What's A Stormwater Vault And Why We Should Care

When it rains, water runs off roofs, streets, and sidewalks, picking up oil, fertilizers, and other pollution along the way. A stormwater vault is a large underground tank which slows that runoff and cleans it before it reaches streams and lakes.

Sammamish’s newest Town Center Regional Stormwater Vault is tucked near 222nd Place SE by Lower Sammamish Commons Park—out of sight but working 24/7 beneath a new park. Despite its compact footprint, it can hold about 1.6 million gallons of runoff—roughly 2.4 Olympic-size swimming pools—from a drainage area of 11.7 acres (nearly nine football fields).

And what is visible on top of the vault, above ground? A new greenspace and trail connection that expands access to Lower Sammamish Commons Park and makes it easier to access, walk and bike through the area.

Spring 2025 construction of the Sammamish Town Center regional stormwater vault near Lower Sammamish Commons Park.

A Public-Private Partnership (PPP) That Multiplies Public Benefits

This project was co-designed and delivered by the City of Sammamish and Innovation Realty Partners, LLC (IRP). Here are some of the public benefits this created:

• Park instead of pavement: A planned roadway was converted into ~¾ acre of park, cutting down on hard surfaces and creating a safe, scenic, car-free connection for people.

• One smart system instead of many small ones: Instead of building seven separate vaults and five separate filters across multiple sites, the PPP team consolidated them into one regional facility. That means fewer things to maintain, better performance, and more land left for parks, homes, and shops.

• Housing choices—including affordability: The regional system supports a mixed-use neighborhood with ~83 townhomes, ~300 apartments (including ~80 affordable units), and ~44,000 sq ft of retail. In other words, the same infrastructure that protects water quality also makes new homes possible, including homes that are more attainable for local workers and families.

Private investment didn’t just fund private buildings—it helped pay for shared infrastructure and create public space that everyone can use.

Built to Protect Lakes, Streams, and Salmon

Sammamish sits within sensitive watersheds that are home to salmon and other aquatic life. The vault is engineered to the highest regional performance standards—Level 3 Flow Control—and provides Sensitive Lake Water Quality Treatment in line with the King County Surface Water Design Manual and the City’s own strict regulations. Downstream, a 5,652-cubic-foot sand filter vault gives the water one more polish before it continues on.

In plain language: the system slows big storms, reduces flooding, and filters pollution—helping protect the lakes and streams that define our region.

More Open Space, Stronger Tree Canopy

Because the vault lives underground, the surface can be a park, not a parking lot. Before construction, the developer worked with Sammamish Stewards to relocate native plants. Twelve existing trees were saved, and one hundred thirty-one additional trees will be planted providing a major boost to the urban tree canopy and a win for birds, pollinators, and shade on hot days.

Why The Regional Approach Matters

• Less duplicate infrastructure: One high-performing facility is easier and cheaper to maintain than many small ones scattered around which supports the budgets of all concerned.

• Room for what we value: Consolidating stormwater makes space for parks, trails, homes, and neighborhood shops—the ingredients of a lively Town Center.

• Future-ready: The vault adds capacity for current and future growth, aligning with the City’s Town Center Plan and reducing long-term costs for taxpayers.

  
  

A First For Sammamish—And A Model For The Region

As one of the newest cities on the Eastside, Sammamish is setting a regional precedent. This is a practical example of how private investment—channeled through a public-private partnership—can deliver public benefits that last: cleaner water, safer streets during storms, more open space, better walking and biking connections, and a wider range of homes.

The Takeaway

This project shows what happens when we stack benefits instead of trade them off:

• Clean water and healthy habitat

• A new park and trail connections

• Efficient, cost-smart infrastructure

• More homes—including affordable options

• A greener, more walkable Town Center

  
  

It’s not just an underground stormwater tank. It’s a blueprint for how Sammamish can grow thoughtfully, protect nature, and deliver public benefits—with help from private partners, like Innovation Realty Partners, who are invested in the community’s long-term success.