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Thread: Floodable area

  1. #1

    Floodable area

    Hi All,

    What values do you use to populate Manhole Parameters? Do you keep system defaults or do you change them? I am particularly interested in levels, cross sectional areas and floodable areas.

    Regards,
    David

  2. #2
    Hi David,

    The default values are there to represent the typical response of a UK catchment.

    As the cone defines the capacity above the flood level it should be edited based on the topography of the drainage area associated with it. Most models have the default values set but for steep catchments the cone can be made thinner with higher gradient sides or for a more level catchment the cone can be made flatter.

    I am not aware of any guidance on this but perhaps some is available that a user from the community could provide access to.

    Kind regards

    Raana

  3. #3
    Raana,

    Thanks for the reply. I have analysed number of models produced by various people. Almost everybody kept cone's default shape. However floodable areas assigned vary. Where subcatchment is connected to node the area of subcatchment is adopted as the floodable area by default. The tricky part is assigning floodable area to manholes where there is no catchment connected to. Some people use standar values varying from 0.1 to 1ha. However in some instances default value of 0ha was kept which is very risky as in that instance no flooding will be indicated even if the hydraulic gradient line is above the ground level at this point.

    What floodable area do you use for manholes where there is no subcatchment connected?

    Regards,
    David

  4. #4
    Hi David,

    For the modelling work we do for Northumbrian Water we set all MHs with #D zero floodable area to 0.1ha.

    Flood cone details are set to Flood 1 0.1m @ 50% Cross sectional area, and Flood 2 0.3m @ 100% Cross sectional area. I think this was to try to represent MHs in a typical road/highway with a 100mm curb better.

    Regards

    Ali

  5. #5
    Alistair,

    Do you also change flood cone details for MHs with catchments attached?

    Regards,
    David

  6. #6
    David,

    All MHs flood cones are set by default to the figures I gave above, including the MHs with subcatchments draining to them. We would only ever deviate from the default values if we are looking at a certain area in a lot of detail, for say a property flooding scheme, where we often include private drainage etc and set floodable areas and flood cone details to be more realistic for the detailed area in question such as to represent a small back yard etc.

    Regards

    Ali

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