Seattle University, WA Rain Garden

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Seattle University, WA, US

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Type: Policy

Status: Initiated in 2007

Source File:


Facilities installs an innovative rain garden to stop buildings from flooding in a large rain event.

In December 2006, the basements of Lynn, Hunthausen, Xavier and the Chapel flooded when 2.5 inches of rain fell in 24 hours. Stormwater came from the surrounding neighborhood and flowed over the intersection of Madison and 11th streets and onto SU’s property. The reason the flooding occurred has to do with humans’ impact on the landscape.

Before settlers moved to Seattle, the forests would store, infiltrate and evaporate rain water and recharge ground water. When the forests were replaced with buildings, streets, sidewalks and parking lots, these impervious, hard surfaces turned rain water into stormwater runoff that needed to be contained and diverted to avoid flooding.

Storm sewers were built to capture the rain water in pipes and hold it in a detention tank until a later time when it can be released to the sewage treatment plant where the water is cleaned of pollutants before being released into Puget Sound. When Seattle gets unusually large rain events, the pipes back up and cause flooding. Parts of Seattle are on the combined sewer overflow system which means during a heavy rain event, the stormwater in the sewer system overflows into Puget Sound untreated.

The large number of impervious surfaces in Seattle will continue to cause flooding during heavy rains. To ensure the four buildings did not flood again, the university had two options, which cost roughly the same: build a conventional stormwater detention system or build an alternative called a rain garden. The Facilities department decided to install an environmentally conscious rain garden that will be a showcase for other property owners.

A rain garden is a natural stormwater management strategy that is a proven solution because it adds pervious surfaces and mimics nature. A rain garden is an excavated depression planted to look like a garden. Stormwater infiltrates through layers of soil and gravel as plants transpire moisture and help attenuate pollutants. SU’s rain garden is ten feet deep, lined with a special fabric and perforated drainage system, and filled with a soil mixture designed to absorb and retain as much water as possible. Two trenches along the entire west wall of the Lynn building route water away from the building. One trench fills the rain garden located on the south lawn. The other trench relieves the water from the building's foundation and flows directly into the City’s combined sewer overflow system.

Because it is deeper than it is wide, only the lawn area and small amount of landscaping was removed and replaced. To bury a stormwater detention tank would have meant digging about a 100 foot by eight foot wide area. This would have required removal of the beautiful mature landscape in front of the Lynn building.

SU’s rain garden is sized to contain the water from a 25-year storm. As the garden reaches capacity, excess water flows into the City’s combined sewer overflow system. The rain garden was completed in November 2007. One month later 5 inches of rain fell within 24 hours. The vast majority of the stormwater from this storm infiltrated into the rain garden and successfully prevented flooding the same four buildings.