Rain Garden in VO does not have an option for entering a underdrain or outflow structure. If these are included in your design, you should use the Bioretention command. The only runoff leaving a Rain Garden is the overflow.
1. Calculate the required total volume
The Rain Garden is designed to capture runoff from a 25 mm storm event “Pickering – 25mm 4hr Chicago”. To size the Rain Garden, we need to know how many cubic meters of runoff coming to the Rain Garden from the upstream command, which is CompoundChannel – 6 in this exercise. We will get this by multiplying the runoff volume and the area of CompoundChannel – 6 .
Select the upstream command CompoundChannel – 6 and then click Hydrograph Result button 
from the Simulation toolbar. Use the pulldown menu to select the “Pickering – 25mm 4hr Chicago”.
Storage required = 5 ha x 10.981 mm x 10000 ha / m x 1 m / 1000 mm = 549 m³
2. Size the Rain Garden
Storage in the Rain Garden is a combination of the storage in the Surface Ponding Layer and in the Engineered Soil Layer. Click the button next to Depth-Area Curve in the Properties menu, and type in the stage and area values as shown in the figure below.
By calculation we get the total volume for Surface Ponding Layer as 585 m³.
| Depth (m) |
Area (m²) |
Inc. Vol. (m³) |
|---|---|---|
| 0 | 1500 | 0 |
| 0.1 | 1700 | 160 |
| 0.2 | 2100 | 190 |
| 0.3 | 2600 | 235 |
| Total | 585 |
The storage for the Engineered Soils Layer is calculated by multiplying the storage depth by the width, length, and porosity:
Storage in the Engineered Soils Layer = 0.5 m x 15 m x 100 m x 0.463 = 347 m³
Therefore, the total storage available in the Rain Garden is 585 m³ + 347 m³ = 932 m³, which is enough to contain the required total volume.
3. Set Native Soil Texture and Infiltration rate
For the Native soil we will input a Soil Texture of Silty Clay Loam and the default infiltration rate for this texture is 0.0051 m/hr. VO provides default infiltration rates for different soil types; however, these values can be manually input as well. This infiltration value determines the rate at which water stored in the Engineered Soil Layer infiltrates into the surrounding soils. As this rate is normally lower then the infiltration rate into the Engineered Soil (i.e. 0.05 m/hr for the default value), the infiltration rate for the native soils generally controls the drawdown rate for the LID.
4. Set Water Quality Removal Efficiency
We will leave the water Quality Removal Efficiency set to default values. The TSS and TP removal rates are 75% and 25% respectively.
