|Click to download Advanced . ] tutorial model files|
Open the file ID_US_Advanced_Tutorial_Ch5.iddx 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.
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. 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. 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, you will size the system to address the Water Quality concerns by using a Runoff Reduction sizing calculator.
- Edit Raingarden and select the Sizing Calculator
- There are several sizing methods that can be used to initially size Stormwater Control (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.
- 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.
- 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. Columns can be shown/hidden by selecting the options at the top of the dialog. Note that the current Outcome is Failure. The SWC could be upsized however space is tight on site and care must be taken to avoid the houses. Click OK to use this required volume to upsize the SWC and see the new outline.
- 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 automatically adjust to accommodate the required Runoff Reduction Volume. Click OK to see the required footprint size of the SWC.
- It is now much larger and may be overwhelmed in higher events. To ensure the smaller Bioretention area is not overwhelmed in higher events, one option is to split the Inflow into two areas, 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.
- Create an Inflow for the remaining portion of the size as was done previously, 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.
- Press Esc to exit the drawing mode. Open the Raingarden again and rerun the Sizing Calculator, selecting Runoff Reduction again and the volume has now reduced. Update the Areas and the Bioretention now just fits into the space between the buildings. Further refinement may be needed in reality to ensure properties are not at risk however, this demonstrates the importance of being able to see the outline on plan.
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, this time change the method to Quick Storage Estimate and the Design Level to Freeboard, this is to allow a 6 inch gap below the top level of the pond for safety.
- Click on the calculator next to the Volume and select the Input Type as User Input, to enable us to enter the whole area rather than only the directly connected portion. 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 within 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 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.
- It is often easiest to calibrate the upstream facilities first in a multi-facility treatment train. Edit the Raingarden SWC Weir outlet, increase the width to 2 ft wide. Then run the analysis again.
- Viewing results for the Raingarden SWC for All Storms within the Stormwater Controls summary shows that increasing the weir width eliminated the flooding at this facility.
- Select Comparison Report under the Results ribbon, and use Update Preview to show a comparison between the Existing and Completed Design Phase outfall results.
- It is shown that the current Dry Pond outlet is discharging less flow than the Existing Phase.
- You can continue to adjust the Treatment Train as discussed above in order to refine the Results comparison to the Existing Phase flows, if desired.
- This tutorial in its entirety has been saved and can be viewed by opening the ID_US_Advanced_Tutorial_Completed.iddx file. 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.