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Friday, March 10, 2017

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.

14 comments:

  1. I’m not totally understanding your note that “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”. I’m trying to have the rating curve determine the flood elevations at the dam but my model is dividing the flow between the rating curve and weir flow over the dam.

    ReplyDelete
    Replies
    1. If the water surface is higher than the crest of the dam, you'll get weir flow in addition to flow from the user rating curve. If you don't want the additional weir flow, artificially raise the top of dam so that water doesn't overtop it.

      Delete
    2. Thanks Chris, this really helped. I was having trouble determining if the weir/rating curve were operating together or if the rating curve took priority. I now see they were operating together for a combined effect and have corrected my model. Thanks!

      Delete
  2. I simulate 2d and breach, but the results on stage and flow hydrogrphs on the tab curves rating and internal boundary curves are very messy and irregular. how can this problem be resolved?

    ReplyDelete
  3. Hi Chris. I'm trying to use the option of Outlet RC but when I try to run the model I get this message:

    SA/2D Connection:
    " - Outlet RC: has GIS coordinates that do not intersect the structure GIS centerline"

    I'm using the Hec-RAS 5.0.7 but I also tried old versions (5.0.6,5.0.5,..)so I believe that's something that I'm doing wrong.

    Do you know this problem? Any ideas about how to fix this? Thanks!

    ReplyDelete
    Replies
    1. I have not experienced that problem before, but the error message sounds helpful. Check your GIS coordinates.

      Delete
  4. I simulated a flood plain area which water goes in from upstream (with Flow Hydrograph B.C) and get out from downstream outflow (with Normal Depth B.C).
    My question is how can I limit the maximum amount of out flow can I do that with outlet rating curve? Lets say my inflow might goes up to 1000 cfs but outflow can not go more than 500 cfs at maximum. How can I set this boundary condition?

    ReplyDelete
  5. Hi Chris, I have a question to ask about the inclusion of inline structures within a 2D model. I need to insert two structures that regulate a reservoir: the first is a barrier in line with a Creager type overflow and four fixed bottom lights, the second is a wide threshold lateral overflow. In particular I need to insert an RC Outlet for each one, but the model keeps giving me this error. How can I solve it?
    Thank you for your kind attention.

    "ERROR with Connection(s)

    The SA 2D Connection at lateral

    has a rating curve with a starting flow elevation lower than the cell
    it is connected to (this is not allowed in HEC-RAS):
    cell 1048 cell elev 45.00
    between Face Points 1418 1419
    for 2D Area 2D

    Error starting unsteady simulation - Stopping compute process"


    ReplyDelete
    Replies
    1. Raise up the first point on your rating curve so that it is not lower than the cell(s) it is connected to. In this case, 45.00.

      Delete
  6. Hi Chris, I have a question to ask about the inclusion of inline structures within a 2D model. I need to insert two structures that regulate a reservoir: the first is a barrier in line with a Creager type overflow and four fixed bottom lights, the second is a wide threshold lateral overflow. In particular I need to insert an RC Outlet for each one, but the model keeps giving me this error. How can I solve it?
    Thank you for your kind attention.

    ERROR with Connection(s)

    "The SA 2D Connection at lateral

    has a rating curve with a starting flow elevation lower than the cell
    it is connected to (this is not allowed in HEC-RAS):
    cell 1048 cell elev 45.00
    between Face Points 1418 1419
    for 2D Area 2D

    Error starting unsteady simulation - Stopping compute process"

    ReplyDelete
  7. Hi Chris

    I have the same issue as above in a 2D/SA connection between a storage area and 2D domain:
    "Outlet RC: has GIS coordinates that do not intersect the structure GIS centerline"
    I have checked and double checked my GIS coordinates, and the RC line definitely intersects the structure line, drawn from upstream to downstream across the structure. I see there are a few posts on the forum with the same problem - what's the solution? Is this a bug or am I doing something wrong?

    ReplyDelete
  8. Hi Chris

    I have the same problem as above with a SA/2D connection between a storage area and a 2D domain: "Outlet RC: has GIS coordinates that do not intersect the structure GIS centerline". I have double checked the GIS coordinates and the RC centreline definitely intersects with the structure line, drawn from upstream in the storage area to downstream in the 2D area. I see there are a few posts in the forum with the same issue - what's the solution?

    ReplyDelete
    Replies
    1. Not sure. However, if your structure line is drawn backwards, I have seen weird stuff like this happen. Make sure your structure line is drawn from left to right from the perspective of looking downstream (i.e. the "from" location is behind you, the "to" location is in front of you. See if that works.

      Delete
  9. Hi Chris

    I have the same problem as above with a SA/2D connection between a storage area and a 2D domain: "Outlet RC: has GIS coordinates that do not intersect the structure GIS centerline". I have double checked the GIS coordinates and the RC centreline definitely intersects with the structure line, drawn from upstream in the storage area to downstream in the 2D area. I see there are a few posts in the forum with the same issue - what's the solution?

    ReplyDelete

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