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Stormwater Retrofit Program

Example 2 of a pond

Public Works

Associate Stormwater Engineer

Jayden Abrams
(425) 295 0551

Overview

The City of Sammamish initiated a stormwater retrofit program to modernize existing drainage facilities. Sammamish was largely developed before stormwater standards prioritized flood control over stream protection and pollution control. Regulatory advances now prioritize resource protection and water quality enhancement in addition to flood control.

The program will update facilities that do not deliver water quality treatment  and resource protection meeting today's standards.

NPDES Permit

The State Department of Ecology authorizes Sammamish to operate a Municipal Separate Storm Sewer System (MS4) under the Western Washington Phase II Municipal Stormwater Permit (Permit). Ecology continues to increase the emphasis on retrofit work as we learn about the impacts to stormwater from outdated drainage systems. Public Works staff developed the City’s retrofit program to ensure consistency with Permit requirements.

Goals and Policies

In addition to the Permit, the City’s Retrofit program advances the specific recommendations established by several City programs and plans including:

Project Selection

The City of Sammamish Retrofit Strategy and Guidance Manual (Guidance Manual) serves as the foundation for the 2024 Retrofit Program. This report evaluated the watersheds in Sammamish and identified high priority areas for retrofitting. The Guidance Manual evaluated 20 different capital projects against the City’s methodology for prioritizing stormwater capital improvement projects (City Council Resolution R2018-804).

2027 Program

Locations

The current Retrofit Program will target the two highest priority projects identified by the Guidance Manual. The project goal is reconstructing the following two storm water facilities:

  • Site 1: An existing stormwater vault located within the Demery Hill subdivision (sketch). The Guidance Manual identifies this as Retrofit Site #2131 on King County Parcel 95440-0530.
  • Site 2: An existing stormwater pond located at the southeast corner of the intersection of SE 20th Street and 228th Avenue SE (sketch). The Guidance Manual identifies this as Retrofit Site #3000 on King County Parcel 042406-9131.

Project Map

Map of Sammamish, WA, showing two locations: Site 1 - Demery Hill in the north and Site 2 - 228th Ave NE in the south, each marked with a QR code and city facility number. Lake Sammamish is visible on the west.

Site 1: Demery Hill Vault Retrofit 

The Demery Hill stormwater facility was built in the 1980s as part of the neighborhood’s drainage infrastructure. The existing vault provides approximately 42,000 cubic feet of detention storage but does not provide water quality treatment.

The project will install a new underground treatment vault to reduce phosphorus and suspended solids in stormwater runoff before it flows downstream toward Lake Sammamish. The improvements will supplement the existing detention facility, helping protect downstream water quality.

Construction is anticipated in summer 2027.

Site 2: 228th Pond Retrofit

This stormwater pond was built in 2001 as part of the 228th Avenue widening project. The existing facility provides limited detention storage and treats stormwater through a permanent pool and a media filter vault.

The project will improve water quality treatment by expanding the pond bottom to the north and modifying the pond’s interior slopes. Improvements also include replacing the perimeter fence and access gate. The revised design focuses on water quality treatment rather than increasing flow control storage.

These improvements will help reduce pollutants in stormwater runoff within the Pine Lake basin. Construction is anticipated in summer 2027.

Funding

The City’s adopted budget includes these projects within the Stormwater Capital Fund (438 Fund). The City is using funds allocated to Sammamish from the Sub-regional Opportunity Fund, managed by King County, to cover a portion of the design and construction costs. All remaining project costs are funded by Stormwater Utility ratepayers.

Design and Permitting

PACE Engineers, Inc. of Kirkland, Washington has completed the design for both projects. Required permits have been obtained.

Construction

Each project will be advertised separately for competitive bidding. Construction for both projects is anticipated in summer 2027.