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Each Stormwater Controls - SWC has an associated sizing calculator that allows the user you to either:(a) specify

  • Specify a new volume for the Stormwater Control whilst varying a chosen dimension, e.g. Depth, Width, Length etc.


  • Specify a scale factor to modify the existing volume of the Stormwater Control, again whilst varying a chosen dimension


  • In the case of a Tank or Pond to specify a particular side slope, either from its top downwards or base upwards

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The SWC sizing calculator consists of the following options:

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Choose Quick Storage Estimate or Runoff Reduction Volume to specify a volume in the field below. When choosing either of these a calculator button appears to the right of the Volume field below; if Quick Storage Estimate was chosen then clicking on the calculator button produces the Quick Storage Estimate Calculator (QSE) calculator, if Runoff Reduction Volume is chosen then clicking on the calculator button produces the Water Quality Calculator (WQv) calculator. Using either of these calculators will return a volume into the sizing calculator; in the case of the QSE calculator the returned volume is the centre of the estimated storage volume range, in the case the WQv calculator the returned volume is the Target Volume stated in the WQv calculator's report.
Using the Quick Storage Estimate option also allows the user to enter manually the desired volume rather than using the calculators.


For Tank and Pond SWCs there is a fourth option: Side Slope. Choose this option to impose a side slope to the structure of the Tank or Pond, specifying the side slope in the field below. In addition the third row of the form is changed to Maintain whose combo box contains two options: Top Area and Base Area. Choose Top Area to impose the side slope from the top area downwards; choose Base Area to impose the side slope from the base area upwards.

Design Up To

(This is applicable for all Methods except Side Slope - relevant to Tank and Pond SWCs only). Choose the level that the volume will be provided up to:

  • Exceedence - The Exceedence Level of the Stormwater Control.
  • Freeboard - The Freeboard level of the Stormwater Control. i.e. Exceedence Level - Freeboard depth.

    Note: Where the freeboard is not specified the Exceedence level will be used.
  • Lowest Outlet - The invert level of the lowest outlet with a control specified. 

    Notes: If the control invert level is below the base level of the structure (or Ponding/Swale in a Bioretention/Swale) the control will not be considered. If there are no outlets the Exceedence Level will be used.


(This is applicable for all Methods except Side Slope - relevant to Tank and Pond SWCs only)Choose . Choose a parameter that is to be updated when theSWC is resized to the new volume. This list is specific to the type of SWC. Most of the options are self-explanatory, e.g. Length, Width, Base Level etc. 

Two options perhaps needs a little more explanation: