Stormwater Control Sizing Calculator

Each Stormwater Controls - SWC has an associated sizing calculator that allows you to:

• 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

The SWC sizing calculator consists of the following options:

Method

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 center 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. (Version 2021.5 onwards) For phases containing only Baseflow or Input Hydrographs the following options are not available: Quick Storage Estimate and Runoff Reduction Volume.

Choose Scale to specify a change in scale of the current Total Volume of the SWC (as given in the bottom right of the SWC's Properties form). The figure entered is relative, so 0% is no change in Total Volume, 100% is a doubling in Total Volume, -50% results in halving the Total Volume.

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 Elevation that the volume will be provided up to:

• Exceedance - The Exceedance Elevation of the Stormwater Control.

• Freeboard - The Freeboard Elevation of the Stormwater Control. i.e. Exceedance Elevation - Freeboard depth.

  Note: Where the freeboard is not specified the Exceedance Elevation will be used.

• Lowest Outlet - The Invert Elevation of the lowest outlet with a control specified. 

  Notes: If the control Invert Elevation is below the Base Elevation of the structure (or Ponding/Swale in a Bioretention/Swale) the control will not be considered. If there are no outlets the Exceedance Elevation will be used.

Update

This is applicable for all Methods except Side Slope - relevant to Tank and Pond SWCs only. Choose a parameter that is to be updated when the SWC 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 Elevation etc. 

Two options perhaps needs a little more explanation:

• Areas (for Pond and Tank only). Choosing this option results in the cross-sectional area for each entry in the Tank / Pond's Depth / Area / Perimeter spreadsheet to be scaled by the same scale factor to achieve the specified volume. In addition, the Perimeter value for each entry in the Depth / Area / Perimeter is set to the circular circumference of its new area.
• Number Of Dry Wells (for Dry Well only). Choosing this option results in the Number Of Dry Wells parameter being set to the minimum no. of dry wells of the current dimensions that are required to satisfy at least the specified volume.