File/s needed: 
Click to download IMPERIAL [US Units] model files.

You can carry on from the previous Chapter or switch to the Sizing of Treatment Phase by selecting from the drop-down list on the left Tree View panel if you have not already done so.

There are three sizing components which are typically performed - Runoff Reduction Sizing, Pond and Protection Event Sizing, and Results and Fine Tuning. Select the desired component or work through all three starting with Runoff Reduction Sizing. 

Runoff Reduction Sizing

As you saw from reviewing the results, the treatment train is not currently configured to adequately mitigate stormwater runoff from the site. The design aim is to capture and infiltrate 1 inch of the ‘first flush’ Runoff Reduction volume from the site, in order to address pollution concerns. Further, we must not discharge greater flow rates off site for the 5 and 100 year ARI events post-development than occurred in the pre-development (existing site) conditions.’

  1. First, you will size the system to address the Water Quality concerns by using a Runoff Reduction sizing calculator.
  2. Edit the Green Street Bio SWC and select the Sizing Calculator  button. 
  3. There are several sizing methods which can be used to initially size SWC facilities. In this tutorial, use the default Runoff Reduction Volume option. Further, there are several design levels which can be selected against which a facility can be adjusted. In this example, the default Exceedence level option will be used. Select the calculator icon within the Sizing Calculator dialog to assess Runoff Reduction volume.
  4. There are several different options within the Water Quality Volume dialog. Select Runoff Reduction under Method, and Use Plan Data for Input Type. This will calculate the Runoff Reduction for all Inflow Areas connected to the SWC you are editing. Enter a Precipitation Depth of 1.0 inch and click Calculate.

  5. The calculated results will itemize the Inflow Volume required to be retained for all connected Inflow Areas as well as the current storage volume of the GreenStreet Bio SWC. Columns can be shown/hidden by selecting the options at the top of the dialog. Note that the current Outcome is Failure. The SWC needs to be upsized to adequately retain the Inflow volume of nearly 20,000 cuft. Click OK to use this required volume to upsize the SWC. 

  6. In the Sizing Calculator, the required retention volume is shown. There are several methods to update the facility, such as increasing the Top and Bottom Areas or making the facility Deeper to accommodate the additional volume. The default Areas option should be selected, then click OK to update the SWC.
  7. The SWC dimensions will automatically adjust to accommodate the required Runoff Reduction Volume. Click OK to see the required footprint size of the SWC. 

  8. As commonly encountered during the design process, the initially sized facility does not adequately fit on the site. Note how the SWC extends into the proposed adjacent building areas. One option is to split the Inflow Area into two, and use both the Greenstreet Bio and the Dry Pond facilities to retain and infiltrate the Runoff Reduction Volume. Ensure that the Snap option is turned on under the Plan ribbon, then right-click the Inflow Area and select Replace Outline.

  9. Trace the lower portion of the site area as shown, then press the Esc key to deselect the Inflow Area
  10. Create an Inflow Area for the remaining portion of the size as was done previously, this time select the Urb-Res-Dev template under Build > Add Inflow, as shown below.

  11. Right-click the newly added Inflow Area and select Connect Inflow. Then snap the Inflow to the Northern Manhole. As no additional inlet restriction will be applied for runoff to this inlet, select the existing inlet at the Manhole as shown below.

  12. Try sizing the Greenstreet Bio SWC system again to see if this split will make the required facility footprint more manageable. Edit the Greenstreet Bio, select Sizing Calculator, the Calculator icon, then Calculate. Note the reduction in runoff volume generated by the associated Inflow Area. As the SWC facility is over sized, click the OKbutton three times to apply this smaller required volume to the facility and notice the new footprint extents. 

Pond and Protection Event Sizing

The controlling volume for the Dry Pond will be the 5 and 100 year ARI events, so no Runoff Reduction sizing is required for this facility, though the Sizing Calculator can be used to set an appropriate initial volume for the Dry Pond SWC to appropriately mitigate the larger runoff events.

  1. Edit the Dry Pond SWC then select the Sizing Calculator, this time change the method to Quick Storage Estimate

  2. Select the Input Type as User Input and enter the input as shown below. Select SCS – Greenville for the Rainfall, and then click Calculate.
    • Area = 6.51 ac
    • Volumetric Runoff Coefficient = 0.9 – this is an approximation of the landuse defined within the Inflow Area
    • Discharge Rate = 3.1 cfs – this is approximately the existing site discharge rate

  3. Due to the potential variance in rainfall events and runoff timing there is an approximate storage range indicated. Click OK and note that an average volume for the potential range is selected.
  4. In the Sizing Calculator, change the Design Level to Freeboard to ensure that the Dry Pond is adjusted based on the storage below the Freeboard level.
  5. Click OK twice to see the initially sized top extents of the Dry Pond SWC. 

Results and Fine Tuning 

  1. Run the Analysis in order to compare results between the Existing and Developed Phases by selecting Go under the Analysis ribbon. 

  2. After running the Analysis the Stormwater Controls summary is shown. Note that the first result shown is for the 1 in Water Quality Event, where there is no outflow from the Dry Pond, showing that all of the First Flush is completely retained within the site. 
  3. By changing the rainfall to the 100 year ARI event the Greenstreet Bio SWC is shown to Flood, while the Dry Pond is shown to have almost 40% available (extra) volume capacity. 

  4. The view from the Profile shows slight overtopping of the Greenstreet Bio SWC. Use the Playback Animation toolbar to view the inundation across the system. Note the red horizontal lines indicate the Maximum Hydraulic Grade Line (HGL) for the elements within the system and the orange icon shown in the screenshot below indicates the Water Surface Level (WSL) is within the Freeboard for the SWC facility. 
  5. It is often easiest to calibrate the upstream facilities first in a multi-facility treatment train. Edit the Greenstreet Bio SWC Weir outlet, increase the width to 2 ft wide. Then run the analysis again. 
  6. Viewing results for the Greenstreet Bio SWC for All Storms within the Stormwater Controls summary shows that increasing the weir width eliminated the flooding at this facility. 
  7. Now the Dry Pond and outfall structure needs to be sized. Select Phases Report under the Results ribbon, and use Update Preview to show a comparison between the Existing and Developed Phase outfall results. 

    It is shown that the current Dry Pond outlet is discharging more flow than the Existing Phase. With unused storage volume capacity within the Dry Pond and too high of discharge rates, the outlets should be raised and the weir size reduced. 
  8. Edit the Dry Pond outlets. Select Orifice and raise the Depth Above Invert level to 1.4ft.

  9.  Go to Weir, and update the Width to 0.3 ft and the Depth Above Invert to 3.2 ft. 

  10. Click OK twice and re-run the analysis. The Phases Report now shows that the site discharge is below the Existing Phase rates and the Stormwater Controls Summary indicates that the facilities are not flooding.
  11. You can continue to adjust the Treatment Train as discussed above in order to refine the Results comparison to the Existing Phase flows. The Completed Design Phase can be opened to reference the final model configuration as described in the section above.

  12. The finished layout can be exported to AutoCAD file (*.dxg or *.dxf ). Go to the Site ribbon, and then select the Save CAD button.