Description
Rain gardens are retention or detention areas that are integrated into a site’s landscaping. They are usually in a shallow constructed depression that has a cover crop, typically deep-rooted Florida-Friendly or native plants. Sometimes they include walkways or parking areas. They are located in the landscape or within parking lot islands to receive runoff from hard surfaces such as:
- Roofs
- Sidewalks
- Driveways
- Parking areas
Sometimes they are called planter boxes if located close to a building. The term Green Stormwater Infrastructure (GSIs) is frequently used.
For design, the water holding capacity of the soil or media includes:
- A sustainable void or porosity within the soil/media
- Water storage above the soil/media
Plant selection is done so that water storage above the rain garden ground level does not destroy the plants.
In situations where the Seasonal High Groundwater Table (SHGWT) is high, a rain garden is classified as a detention system and an underdrain is used. However, they do not function like a wet pond and therefore do not receive wet detention pond credit.
When a cover crop is used, credit for removal of TN and TP is assigned and must be documented to establish:
- Maintenance plan
- Ownership
- Credit assignment
When sorption media is used, a separate credit is assigned and must be documented to establish service life.
All credits are applied as if the remaining concentration and mass are being treated (series treatment).
Example Calculation of Series Removal
Assume:
- 100 kg/year input mass
- 60% removed by infiltration (retention)
- 40 kg/year remaining
If Cover Crop Removes 10%:
Overall removal =
60 + 0.10(40) = 64 kg/year
If Media Removes 75%:
Overall removal =
60 + 0.75(40) = 90 kg/year
The general series removal equation is found in the Applicant’s Handbook Vol 1 (June 28, 2024).
Input Data
Six Required Inputs
There are six (6) inputs required to determine annual removal effectiveness:
- Retention or detention
- Media used
- Sustainable void fraction of the media or soil above SHGWT
- Type of media and media volume
- Water storage above the rain garden (with or without media)
- Credit for a cover crop
The function of a rain garden is to:
- Infiltrate runoff (retention)
OR
- Detain runoff (detention)
Sustainable void fractions have been determined over long periods of time and eliminate safety factors based on clean water porosity testing. For most media, a sustainable void fraction of 0.25 is recommended.
References:
Chang, N.B., Wanielista, M., and Henderson, D. (2011). Temperature effects on functionalized filter media for nutrient removal in stormwater treatment. Environmental Progress and Sustainable Energy, 30(3), 309–317.
Hossain, F., Chang, N.B., and Wanielista, M. (2010). Modeling kinetics and isotherm of functionalized filter medium for nutrient removal in stormwater dry ponds. Environmental Progress and Sustainable Energy, 29(3), 319–333.
Example: Retention Rain Garden with Media
This combination of inputs results in an average annual removal of:
77%
With Additional 10% Cover Crop Credit:
77.2 + 0.10(22.8) = 79.5%
Rounded to one decimal place: 79.5%
Rounded to nearest whole number: 80%
Plot Option (Discovery Tool)
The plot option can be used for discovery analysis. It shows the marginal change in annual capture efficiency and allows optimization of design based on:
- Cost
- Performance
- Site constraints
Groundwater Discharge Analysis
When using retention with media, groundwater loading and recharge volume can be determined by specifying a groundwater discharge analysis when setting the performance surface discharge standard on the site information worksheet.
The resulting groundwater discharge and loadings for this example are shown as:
