Design considerations for a rain garden are as follows:
 

Pretreatment Area. Required where a significant volume of debris suspended material is anticipated such as parking commercial areas. Grass buffer strips or vegetated buffers are commonly used pretreatment devices.

Ponding Area. Typically limited to a depth of 6 inches.

Groundcover Area. 3 inches of mature mulch recommended.

Planting Soil.

Depth = 4 feet
Soil mixtures include sand,
Clay content £ 10%.

In-Situ Soil. Infiltration rate ≥ 0.5 inches/hour without underdrains   Infiltration rate ≤ 0.5 inch/hour underdrain required (see detention pond).

Plant Materials. Minimum 3 native species.

Inlet and Outlet Controls. Non-erosive flow velocities (0.5 ft/sec)s

Maintenance. Routine landscape maintenances

Hydrologic Design. Determined by state or local agency.

Sizing. Bioretention area size is based on the drainage area to be controlled and the anticipated rainfall.

Figure 1. Natural hydrologic cycles
 

UFC 3-210-10, Low Impact Development, states that bioretention areas range from $10 to $40 per square foot depending on the site requirements however other factors need to be taken into account when considering this stormwater mitigation method. Costs for bioretention areas are minimal if implemented when the overall landscape design is constructed since additional expenses for bioretention areas are little more than standard landscaping practices. Additionally, the need for stormwater mitigation measures such as oil/water separators, culverts, curbing, and drains may be reduced or replaced by rain gardens.

Figure 2. Rain garden design details

Rain gardens are one of many potential strategies which may be implemented to meet the intent of  LEED Sustainable Sites (SS) Credits 6.1 and 6.2.

Benefits of Rain Gardens

Conventional stormwater control systems typically deliver runoff to adjacent watersheds without any filtration adversely impacting aquatic habitats.  Rain gardens are an effective method to remove pollutants and sediment from stormwater while sustaining natural hydrology in an unnatural, developed environment.  In addition to the potentially reduced construction costs  mentioned above, financial rewards are often realized through the avoidance of pollution and/or erosion remediation.

Figure 3. Existing KMC parking lot
rain garden