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Thread: ICM 7.5 BMPs modeling questions

  1. #1

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    Jan 2018
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    Post ICM 7.5 BMPs modeling questions

    We are modeling several BMPs with ICM 7.5 but are unsure of how to handle the BMP footprint within the catchment. See questions below:

    1/Do we need to subtract the BMP footprint area from the catchment tributary area?
    2/If the BMP is built on an area that’s impervious within the catchment, do we need to recalculate the percent impervious and percent pervious in the catchment once the BMP footprint is subtracted from the catchment tributary area?
    3/If the BMP is built on an area that’s pervious within the catchment, do we need to recalculate the percent impervious and percent pervious in the catchment once the BMP footprint is subtracted from the catchment tributary area?
    4/ Do the responses to the questions above apply to all types BMPs equally (green roofs, infiltration trenches, porous pavement, bio-retention…)?

    Thanks,

  2. #2
    Senior Member

    Innovyze Employee



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    Quote Originally Posted by Eudora View Post
    We are modeling several BMPs with ICM 7.5 but are unsure of how to handle the BMP footprint within the catchment. See questions below:

    1/Do we need to subtract the BMP footprint area from the catchment tributary area?
    2/If the BMP is built on an area that’s impervious within the catchment, do we need to recalculate the percent impervious and percent pervious in the catchment once the BMP footprint is subtracted from the catchment tributary area?
    3/If the BMP is built on an area that’s pervious within the catchment, do we need to recalculate the percent impervious and percent pervious in the catchment once the BMP footprint is subtracted from the catchment tributary area?
    4/ Do the responses to the questions above apply to all types BMPs equally (green roofs, infiltration trenches, porous pavement, bio-retention…)?

    Thanks,
    Hello Eudora,
    Aren't you actually asking about how SuDs or LID's are modeled in ICM?
    Regards,
    Bob Dickinson

  3. #3

    Join Date
    Jan 2018
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    Hi Bob,

    Yes. Could you clarify it for us? Thanks,

    Eudora

  4. #4
    Senior Member

    Innovyze Employee



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    Quote Originally Posted by Eudora View Post
    Hi Bob,

    Yes. Could you clarify it for us? Thanks,

    Eudora
    Hello Eudora,
    Here is link that explains the internal calculations.
    https://www.openswmm.org/Topic/9951/...ers-with-a-lid
    and a link with images
    https://swmm5.org/2017/04/17/swmm5-s...deling-part-1/
    Regards,
    Bob Dickinson

    Key passage from Lew Rossman


    Lew Rossman

    [Original email]

    Just to be clear, there is no internal adjustment made to the Percent Impervious value of a subcatchment with LID units. It’s up to the user to make the proper adjustment. Take for example a 10 acre subcatchment which has 50% imperviousness before any LID units are added (i.e., 5 acres). Then suppose 20% of the original pervious area (1 acre) and 40% of the original impervious area (2 acres) are replaced by LID units. The amount of non-LID area is now 7 acres of which 5 – 2 = 3 acres is impervious. The user (not the program) has to re-set the subcatchment’s Percent Impervious to 3 / 7 = 43%. The percent of the subcatchment occupied by LID is 3 / 10 = 30%.
    The percent impervious adjustment can’t be done internally by SWMM because it has no way of telling how much pervious v. impervious land was replaced by LID. The user has to bring this information to the particular LID setup they are trying to model. The effect that LID usage has on the resulting non-LID area Percent Impervious will depend greatly on the type of LID employed and various site configuration constraints.



  5. #5

    Join Date
    Jan 2018
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    Thanks for clearing this up. This is very helpful!

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