Building a Rain Garden

At the Dungeness River Nature Center

A rain garden is a bioretention system characterized by a shallow, flat bottomed, bowl-shaped landscape feature filled with a special soil mix.

Its purpose is to collect, filter, and infiltrate stormwater runoff from roofs and pavements. Typically, an engineer isn’t required to do the design work which means rain gardens can be installed by homeowners with a basic level of training.

A demonstration rain garden was built in 2021 at the newly expanded Dungeness River Nature Center in Sequim, Washington. This rain garden will protect the salmon-bearing Dungeness River by absorbing and filtering stormwater runoff from the roof, paved patio, and sidewalks.

Here’s the story of the building of the rain garden, from start to finish. Read the whole thing or jump to your area of interest: planning and design, construction, planting and mulching, maintenance or build your own. Only have 30 seconds? Watch the volunteers planting the garden in this time-lapse video.

About the Location

The Dungeness River Nature Center is located on the Olympic Peninsula on land owned by the Jamestown S’Klallam Tribe. A non-profit organization manages the Center, partnering with the Tribe, the Olympic Peninsula Audubon Society, and National Audubon. The Center is adjacent to the Olympic Discovery Trail and features exhibits, low-bank access to the Dungeness River, and a historic railroad bridge.

In 2022, the Center expanded, increasing the size of the building from
1,600 square feet to more than 7,400 square feet. Patios and sidewalks were installed along with a new entry road and a 75-car parking lot with an engineered stormwater management system. The Tribe and partners elected to treat remaining stormwater runoff with a rain garden.

Planning and Design

WSU Extension partnered with the Dungeness River Nature Center and the Jamestown S’Klallam Tribe to build a rain garden as part of the Center’s building expansion. The rain garden will protect the Dungeness River by absorbing and filtering stormwater runoff from the new roof, paved patio, and sidewalks.

The team determined overarching goals for the garden including:

• building a functional rain garden to treat stormwater runoff
• providing an opportunity to educate visitors and encourage them to consider building a residential rain garden
• creating habitat for wildlife including pollinators.

The rain garden site was used as a staging area for construction equipment during the building of the new nature center and was heavily compacted. But, within a few months it was transformed into a beautiful rain garden.

Area before construction of rain garden.

The rain garden was designed following the steps in the Rain Garden Handbook for Western Washington (Handbook) including:

• determining the “contributing area” that would drain to the rain garden
• testing the soil and digging a perk hole to test the infiltration at the site
• determining the ponding depth and size of the garden
• developing a design for the rain garden shape and size
• locating the inflows and overflow location

Soil testing

The garden was designed to manage approximately 40,000 gallons of stormwater runoff a year from approximately 5,900 square feet of hard surfaces and the roof. Stormwater is directed from the roof and patios to the 650 square foot garden through three inflow pipes and from direct flow from hard surfaces. The water is absorbed, usually within 48-72 hours.

An overflow ditch allows water to leave the garden, in the rare event of a storm that exceeds the garden’s capacity. The other component of the site’s stormwater management plan is an engineered, underground system in the new parking lot on the east side of the building that collects rooftop, sidewalk, and parking lot runoff.

Construction

Preparation for constructing the rain garden began by outlining the rain garden edge and marking the inflow and overflow. Following the Raingarden Handbook, next steps included:

• excavating the rain garden
• flattening and leveling the garden bottom
• filling with a rain garden soil mixture

Excavation work began on July 29, 2021 and was completed in a day and a half. The rocky and heavily compacted soil was excavated to a depth of about three feet by the Tribe’s Jamestown Excavation company. The bottom of the rain garden was flattened and leveled. A mix of soil and compost was added to the site to a depth of approximately 18 inches – until the inflow pipes were level with the bottom of the rain garden. A rock-lined overflow was constructed in case the water overflows.

Construction sequence

Outlining the edge of the garden

Excavation begins!

Checking the depth

Excavation continues. View looking west

Checking the depth again

Creating the inflow

Inflow pipe before extension is added

Inflow extension

Inflow pipe cut back and supported with rock

Soil mix spread over entire area, 18 inches deep in the rain garden depression

Rock are placed around inflow areas to dissipate flow

View of two inflows and overflow prior to planting

Planting and Mulching

1,075 plants, two days, and 50 volunteers! Instant garden!

In early October, with the help of a team of 50 volunteers, the planting went quickly. After planting, volunteers placed layer mulch three to four inches deep to reduce weeds and prevent erosion.

A variety of small trees, shrubs, groundcovers, and grasses suitable for a sunny location were selected. Zone 1, the infiltration zone at the bottom of the rain garden, contains plants able to withstand being underwater for short periods of time while still being drought tolerant. Zone 2, along the garden sides, has plants that can tolerate occasional standing water. And the Zone 3 surrounding area is populated with native and ornamental water-wise plants.

After planting, the entire area was mulched with bark chips to a depth of three to four inches, for weed reduction and moisture retention. The mulch works with the compost in the soil mix and the plants to remove contaminants, infiltrate water, and provide other ecosystem and aesthetic benefits.

Because increasing habitat for birds and insects was an important goal for the garden, moss covered logs, snags, rocks, bird feeders, and a water feature were added. The garden can be viewed from both outside and inside the building in a dedicated wildlife viewing room.

Plants in Zone 1 and 2

Fifty-six species were planted. View and download the plant list.

Plants arrive – over 1,000

Tools at the ready

Volunteers arrive and get to work

Rocking the inflows and overflow

Planting is done

Mulching begins

Maintenance

A rain garden, like any garden, needs maintenance. Volunteers will ensure the garden is adequately watered for the first few years to establish plants, perform general weeding, and check that the inlets and overflow are clear of debris.

Build your Own

For information on how to build your own raingarden, please visit our Rain garden resources page.

After hail storm April 2022