Rating Curves for Dams

Written by Jesse Rufener, P.E., CFM | GPD Group
Copyright © The RAS Solution 2017. All rights reserved.

Version 5 of HEC-RAS allows the use of rating curves for inline structures (and lateral structures).  Rating curves can be added through the Outlet RC feature in the inline structure editor.  Inline structure rating curves can be useful for evaluating the impacts of structures, such as labyrinth dams, where it may be difficult to correctly capture the geometry and/or flow properties within the inline structure editor.

Click the Outlet RC button and in the following menu you can add your rating curve based on upstream flow or water surface elevation. 

Please note that the rating curve must account for any influence of downstream tailwater as HEC-RAS does not with the rating curve option. 

When using the inline structure rating curve, the top of the dam must be above the highest elevation on the rating curve if you only want the rating curve to account for flow over the outlet feature.  You can check the Stage and Flow hydrograph to see how the rating curve is contributing to flow past the inline structure.  In the image below, the total flow is a combination of the rating curve and flow over the structure as I didn’t have the top of dam elevation set high enough in the first iteration.

Note that the peak flow is 34,471 cfs with a peak HW stage of 927.86.  When I raised the top of the dam to be above the top elevation of the rating curve, the peak flow is 33,967 cfs with a peak HW stage of 930.47 as shown in the Stage and Flow hydrograph below.  The peak flow rates are within 1.5%, but the peak HW elevations differ by almost 3’.

The Outlet RC feature is also available for lateral structures.

Transect for Extracting Flow in a 2D Area

Written by Christopher Goodell, P.E., D.WRE  |  WEST Consultants
Copyright © The RAS Solution 2015.  All rights reserved.

*Update:  Version 5.0.2 now does flow transects in RAS Mapper with profile lines.  No longer necessary to use a 2D Area Connection to do this.

A very common output request in 2D modeling is for flow over (through) a transect line.  HEC-RAS 5.0 does not have a specific tool for extracting flow data along a line in a 2D area, however it CAN be done.  Here’s how you do it.
Inside your 2D area, draw a 2D Area Connection where you would like to extract flow.  In the figure below, I’ve drawn my 2D Area Connection line from bottom to top, which represents left to right, looking in the downstream direction. 

Notice that with the completely orthogonal cell orientation, but with a skewed 2D Area Connection line, the black and red striped connection line is “jagged”, following cell faces between the connecting cells.   To fix this, we need to use a breakline.  You could draw a break line on top of the 2D Area Connection (click the 2D Area Break Lines button on the top of the Geometric Data window), however, HEC-RAS also has a short cut for putting breaklines on 2D Area Connections.  If you right click on the 2D Area Connection (I’ve named if “Flow Line”), you’ll see an option to “Create 2D Flow Area Breakline”.  Select this for quick cell re-orientation around the 2D Area Connection Line.


Now you’re ready to enter data to the 2D Area Connection that you’ll use for extracting flow.  Click the SA/2D Area Conn button on the left side of the Geometric Data window. 
Make sure you connect “From” and “To” to the same 2D flow area.  Then choose “Normal 2D Equation Domain”.  This will force RAS to use the same 2D St. Venant equations over the terrain under the 2D Connection line as if the connection line wasn’t there.  BUT…you still have to define the Structure Type as a Weir and provide station-elevation data to define the crest of the weir, even though RAS will not use it.  Maybe this gets fixed in a later version.  HEC makes this easy though as you can extract the terrain profile from the 2D Connection cutline by clicking the button “Terrain Profile” as shown below.  Just copy and paste the terrain profile into your station-elevation table and you will  have defined a “zero-height” weir.  I guess it’s possible to use any crest definition, since RAS shouldn’t use the weir equation-it will use the normal 2D equation domain, but I haven’t tried it. 

Now you are ready to compute.  After computing, go to the Stage and Flow Hydrographs output window.

Make sure you select Type…Storage Area Connections (yes, it is technically an SA/2D Area Connection, but HEC hasn’t gotten around to renaming this yet in this window).  Now you can see the flow hydrograph for the flow line you’ve drawn.  You can check, but you’ll see the results are exactly the same as before including the flow line.

Tuesday Tip of the Week–Column Header Select

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

Here’s a nice feature in HEC-RAS input tables that will save you a little bit of time.  Let’s say you want to highlight an entire column of data.  Maybe you want to delete all of the values in that column or paste in a block of data.  Or perhaps you want to highlight all of the cells so that you can perform an operation (multiply by a factor, add a constant, etc.) to all the values in the column.  Instead of clicking and dragging from top to bottom in the column (this could take a long time if you have a lot of entries), click the column header.  This will highlight the entire column instantly, allowing you to quickly perform the desired task.  This feature is particularly useful when working in boundary condition hydrographs, or geometry tables, like Manning’s n values.

Tuesday Tip of the Week–Axis Zooming for True Peak Discharge

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

Copyright © The RAS Solution 2014.  All rights reserved.

Another simple, yet very convenient tool not found discussed in the manuals is the ability to zoom in on one of the axes of a graphical plot.  This is especially useful when trying to determine if you’ve captured the true peak discharge in a steeply rising hydrograph.  Take the following example: an unsteady flow dam breach output flow hydrograph.

Here we have a steeply rising flow hydrograph from a breach at the inline structure that appears to have a peak discharge of about 35,000 cfs.  To visually inspect whether or not this hydrograph captures the true peak, we can zoom in on the x axis by clicking and dragging a “bar” on the axis itself.  Simply place your cursor just below the x axis line, on the axis labels (in this plot, the time, in hours), then click and drag a red bar bracketing the portion of the hydrograph you wish to zoom in to.

Once you release the mouse button, the plot zooms in only on the x axis, in effect “spreading” out the hydrograph so that the peak is more apparent.  Notice that the full range of discharges still appear in the plot, we’ve simply truncated the time window to a more visually convenient range. 

Now notice we can see that because the hydrograph output interval is set too coarse (in this case every 10 minutes), we miss out on the true peak of the discharge hydrograph.  It happens somewhere between 1950 hours and 2000 hours.  By rerunning the model with a smaller hydrograph output interval, we can capture a better representation of the true peak discharge.  In the next figure, the discharge hydrograph is shown for the same location with a 2 minute hydrograph output interval.

After zooming in again on the x axis, we can see that we have a much better representation of the true peak of the discharge hydrograph, which is closer to 42,000 cfs, versus the 35,000 cfs shown in the first simulation:

The axis zooming feature works on any plot (input or output) and you can zoom on either the x or y axis.  So next time you’re evaluating hydrographs, or simply wish to have a better view of a curve in any of the plots, give the axis zooming feature a try.

Initial Conditions Trick

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2011. All rights reserved.
I’ve mentioned many times on this blog and in class about the importance of having your initial conditions flow match your first time step flow, when running an unsteady flow model.  As with any “rule” in RAS modeling, there are exceptions to this, but generally speaking, if your first timestep flow is “X”, then your initial conditions flow should also be “X”. 

This is generally very easy to do-if you have a single reach.  The hard part is just remembering to do it.  So, if you know you’re going to be changing your initial flow a lot, maybe while testing different hydrologic scenarios, or just trying to stabilize your model, you can save yourself some time and extra mouse clicks by leaving the initial flow cell blank.  By leaving it blank, you’re telling RAS to just use the first timestep flow provided in the flow hydrograph.  Give it a try, it works great.

This works perfectly for single reach systems.  For more complex systems, there is a catch.  For dendritic systems, this trick only applies to the upstream end of Rivers, not at junctions.  For looped systems, this trick only applies to the upstream end of Rivers that do not originate as distributaries from a junction.  Don’t worry, if you fail to enter an initial flow value where one is required, RAS will tell you about this when you try to compute. 

Dam Failure of a Coal Slurry Impoundment

Written by Chris Goodell, P.E., D. WRE | WEST Consultants
Copyright © RASModel.com. 2011. All rights reserved.
I was recently asked my opinion on a good way to model the following dam breach event:
“I …recently completed three consulting projects where I simulated the breach of three proposed coal slurry impoundments.  The permitting agency required a RAS model of an “instantaneous” hypothetical breach (over full depth, almost 80 ft for one of the impoundments).  I was able to achieve a stable model with brief breach formation time and satisfy the permitting agency.  The client (a coal company) considered the results to be unrealistic due to the rapid failure time and the fact that much of the impoundment is very viscous slurry; they have asked me to revisit the problem.  They asked me to model a partial breach of the top 10 ft, which they estimate is the distance from the top of slurry to the top of impoundment and occupied by water for the failure scenario, followed by the viscous slurry.  I was wondering if HEC-RAS could model such a complex situation.  I was thinking it might be modeled using the sediment transport capabilities within RAS.  I do have properties of the slurry, including particle size distribution, etc.  I suspect a more complex model is needed, but wanted to get your opinion, since I frequent your blog and have seen many complex issues addressed with RAS.
Thanks to Jason Hill, Ph.D., P.E. for sending in this interesting problem.  I don’t know if it is ultimately the best solution, but one that I think may work is as follows:
“…to model the breach of a partial water, partial slurry impoundment, you’re going to have to get creative.  First of all, RAS technically cannot model highly viscous fluids, like mud or slurry flows.  Really your only option for a “RAS-Only” model is to bump up Manning’s n values to account for the highly viscous flow.  Without a means of calibrating these high n values, you really are just guessing when you increase them. 
Here’s my suggestion:  Not sure if this would work, but what I would explore is the use of a combination of HEC-RAS, NWS BREACH, and FLO2D.  First, assume the first “pulse” of flow (water flow) will be separate and distinguishable from the second pulse (slurry flow).  The initial (water) part of the breach and the first pulse can be modeled and mapped using HEC-RAS exclusively.  For the second pulse of flow, I would model the remainder of the breach using NWS BREACH.  This model will simulate the breaching process and will generate a breach outflow hydrograph for you.  An advantage of NWS BREACH over RAS is that it provides an input for sediment concentration of the breach flow.  Once BREACH has provided you with a breach outflow hydrograph, use that as the inflow to a FLO2D model.  I say FLO2D only because I’m familiar with it and it can model highly concentrated mudflows.  But any model that you can find that models mudflows will work in this case. 
In summary, you’ll end up with two hydrographs to route downstream and to map independently:  the water hydrograph, and the slurry hydrograph.  The “water” breach will be modeled, and the water hydrograph will be routed using HEC-RAS.  The “slurry” breach will be modeled with NWS-BREACH, and the slurry hydrograph will be routed using FLO-2D (or other model capable of simulating mudflow).”
Although I know a little bit about NWS BREACH and FLO2D, I freely admit I haven’t tried this before. I think it can be made to work but I can also foresee a few hurdles.  Namely, what happens when/if the slurry flow and the water flow ultimately mix together somewhere downstream?  How do you map that condition?    If any of you out there have other suggestions, please feel free to comment to this post.