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Q & A: Flow Attenuation

June 28, 2012, 9:53 am
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Question: 
When running an unsteady flow model with a single inflow hydrograph, why does my discharge decrease in the downstream direction for a given output profile? 

Answer: 
This is called flow attenuation.  You see this to varying degrees in all unsteady flow models and it is a real phenomenon.  The shallower the reach, or the wider the floodplain, the more pronounced this effect will be.  In very steep streams, you may not notice flow attenuation at all. 
The physical process is as follows:  As the flood level rises, water moving downstream fills in available volume.  This volume is called storage.  Water going into storage is taken away from the flow going downstream and that is why you see a decrease in discharge as you move in the downstream direction.  Wider floodplains and shallower reaches have more available storage volume, which is why flow attenuation is pronounced under these situations.  Once the flood wave passes, and you are on the receding limb of the flood hydrograph, the water that had gone into storage now returns to the active discharge.  In this case you’ll see an increase in flow as you move in the downstream direction.  Notice in the figure below that the discharge at time 0042 (before the peak of the flood wave) decreases in the downstream direction, while the flow at time 0124 (after the peak of the flood wave) increases in the downstream direction.
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You can also see this effect when viewing hydrographs in the figure below from two cross sections, one upstream (River Station 2500) and one downstream (River Station 2400).  The attenuation of flow is the difference in peak discharge between these two hydrographs-in this case, about 2.3 cms.  The area between these two curves represents a volume of water.  The area to the left, where the upstream discharge is greater than that downstream discharge, represents water going into storage.  The area to the right, where the upstream discharge is less than the downstream discharge, represents water leaving storage.
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Attenuation is included in the conservation of mass equation, which is one of the two equations (the St. Venant equations) used to define the movement of water through a reach in HEC-RAS-the other being conservation of momentum.    From the HEC-RAS Hydraulic Reference Manual (Page 2-22), “Conservation of mass for a control volume states that the net rate of flow into the volume be equal to the rate of change of storage inside the volume.”    In other words, Inflow minus outflow equals the change in storage over time.  The equation is:
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where A = flow area, Q equals discharge, t = time, and x = length. 
The discretized form of this is more practical to use and may be more familiar: 
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Where I = Inflow to a discrete control volume, O = Outflow, DS = Change in Storage, Dt = time duration (i.e. time step).
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How to draw cross sections.

July 31, 2012, 8:24 am
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Cross sections must be perpendicular to the flow lines at all locations.  And they cannot intersect with each other.  That is why it is common to see cross sections snap at different angles outside the main channel (we call this doglegging).  The trick is to keep them from intersecting, while also staying perpendicular to flow lines.  In the figure below, the dark blue line represents the main channel.  The brown lines represent the edge of the flood plain.  The light blue lines are my impression of the flow lines through this terrain, if water were flowing appreciably in the floodplain.  The green lines are cross sections.  Notice that the cross sections are drawn so that they are not only perpendicular to the main channel, but also to my perception of the flow lines in the floodplain.  It can be very helpful to draw these flow lines before cutting cross sections. 
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It takes a little bit of practice to do this correctly, and most of the time some trial and error, but as long as you remain perpendicular to the flow lines and don’t intersect, you’ll have a good set of cross sections. 
Where it can get tricky is at a junction.  The following RAS Bloggery article will help with junctions.  http://hecrasmodel.blogspot.com/2009/02/how-to-best-model-junction.html
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Flow spike after peak of dam breach floodwave.

August 17, 2012, 7:46 am
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Just ran into an interesting phenomenon.  I was running a dam breach model with fairly typical breach parameters and piping failure mode.  After the peak of the breach hydrograph, there appeared a mysterious “spike” as shown in the figure below. 
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This happens to be right at the same time the flow through the developing breach becomes freeflowing (as opposed to pressure flow through the breach opening) as shown in the next two figures.
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This is the point at which RAS switches from using the orifice equation to the weir equation.  The breach editor allows you to specify a breach weir coefficient.  By lowering this weir coefficient, you can get rid of the spike and have a smoother hydrograph.  I'm not concluding that this spike is incorrect. In fact, it is quite possible that you do experience a real surge when the breach collapses in and goes freeflowing.  In that case, you may find the spike acceptable.  Whichever result you use, be sure you can back it up with sound reasoning.  Given the extreme uncertainty in both breach weir coefficients and piping coefficients, it's probably best that you run a sensitivity analysis to gain a full understanding ot the effects these coefficients have on the dam breach hydrograph. 


Breach weir coefficient = 3.0
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Breach weir coefficient = 2.0
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Breach weir coefficient = 1.0
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Looks like a breach weir coefficient of 1.0 does the trick. 
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Inline Structure Flow Stability Factor

October 31, 2012, 3:17 pm
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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?
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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. 
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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…
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Connecting a River to an Off-channel Storage Area using a Lateral Structure

November 7, 2012, 9:01 am
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An off-channel storage area in HEC-RAS can be a very useful way to simulate flooding in interior areas, adjacent ponds and lakes, urban areas next to rivers, green storage, or just about any area that you expect to flood but will be better represented as ponded water versus actively conveying water.  Connecting rivers to off-channel storage areas is done via lateral structures.  Although it is possible to use lateral structures and storage areas in steady flow modeling, typically lateral structures and storage areas are used in unsteady flow modeling, where quantification of storage and hydrograph attenuation are very important.

Here’s a simple example of using a lateral structure to connect a river to an off-channel storage area (this happens to be the LeveeBreach.prj project that comes with the HEC-RAS installation).



To begin, first make sure your cross sections are all included and correctly entered.  Then you draw in (or import from GIS) your storage area.  To do that, simply click on the Storage Area button on the top of the geometry schematic and start clicking points to define the perimeter of the storage area.  Double-click to complete the storage area.  Now you are ready to connect the river to the storage area.
Select the Lateral Structure button on the left side of the geometry schematic.    When you do this the first time, the following graphic will be blank, but in this case, the lateral structure (which is being used to simulate a levee) is already entered in.  The figure below shows the lateral structure in profile view.  The stationing plotted on the x-axis is the lateral structure stationing (which you will define in the Weir/Embankment editor).  It is NOT the same as the river stationing.

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The River, Reach, and HW RS (Headwater River Station) define the location of the lateral structure in your system.  The upstream end of your lateral structure will be located at the HW RS (but can be shifted downstream of this station in the weir/embankment editor).  Notice that lateral structures are stationed from upstream to downstream (i.e. 0 is the most upstream point on the lateral structure).  The vertical lines in the graphic represent cross sections that are spanned by the lateral structure.  The vertical line that sits at station 0 is the HW RS.  The boxes on the bottom of the vertical lines represent the invert elevation of the respective cross sections, and the boxes on the top represent the end points of the cross section (on the side of the cross section that the lateral structure is located: left or right).  The red dots represent the bank stations of the respective cross sections.

Next, give a description for the lateral structure in the Description box and then define where its headwater position is.  You can place the lateral structure in either of the overbanks (left or right side) or adjacent to either bank station (left or right). 

The plan data Optimization button is just a shortcut to the plan file to quickly define whether or not you want to optimize the flow split over the lateral structure during the initial conditions run.  Typically you will want to optimize this if you have flow over the lateral structure at the beginning of the simulation.  If your initial conditions are below the lateral structure, leave this off.  The Breach button is a shortcut to the breach editor, if you want to breach this lateral structure during the simulation.

The “Tailwater Connection” is really the subject of this post-this is how you connect the river to the storage area.  Make sure you select “Storage Area” as your Type and then go choose the storage area you want to connect to by clicking the “Set SA” button.  Alternatively you could connect a lateral structure to another river/reach or you could connect it to nothing (send the flow over/through the lateral structure out of the system). 

There’s still work to be done to define the Weir/Embankment (if not already done), but the Storage area and the river are now connected via the lateral structure.  If you want to make sure you are connected, look at the points of the lateral structure on the geometry schematic.  If you see thin black lines connecting the end of the lateral structure to the storage area, then you know RAS recognizes them as being connected (sometimes you have so zoom in close to see the “connection lines”).

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If you are having difficulty connecting lateral structures to rivers and/or storage areas, I highly encourage you to open up this example in HEC-RAS and have a look around.  Normally you will find the example projects in C:\Program Files\HEC\HEC-RAS\4.1.0\Example Projects.

The “4.1.0” might be different if you’re using a different version of HEC-RAS.  If you don’t see the example projects here, go to the Help menu item on the main HEC-RAS window and select “Install Example Projects…”

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HTAB Problems with using the Drawdown Scheme for troubleshooting.

December 26, 2012, 5:18 pm
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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:
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It looks pretty good.  However, if you zoom in on the end point, you can see that there is a discontinuity in the curve. 
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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.image
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). 
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This will provide a much better linear extrapolation and get rid of the “stairstep” problem in the stepdown scheme. 
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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.  
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Dealing with dry bed conditions for Creek/Spillway linked to a Reservoir Storage Area

February 8, 2013, 1:57 pm
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Written by Daniel Christensen, P.E.   WEST Consultants, Portland Office
 

Our office ran into (what I thought was) an abnormal modeling situation this past month.  Here is a description of the situation.  The project is a dam breach model, and the spillway for the dam diverts water to an adjacent basin.  The spillway is actually more like a canal that runs about a mile until it empties into the adjacent basin. The dam the impounds the main creek has a typical regulating outlet system that supplies water to the creek, and the adjacent creek  (supplied by the spillway) remains dry except for large events when the pool rises above the invert of the spillway.  The unsteady model was supplied to our office, and the creeks were modeled as separate reaches, both connected to a storage area representing the reservoir (see fig 1 below).  Both the dam and spillway were modeled as an inline structures.  Figure 2 shows a close-up view of the spillway.
FIGURE1 

FIGURE2
  

As you have probably already guessed, how do we handle the dry-bed situation in the adjacent creek when the pool elevation in the reservoir is below the invert of the spillway?  The starting reservoir elevation for the PMF/Flood Scenario dam breach was actually 20 ft below the spillway’s invert.   If you tried to the run the model with a boundary condition that was 20 feet below the main stream bed, the model would crash immediately because of the negative water surface profile (see Fig 3).

This was when modeling creativity came into play and helped resolve the dry-bed issue while still allowing water from the reservoir(storage area) to enter the creek.  Instead of connecting the adjacent reach to the reservoir storage area, we disconnected it and assigned a constant, low base flow to the adjacent creek as a boundary flow condition (100 cfs.  We knew the max flow for the spillway was 20,000 cfs so 100 cfs wouldn’t make a difference in the max stage in the creek).  This simple change fixed the dry bed issue in the creek.  Now, how do we connect the reservoir storage area to the reach?  

FIGURE3


To connect the storage area to the adjacent creek, we defined a lateral structure on the left bank that was 2 or 3 cross sections downstream from the top of the reach (see fig 3).  The reservoir was defined as the lateral structure’s tailwater condition, and the spillway cross section was assigned to the lateral structure weir.  The weir coefficient was tested for multiple runs until the flow out of the lateral structure matched the spillway rating curve.  A final wier coefficient of 1.2 was used.  Also, to prevent water from flowing back into the reservoir from the creek, “Flaps to prevent Positive Flow” were assigned to the lateral structure.

FIGURE4
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Quasi Two-Dimensional Modeling in HEC-RAS

March 25, 2013, 12:15 pm
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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.
  
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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. clip_image004 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. clip_image006Areas 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. clip_image008Alternatively, 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
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Extending your Cross Sections to High Ground?

March 26, 2013, 10:46 am
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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. 
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Two-Dimensional modeling in HEC-RAS

April 5, 2013, 10:53 am
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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...

 



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HEC-RAS File Types

April 9, 2013, 1:55 pm
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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. image
  • .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. clip_image004
  • .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.









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

April 9, 2013, 4:49 pm
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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
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New Forum Up!

April 18, 2013, 10:08 am
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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

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

April 29, 2013, 1:27 pm
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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.

clip_image002

  • 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.

clip_image004

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.

clip_image006

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).
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Skew Angle for Bridges

May 31, 2013, 4:40 pm
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FIGURE 1



If a bridge is not oriented perpendicular to the flow lines going through the bridge, HEC-RAS has an option for skewing the bridge deck and/or piers.  (A user can also skew cross sections in a similar manner)  When skewing a bridge, the user reduces the bridge’s deck/roadway stationing by multiplying the values by cos Θ *b.  The new bridge opening width becomes the equivalent length that is perpendicular to the flow lines passing through the bridge.  Figure 1 shows an example of a bridge to be skewed.  

The HEC-RAS Reference manual makes a note that the skew angle (Θ) should be based on the angle of the flow path as it goes through the bridge compared with a line perpendicular to the cross sections bounding the bridge.  It should not be based on angle upstream of the bridge as shown in Figure 1 with the gray lines, which would overestimate the skew angle.  As the water approaches a bridge that is highly skewed, it’s common that the flow lines will turn before going through the bridge.   A field visit is very important to visualize the flow pattern at the bridge and to help estimate the skew angle.   

In determining a bridge skew angle, a modeler should also consider the question, “Will I be modeling large flows or small flows and will the flow paths through the bridge vary between the two?”  If you are modeling a dam breach, it’s more likely that the flow path during the peak of the flood won’t turn as much through the bridge, and therefore the hydraulic skew should match the geometric skew of the bridge.   If you are only modeling lower flows, the HEC-RAS Reference Manual suggests that skew angles below 20º to 30º do not appreciably affect the flow patterns through a bridge – the reason being that during lower flows the water/flow lines will be able to turn or meander more easily through the bridge opening than during large flows.

Users should also remember to apply the same skew angle to the bounding bridge cross sections.  If for some reason you don’t want to skew the bounding cross sections, you may have to manually alter either the cross section or bridge deck stationing to ensure that the bridge opening correctly aligns with the cross section.



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Region and Language Settings

June 24, 2013, 10:11 am
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For my international friends out there, just a friendly reminder that to get the most out of your HEC-RAS experience (and to avoid some errors and crashes), make sure that while working in HEC-RAS, you use the English (United States) format in your Windows Region and Language settings.  The most common sources of regional language errors that happen in RAS seem to be the date format and the period/comma usage for decimal symbol and digit grouping. 

 
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Quickly Edit Reach Lines in the Geometry Editor

June 27, 2013, 3:32 pm
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Written by Aaron A. Lee | WEST Consultants

Editing the positioning of your geometry in HEC-RAS can sometimes be a pain, especially with large systems. This tip will show you how to quickly (and easily!) remove sections of a reach. If you import reach stream lines through ArcGIS, or a similar program, your schematic may look similar to the figure below.

image

In this example I want to remove a large section of the reach, and the method of clicking each node to would be very time tedious and consuming. Manual edits can be useful for editing small portions of a reach or storage area, but not for hundreds or thousands of points!

Instead, navigate to Reach Invert Lines Table … under GIS Tools in the Geometry window. This will bring up a table of all the points in the reach, which are organized by XY coordinates. From here you can remove or edit a large portion of points. XY coordinates are shown in the bottom-right corner of the Geometry window when hovering the mouse around the area you are interested in.

image

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HEC-RAS - Then and Now

August 2, 2013, 3:54 pm
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Written by Chris Goodell | WEST Consultants

The first version of HEC-RAS I ever used was Version 2.2 back in 1999.  There are very few of us who actually used the first version of HEC-RAS which came out in 1995, but for those who do remember that, here's an image showing the very first HEC-RAS Version 1.0:


Version 1 was very simple, only allowing steady flow water surface computations with very few optional features.  Most users probably ran this on Windows 3.1, NT, or 95.  It's actually suprisingly similar to today's current version 4.1:



Notice a lot of the buttons are still there from the original version.  I think one of the reasons HEC-RAS is so successful and widely used is the consistency in form and usability that the developers at HEC have adhered to when developing new versions of HEC-RAS.   
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Careful with Flow Inconsistency on the Max WS Profile

August 15, 2013, 2:30 pm
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Written by Chris Goodell | WEST Consultants
I’m a big proponent of checking flow consistency in your results.  Anyone who has taken a RAS class from me has heard me go on about Standard Table 2 and the benefits of maintaining a consistent distribution of flow not only between sub sections (left overbank, main channel, right overbank) in a cross section, but from cross section to cross section.  Any significant change in flow, or flow distribution should be questioned and explained.  Generally there is a problem with ineffective flow definitions, Manning’s n values, bank station placement, or your model is simply unstable (for an unsteady flow model). 
image
After running an unsteady flow model, and you open up the profile output table (also called summary output table), the first profile that pops up is the Max WS profile.  Now, you have to be careful when checking flow distribution with the Max WS profile.  Although it shows up in the plot and tables along with all of the other profiles, the Max WS profile is not a real profile.  It never happened.  It is actually a compilation of all of the highest water surface elevations that happened during the simulation for each cross section-regardless of time.  A “Greatest Hits” of water surface elevations, if you will.  This is exactly what you would plot when producing a maximum inundation map. 
image
For many models, the Max WS profile will do just fine in identifying flow distribution problems with an important exception– reaches that have significant lateral inflows whose peak flows do not line up temporally with the main channel peak flow.  If that lateral inflow sets up a backwater in the main channel, prior to the arrival of the flood in the main channel, it could actually produce a higher peak water surface elevation than the elevation that corresponds to the peak of the main channel flood. 
Here’s an example from a question I recently got from a former attendee of one of my RAS courses.  Warning, this gets a bit detailed and specific-make sure you’re wide awake before reading on… 
Question:  “I’m currently working on an unsteady model where I have my initial flow hydrograph and then two lateral inflow hydrograph’s further downstream.  My question is that at my first lateral inflow hydrograph location the next couple of cross-sections upstream of the inflow point have greatly reduced peak flows.  For instance, the peak flow of the hydrograph entering the upstream end of the reach is around 646 cfs and at the cross-sections just upstream of the lateral inflow that number is reduced down to around 387 cfs.  There is not a drastic change in cross-section shape or stream slope in the area of the lateral inflow.  Have you run across this type of thing before?  Is this realistic or is something in the model not quite right?  Any thoughts would be greatly appreciated.”
Response:  What looks like an inconsistency, really is not.  In fact your results look great.  When checking flow consistency, be careful doing this with the MaxWS profile in the summary output table.  This is where you saw the drop from 646.61 cfs to 387.53 cfs from RS 3936.90 to RS 3899.86.  The problem with checking flow consistency on the MaxWS profile is that the maximum water surface does not necessarily correspond to the maximum flow, especially if you are in a backwater area like below.  This backwater is set up by the lateral inflow entered just downstream at RS 3726.34 and happens prior to the arrival of the peak flow in the main channel.  A very common occurrence when modeling a flood in large systems with multiple tributaries.  clip_image001 The peak of the lateral inflow at 3726 happens at 1250 hrs on the 13th.  This sets up a backwater that produces the max ws elevation of 976.52 ft for RS 3899.86.  However, the flow at RS 3899.86 at this moment is only 389.13 cfs.  The peak flow in the main channel has not arrived yet.  clip_image002 The peak flow in the main channel at RS 3899.86 happens after the lateral inflow peak by about 1 hour and 10 minutes.  At 1400 hrs, the backwater effect from the lateral inflow is almost completely gone at RS 3899.86 and the peak flow is 647.59 cfs with a corresponding ws elevation of 976.25 ft.  clip_image003clip_image004 As a result, what you see in the max ws profile is not the max flow at RS 3899.86, but the flow that is happening during the max ws elevation.  You can see all of this by checking around the stage and flow hydrographs. If you see inconsistencies like this for the max ws profile, you can verify your results are still good by scanning through each individual real profile in the summary output table (cumbersome), or go to the dss file and open up the max flow profile like so: clip_image006 This gives you a plot like this: clip_image008 Notice that the max flow at 3899.86 is indeed in the 650-ish range, which is where it should be.  The big changes in max flow are where you have your lateral inflow hydrographs. 













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Negative Flows over Inline and Lateral Structures

September 4, 2013, 2:29 pm
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Written by Chris Goodell | WEST Consultants.

Though it’s not very common, it is possible to get negative flows over inline structures for unsteady flow simulations.  It’s much more likely you’ll see negative flows over a lateral structure though.  Negative flows are simply discharges in your model going the opposite direction than what you have defined as the downstream direction (or primary flow direction).  This can occur in tidal areas, backwater, or complex looped networks.  For lateral structures, you may have water spill out of your river over a lateral structure and into a storage area.  Once the flood wave passes, the head level in the storage area is higher than the river and water flows back into the river (negative flow).  It’s called negative flow because RAS will show it in tables and plots as a negative number. 

Recently, an HEC-RAS output table issue was brought to my attention, directly related to negative flows over weirs. 

“I was looking through the HEC-RAS file for the XYZ model for the 72-hr PMF and came across the following.  Do you know what -1.#INF means?”

image

In this example, the water surface elevation downstream of the inline structure exceeds the water surface elevation upstream of the inline structure between the time of 1420 hrs and 1900 hrs on 02 January, 1913.  During this time range, we have a negative head differential and thus a negative flow over the inline structure (from downstream to upstream).  During a simulation, when an inline structure is overtopped by water, RAS uses the same equation (the weir equation with adjustments for submergence) regardless of the direction of flow over the inline structure. 

The -1.#INF is the double precision floating point default return in the Visual Basic programming language for any unreal number (i.e. you may remember imaginary numbers, “i” from high school math class).  Well RAS is trying to take a negative number (in this case the head differential) to a non–integer power, (1.5 for the weir equation, Q = CLH^1.5).  There is no real solution for this.  The good news is, this is only happening in the table, as RAS recomputes the Q Weir to populate the table using the steady flow engine.  Presumably, that is why RAS cannot deal with negative head differentials, since there is no negative flow in steady flow modeling.  During the unsteady flow computations, RAS correctly removes the negative sign to do the computations and then replaces it to show flow in the “negative” direction.  That’s why the computations work and why the Q Total is negative (which is presumably what Q Weir should be in this example). 

So, in short, the unsteady flow computations are fine-the solution is correct.  The problem comes when reporting results to the Inline Structure Table.  Hopefully this will be fixed for the next version of HEC-RAS.  Until then, you can use the stage and flow output hydrographs to verify the total and weir Q’s. 

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