Stormwater Code – Seattle’s Latest Updates and Impacts

Seattle’s new stormwater code takes effect July 1, 2021. Our civil team has been tracking the city’s discussions and decisions, providing feedback to officials, and advising clients about the most impactful changes in their markets.

What’s driving this change?

Because our population continues to grow, it’s more important than ever to safeguard our Pacific Northwest natural resources.

Seattle’s new stormwater code will now comply with the Washington State Department of Ecology (WADOE) 2019 Stormwater Manual. The 2016 stormwater code adopted a more restrictive drainage standard, requiring Green Stormwater Infrastructure to increase water quality.

Many site-related design changes precipitated by the 2016 code changes are addressed in the new stormwater code. New standards for detention, rainwater harvesting, bioretention, and storm main extensions are addressed.

How will the changes affect my project?

Increased detention sizing and an emphasis on stormwater design in the Master Use Permit are the two biggest impacts to many Seattle clients.

To accommodate the new peak flow standards – that is the amount of runoff experienced – the detention vault must be significantly larger. For example, in commercial and multi-unit residential projects, the change roughly translates to the space of six parking stalls per acre of development when the vault is located within a parking structure.

These changes do include unique implications for our school district partners who are already challenged by tight urban sites. Larger detention systems will need to be located deeper in the ground, creating associated cost and schedule implications for additional soil movement. Preliminary drainage reports must be included with the Master Use Permit application, several months earlier than the current code requires, meaning the design team will need to collaborate closely on these plans at the onset of design.

A win for pups and city sidewalks.

There are some fun wins amid these new regulations. No more limit on the size of dog runs at multi-unit residential properties. For Seattle, with a dog population that exceeds 150,000, this is certainly a win.

Also, for clients who’ve agonized with us about controlling runoff across public sidewalks in a region where it always rains, the new code provides some relief from complete collection of the runoff.

Alternatives worth pondering.

A Landscape Management Plan (LMP) may be used as a strategy to treat landscape areas in lieu of a water quality system. A more sustainable approach than previous codes, the LMP defines the layout and long-term maintenance of landscaping features with an emphasis on safety and environmental goals. These goals include minimizing pesticide and fertilizer use and reducing runoff that contains visible contamination, organic materials and other pollutants. From woonerfs, Green Streets, and landscaped public plazas to extensive rain gardens, bioswales, and outdoor learning areas for all ages, Seattle has stellar examples of sustainable and attractive landscaping.

Rainwater harvest sizing criteria is less stringent, making it easier for owners and developers to achieve. Rainwater collection-reuse and green roof systems are optional and may contribute toward stormwater mitigation design.

Project Highlights

Explore Seattle pedestrian pathways that showcase stormwater control, attract pollinators, and feature work of local artists! Some of our favs:

Do I need to be concerned if my property is outside Seattle?

Traditionally, communities outside Seattle have longer timelines to adopt changes initiated by the Dept. of Ecology. Not all municipalities operate combined sewer overflow (CSO) systems, so some aspects of Seattle’s new code will not apply to them. Updated regulations relative to detention sizing and rainwater harvesting are likely to be implemented first. So, while we can expect other municipalities to either directly adopt WADOE code or create a slight modified version, it’s Seattle who is leading the charge.

Learn more!

Three members of our civil practice contributed their expertise to this article and are facilitating client conversations on this topic. We’re eager to help you decipher the new code implications. Drop a note to Kyle to schedule a meeting.

Images © Benjamin Benschneider / Kevin Scott / Lara Swimmer / Michael Walmsley / Moris Moreno