Welcome to the RAS Solution › Forums › HEC-RAS Help › 10% Slope limitation (in 2D) – solution?

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  • January 25, 2019 at 1:22 pm #7189
    Mark
    Participant

    Hello All

    I am aware of the 10% slope limitation with slopes in excess giving potentially erroneous results.

    In my 2d rain-on-grid case, the results are hugely off in steep areas. One example shows a depth of approximately 8m relating to a flow of 0.1 cumecs (using a 20m mesh size, roughness = 0.06, with a slope of >50% for the cells in question).

    That said. Are there solutions to this error? Perhaps a smaller grid to limit affected cells – but the problem will still persist. Or do we need to just accept this as being a limitation, in which case, would there be a need to consider alternate software (i really hope not).

    Thanks

    January 25, 2019 at 7:08 pm #12020
    Anonymous
    Guest

    The slope limitation is based on the background equation for which the cosine of an angle is calculated.

    I have not had to model in regions with this much slope. BUT i have read about people solving this issue by making the cell size much larger in these regions. So the software sees the top of the slope as 1 cross section and only the bottom of the slope as 1 cross section confined in 1 cell.

    Did that make sense?

    January 27, 2019 at 3:35 pm #12021
    Mark
    Participant

    Hi Luis

    Thanks for that feedback. Yes – that makes sense. It may not be an option in my study due to the substantial variation in elevation… but good to know for future model builds. Could even be worth double checking cell faces on a slope grid.

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