5.4.5 Rain Gardens

5.4.5.1 BMP Description

Rain gardens (BMP T5.14 in Volume V, Chapter 11 of the Ecology Manual) are non-engineered, shallow, landscaped depressions with compost-amended soils and adapted plants. The depressions pond and temporarily store stormwater runoff from adjacent areas. A portion of the influent stormwater passes through the amended soil profile and into the native soil beneath. Stormwater that exceeds the storage capacity is designed to overflow to an adjacent drainage system.

5.4.5.2 Performance Mechanisms

While rain gardens cannot be used to achieve runoff treatment to satisfy Minimum Requirement #6: Runoff Treatment, some treatment is provided in the form of filtration, sedimentation, adsorption, uptake, and biodegradation, and transformation of pollutants by soil organisms, soil media, and plants. Rain gardens also provide some flow control in the form of detention, attenuation, infiltration, interception, evaporation, and transpiration.

5.4.5.3 Applications and Limitations

Rain gardens can be used to satisfy the On-Site List Approach for Minimum Requirement #5: On-site Stormwater Management. They cannot be used to satisfy the LID Performance Standard Approach to MR #5, nor can they be used to satisfy Minimum Requirement #6: Runoff Treatment or Minimum Requirement #7: Flow Control. Bioretention may be used instead to meet all of these requirements (MR #5–7), as described further in Vol II–5.4.6 Bioretention Cells, Swales, and Planter Boxes and as summarized in Table II-5.9.

For projects eligible and electing to use List #1 (Vol I–4.2.5 Minimum Requirement #5: On-site Stormwater Management), rain gardens are to be used to the extent feasible for runoff from roofs and other hard surfaces unless a higher priority BMP is feasible.

Infeasibility criteria for rain gardens are the same as for bioretention. See Bioretention Infeasibility Criteria in Appendix H.

Although not required, installation by a landscaping company with experience in rain garden construction is highly recommended.

Rain gardens constructed with imported compost materials shall not be used within 0.25 mile of phosphorus-sensitive water bodies. Preliminary monitoring indicates that new rain gardens can add phosphorus to stormwater. Therefore, they shall also not be used with an underdrain when the underdrain water would be routed to a phosphorus-sensitive receiving water.

Table II-5.9. Rain Gardens Applicability.

BMP	MR #5: On-site Stormwater Management		MR #6: Runoff Treatment				MR #7: Flow Control
	List	LID Performance Standard	Basic	Enhanced	Oil Control	Phosphorus
Rain Gardens	X

5.4.5.4 Site Considerations

See the LID Infeasibility Criteria (Appendix H) for site considerations associated with rain gardens, and Vol II–5.3.2 Determine Infiltration Feasibility for general infiltration feasibility criteria.

5.4.5.5 Design Information

See BMP T5.14 in Volume V, Chapter 11 of the Ecology Manual and the Rain Garden Handbook for Western Washington (Hinman et al. 2013) for detailed design guidance on rain gardens.

See BMP T5.14 in Volume V, Chapter 11 of the Ecology Manual for guidance on Runoff Model Representation.

5.4.5.6 Minimum Construction Requirements

To help prevent clogging and over-compaction of the subgrade, bioretention soils, or amended soils, do not excavate, place soil, or amend soil during wet or saturated conditions.

5.4.5.7 Operations and Maintenance Requirements

See the Rain Garden Handbook for Western Washington (Hinman et al. 2013) for tips on mulching, watering, weeding, pruning, and soil management. See the Maintenance section in BMP T5.14 in Volume V, Chapter 11 of the Ecology Manual for additional maintenance guidance.