City of Sammamish

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 Ecology continues to increase the emphasis on retrofit work as we learn about the impacts to stormwater from outdated drainage systems.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).

2024 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 20^th Street and 228^th Avenue SE (sketch). The Guidance Manual identifies this as Retrofit Site #3000 on King County Parcel 042406-9131.

Project Map

Site 1: Demery Hill

Contractors installed the Demery Hill vault in the 1980’s as part of the infrastructure improvements for the neighborhood. The facility provides roughly 42,000 cubic feet of detention storage, and no means of removing runoff pollutants. The goals for this facility are two-fold:

Increase detention storage and reduce downstream flows. This will lessen the site’s contribution to downstream erosion in George Davis Creek.
Provide water quality treatment to reduce discharges off phosphorous and total suspended solids into Lake Sammamish.

The Guidance Manual recommended expanding the existing vault to provide up to 100,000 additional cubic feet of storage. A separate facility will provide water quality treatment.

Site 2: SE 20th St and 228th Avenue SE

This existing pond was built as part of the 228th Avenue widening in 2001. The design provided limited detention storage to buffer against peak rainfall events. The facility also provided water quality treatment through a permanent pool in the pond coupled with a media filter vault.

The Guidance Manual recommends expanding the detention volume as practical to improve floc control characteristics. Both the discharge piping and treatment approach will be evaluated to ensure pond performance is consistent with current runoff treatment standards within the Pine Lake basin.

Funding

This City's adopted budget includes this project within the Stormwater Capital Fund (438 Fund). All project costs are paid for by the Stormwater Utility's rate payers.

Design

PACE Engineers, Inc. of Kirkland, Washington is under contract to provide design services for this project.

Construction

A separate contract will be advertised for construction work.

Stormwater Retrofit Program

Senior Stormwater Engineer