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

Open the file XPD_US_Advanced_Tutorial_Start.xpdx or continue working from your previous file.

You can carry on from the previous Tutorial 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 Protection Event Sizing, and Results Fine Tuning.

As you saw from reviewing the results, the Treatment Train is not currently configured to adequately and efficiently 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. Furthermore, 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. Select the desired component or work through all three starting with Runoff Reduction Sizing.

Runoff Reduction Sizing

This chapter addresses the design aim to capture and infiltrate 1 inch of the ‘first flush’ Runoff Reduction volume from the site, in order to address pollution concerns.

First, we will size the system to address the water quality concerns by using a Runoff Reduction sizing calculator.
Double-click the Raingarden SWC and select the Sizing Calculator button.
There are several sizing methods which can be used to initially size SWC facilities. In this tutorial, use the default Runoff Reduction Volume option. Furthermore, there are several design levels that 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.
There are several different options in 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.
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 Raingarden SWC. The columns can be shown or 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 ft³. Click OK to use this required volume to upsize the SWC.
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.
The SWC dimensions will be automatically adjusted to accommodate the required Runoff Reduction Volume. Click OK to see the required footprint size of the SWC.
As is 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.
To reduce the size of the Bioretention area, one option is to split the Inflow Area into two, and use both the Raingarden and the Dry Pond facilities to retain and infiltrate the Runoff Reduction Volume. This also more accurately represents how the site will drain. Ensure that the Snap option is turned on under the Plan ribbon, then right-click the Inflow Area and select Replace Outline > Free Form.
Trace the lower portion of the site area as shown, then press the Esc key to deselect the Inflow Area.
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.
Notice that the newly added Inflow Area has been automatically connected to the adjacent Standard MH junction. No addition inlet specification or connection is needed.
Try sizing the Raingarden SWC system again to see if this split will make the required facility footprint more manageable. Double-click the Raingarden, select Sizing Calculator, then the Calculator icon, and then click Calculate. Note the reduction in runoff volume generated by the associated Inflow Area. As the SWC facility is over sized, click the OK button three times to apply this smaller required volume to the facility and notice the new footprint extents no longer overlap the buildings.

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.

Edit the Dry Pond SWC then select the Sizing Calculator. Change the Method to Quick Storage Estimate and the Design up to to Freeboard, this will allow a 6 inch gap below the top level of the pond for safety.
Click the calculator next to the Volume and 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
Due to the potential variance in rainfall events and runoff timing, there is a wide approximate storage range indicated. However, the average will give us a good starting point. Click OK and note that an average volume for the potential range is selected.
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.
Click OK twice to see the initially sized top extents of the Dry Pond SWC.

Results and Fine Tuning

Run the analysis in order to compare results between the Existing and Developed phases by selecting Go under the Analysis ribbon.
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 in the site.
By changing the rainfall to the 100 year ARI event the Raingarden SWC is shown to Flood, while the Dry Pond is shown to have available (extra) volume capacity.
The view from the Profile shows slight overtopping of the Raingarden 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 in the system and the orange icon shown in the screenshot below indicates the Water Surface Level (WSL) is in the Freeboard for the SWC facility.
It is often easiest to calibrate the upstream facilities first for a multi-facility treatment train. Edit the RaingardenS WC Weir outlet, increase the width to 2 ft wide. Then run the analysis again.
Viewing results for the Raingarden SWC for All Storms in the Stormwater Controls summary shows that increasing the weir width eliminated the flooding at this facility.
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 less flow than the Existing phase. With some storage volume capacity within the Dry Pond and the maximum discharge not reached, the pond size could be slightly reduced.
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.
The finished layout can be exported to AutoCAD file (*.dxg or *.dxf ). Go to the Export ribbon, and then select the To CAD button.