Bioretention Pond Design

How Does Bioretention Work?

Bioretention areas (also referred to as bioretention filters, infiltration basins, or rain gardens) are structural stormwater controls that capture and temporarily store a pre-determined water quality volume (WQv) using soils and vegetation in shallow basins or landscaped areas to remove pollutants from stormwater runoff. They typically have a maximum contributing drainage area of about 5 acres. Highly impervious drainage areas should be 2 acres or less. Rain gardens are usually limited to residential lots and smaller areas.

Bioretention pond design

Always remember that the Infiltration (a.k.a. Exfiltration) should be viewed as any other outlet device in the pond.

The best way to create a bioretention pond in Hydrology Studio is to use either the Contours and/or Trapezoid shapes for the storage definition. The engineered soil layer (bioretention soil and gravel bed) should be sized to contain the required WQv but should not be included in your storage definition.

Don’t Include Infiltration Soil in Storage

Here’s why. Inflow from runoff exits the pond (outside of any structural devices such as weirs, orifices, etc.) through the bottom soil layer via infiltration. The infiltration is considered a control outflow device, just like any other outflow device. The bioretention soil layers ARE the outflow device and are technically downstream. Including their voids volume violates the assumptions of the Storage Indication or Level Pool routing procedures. With or without an underdrain, the bottom of the pond as shown above should be stage zero.

Outlet Devices

You’ll add the outlet devices in Step 3: Add Outlets when building your detention pond. The example bioretention pond above shows to have two outlet devices:

  1. Exfiltration (because it exits the pond) entered as a simple percolation rate typically in inches/hour per field tests. (A minimum percolation rate of 0.50 in/hr (13 mm/hr) is accepted by most jurisdictions.) This rate will be applied to the contour areas of the pond thus producing an outflow in cfs (cms) at each stage in the pond.
  2. An overflow weir. Typically a broad-crested weir. The size and crest elevation to be set to satisfy target outflow and drain-time requirements. The WQv will need to be 100 percent contained below the weir crest elevation.


If the native soil beneath the bioretention layer cannot convey water at a minimum infiltration rate (usually about 0.5 inches/hr), an underdrain should be included in your design. Underdrains re-introduce infiltrated water back into the outflow system. To account for this flow, uncheck the option, “Extract from Outflow” hydrograph as shown below.

Using a Bioretention Pond Forebay

Forebays are often used to capture a first-flush or water quality volume (WQv) before the runoff hydrograph reaches the main detention pond. There are two approaches you can use in the software:

1. Remove the WQv from the inflow runoff hydrograph

When off-line structural controls such as bioretention areas and infiltration trenches capture and remove the water quality volume (WQv), downstream structural controls do not have to account for this volume during design. That is, the WQv may be subtracted from the total volume that would otherwise need to be routed through the downstream structural controls.

From a calculation standpoint this would amount to simply removing the initial WQv from the beginning of the runoff hydrograph – thus creating a “notch” in the runoff hydrograph. This can be accomplished by Diverting the inflow hydrograph using the “First Flush Volume” option. For more information, see, Diverting Hydrographs.

2. Setup two ponds (forebay and detention)

The second option includes setting up two ponds, one for the Forebay or Bioretention pond, and the second for a more permanent pool Detention Pond as shown below.

This is a multi-purpose infiltration-detention system. The Bioretention Forebay would be sized just large enough to contain the WQv and would have Exfiltration as an outflow device as well as an overflow Weir which would divert excess runoff into the lower, traditional detention pond.

Setting Up the Basin Model for Bioretention

The most upstream inflow hydrograph would be routed through the Bioretention Forebay, creating an outflow hydrograph. That outflow hydrograph would then be used as the Inflow hydrograph into the second (Detention) pond. A second routing would produce the Final Outflow hydrograph.

Your Basin Model will look like the following where your runoff hydrograph is used as the Inflow into the first pond (Forebay). That routing will produce a hydrograph that will be used as the Inflow into the second pond (Detention Pond). Thus two pond routings.

When using a Forebay be sure to “Extract” the exfiltration when setting up the Forebay pond unless an underdrain is used and daylights into the second pond. You do not want the exfiltration to be included into the “inflow” to the downstream pond.

Drain Time, WQv and Extended Detention

To learn more about modeling for Water Quality Volume (WQv), Channel Protection Volume (CPv) and Drain Time, read this article regarding Full Spectrum Detention.


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