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Modeling 2D area connections in HECRAS

  • 1.  Modeling 2D area connections in HECRAS

    Posted 01-09-2020 11:46 AM
    Folks:

    We are attempting to model two-dimensional flow in two channel transition areas connected by a culvert. The two areas are separated by an embankment. I noticed that if we add a 2D area connector to the model and specify the embankment properties in the same window used to enter the culvert properties, the computed flow field does not "see" the embankment. That is, it appears in the model results that the two areas are separated by an invisible, thin wall with the culverts passing through it. On each side, water can be seen circulating under the culverts, which does not reflect reality since the barrels are buried within an embankment.

    Given this, it appears that we have to first modify the terrain to include the embankment. Is this correct? Or, have we erroneously omitted something else in the model geometric input?

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    Mark Wilsnack P.E., D.WRE, F.EWRI, F.ASCE
    PRINCIPAL ENGIN
    South Florida Water Management District
    West Palm Beach FL
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  • 2.  RE: Modeling 2D area connections in HECRAS

    Posted 01-10-2020 10:11 AM
    Mark,

    The embankment will behave as a wall in the 2D model provided the WSE on either side of the embankment remains lower than the embankment crest elevation. If the computed WSE rises above the crest the model will then "see" the embankment and calculate a discharge based on the weir parameters you specify in the embankment tab.  Adding embankments can sometimes be tricky given the the adjacent 2D cell elevations as well as getting the limits of the embankment correct.

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    Breagan Eicher P.E., M.ASCE
    CE
    Lawson-Fisher Associates PC
    South Bend IN
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  • 3.  RE: Modeling 2D area connections in HECRAS

    Posted 01-13-2020 08:57 AM
    Good Morning Mark,

    It is hard to answer questions like this without being able to see your model, so take these with a grain of salt if I am way off on some of these assumptions.

    Yes the embankment needs to be included in the underlying terrain as well as the 2D grid. I prefer to insert break lines in the grid down the center of embankments to realign cells so that one face of each cell matches the embankment alignment. I usually decrease the cell size along break lines to better capture the slopes of embankments.

    If you are using version 5.0.4 or later (preferably 5.0.7), I suggest using only one 2D grid and modeling the culvert as an internal boundary condition. This should theoretically remove the invisible wall and allow smoother transfer across the embankment. However, if you are stuck with an older version (5.0.3 or earlier don't allow internal boundary conditions), make sure that the structure representing the embankment doesn't have elevations significantly higher than the grid cells on either side.

    Hope this is helpful!

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    Maegan Nunley P.E., M.ASCE
    Senior Project Engineer
    Luna Engineering
    Columbus OH
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