reported error suggests you are overtopping that lateral structure, make sure that the connecting storage area doesn’t have a steeply rising storage elevation curve.

Here’s an example:

Maximum iterations of 20 at: RS WSEL ERROR

03JAN2010 09:50:38 River Upper 34454.76 4423.88 0.055

03JAN2010 09:51:00 River Upper 34602.8* 4424.34 0.027

03JAN2010 09:51:08 River Upper 34602.8* 4424.39 0.048

03JAN2010 09:51:23 SA Area47 4392.44 0.052

03JAN2010 09:51:30 River Upper 34602.8* 4424.34 0.026

03JAN2010 09:51:38 River Upper 34602.8* 4424.39 0.047

03JAN2010 09:51:53 SA Area47 4392.66 0.058

03JAN2010 09:52:00 River Upper 34454.76 4423.93 0.028

03JAN2010 09:52:08 River Upper 34602.8* 4424.39 0.048

03JAN2010 09:52:23 SA Area47 4392.91 0.065

03JAN2010 09:52:30 River Upper 34454.76 4423.93 0.027

03JAN2010 09:52:38 River Upper 34602.8* 4424.40 0.048

03JAN2010 09:52:53 SA Area47 4393.19 0.073

03JAN2010 09:53:00 River Upper 34454.76 4423.94 0.030

03JAN2010 09:53:08 River Upper 34602.8* 4424.40 0.050

03JAN2010 09:53:23 SA Area47 4393.50 0.080

03JAN2010 09:53:30 SA Area47 4393.58 0.083

03JAN2010 09:53:38 River Upper 34602.8* 4424.40 0.051

03JAN2010 09:53:53 River Upper 34602.8* 4424.38 0.022

03JAN2010 09:54:00 River Upper 34454.76 4423.94 0.023

03JAN2010 09:54:08 River Upper 34602.8* 4424.41 0.050

03JAN2010 09:54:30 River Upper 34454.76 4423.96 0.037

Notice how the errors are bouncing between Storage Area 47 and a specific part of the reach “River Upper”. If we check storage area 47, sure enough, its storage elevation curve rises very quickly, and could probably be adjusted to removed the stability issue in the range of stages shown in the computation message log (about 4392 to 4393 ft).

Dam Breach Modeling Q & A

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2010. All rights reserved.
Some questions and answers related to dam breach modeling in HEC-RAS…

Question. The Sunny Day model has a consistent water surface elevation from the very start of the model – it only decreases once the breach occurs. How is HEC-RAS setting this starting WSEL?

Answer. You define the starting water surface elevation either by equalizing the flow from your outlet works with the reservoir inflow, or by setting an initial conditions water surface elevation in your flow editor and a pilot flow through the dam equal to the reservoir inflow at the beginning of the simulation.

Question. My breach models show a dramatic decrease in max Q from the cross-section immediately downstream of the dam to the end of the model. I know that HEC-RAS has an inherent storage routine that attenuates the flow throughout the model but is it reasonable to have a result that shows a beginning max Q of 12,370 cfs and an ending Q of 275 cfs (the reach is approx. 3.8 miles long with a slope of 0.02 ft/ft upstream and 0.001 ft/ft downstream)? This is an arroyo about 800-900 ft. wide, Manning’s at .055.

Answer. I would be skeptical of those results. Perhaps there is an error somewhere in the simulation, or you have a lot of flow leaving the system. Sometimes, if your model is not properly constructed, you can develop a large “wall of water” in profile view. A lot of times this is due to poorly defined HTAB parameters. This will create an artificial pool of water behind the wall, which will drastically attenuate your flood wave. Look in the profile plot and animate through your simulation. If you see an unexplainable wall of water backing up flow, that would be the cause.

Question. My models are stable but still have inherent errors (max iterations) and critical depth defaults to varying degrees. Does this have a significant effect on the model results? Changing parameters at this point to reduce inherent errors most likely will cause instability.

Answer. Max iterations are not necessarily a problem as long as the associated errors are small and it is not causing visible instabilities or obvious errors in your results. I try to get rid of all max iterations where possible. If not possible, I try to get the errors below 0.1 ft as much as I can (my own rule of thumb). RAS does not typically default to critical depth in unsteady flow (like it does in steady flow). But it sounds like you have areas that have flow close to critical depth. This can cause instability problems. If you believe flow should be close to critical depth in these locations, try turning on the Mixed Flow option and adjusting your LPI factor. If you do not believe flow should be near critical in these locations (most of the time in natural streams you should not see critical or supercritical flow), then you may be underestimating your Manning’s n values. Manning’s n values for the front end of dam breach flood waves and steep reaches are frequently underestimated. Check Jarrett’s equation if in a steep reach. Your reach slope of 2% is quite high. An n value of 0.055 is possibly too low during the low flow period preceding the dam breach flood.

Question. Does the number of vertices defining a cross section matter, in another words, does the model run better with cross sections that have fewer vertices but still accurately define the section, vs. similar sections that have many redundant vertices?


Answer. Better definition is usually advantageous. RAS does not like to have long horizontal portions of cross sections which is common for coarsely-defined cross sections. It can cause numerical problems. These days, having the maximum number of points in a cross section (500) typically does not noticeably slow down computation speed. I recommend getting as much detail as you can in your cross sections.