Rain Gardens and the New Stormwater Manual

by Anne Cline

Under the recently updated Stormwater Management Manual for Western Washington, many projects creating or replacing between 2,000 and 5,000 square feet (sq. ft.) of hard surfaces such as a driveway or new building addition, or disturbing soils within an area ranging from 7,000 sq. ft. to 33,000 sq. ft. may use a rain garden to manage stormwater. A rain garden is a Low Impact Development (LID) tool used for managing stormwater by passing runoff through a designed landscape area, thus reducing runoff by evapo-transpiration and infiltration to the local subsurface environment.

The updated stormwater manual identifies nine minimum management requirements for development and redevelopment projects. Not all of the requirements necessarily apply to a project. The size and configuration of the project determines which need to be met. The requirements are: 1) preparation of a stormwater site plan, 2) construction stormwater pollution prevention, 3) source control of pollution, 4) preservation of natural drainage systems and outfalls, 5) on-site stormwater management, 6) runoff treatment, 7) flow control, 8) wetlands protection, and 9) operations and maintenance.

If only minimum requirements 1 through 5 apply, a rain garden is a LID tool that may be used to satisfy minimum requirement #5. Rain gardens collect stormwater from impervious surfaces such as driveways, roofs, or turf and filter many pollutants through interaction with soil, water, and plants to prevent damage to water quality and erosion in local creeks. If constructed as part of a Stormwater Management Plan, the rain garden is listed on the title of the property, and must be maintained as a rain garden.

Raedeke Associates, Inc. staff have been trained in rain garden design, construction oversight, and maintenance. Our soil scientists have the knowledge and experience to determine the suitability of using rain gardens for a specific project, and we have extensive knowledge and experience in plants suitable for the different hydric regimes encountered in a rain garden.