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Green River Natural Resources Area

Introduction

GRNRA Entrance Sign 

The Green River Natural Resources Area project transformed an abandoned sewage lagoon system into a combined stormwater detention and enhanced wetland facility that provides a rich diversity of wildlife habitat. The 304-acre site is one of the last remaining open tracts of land in the Kent Valley and incorporates state-of-the-art techniques of wetland creation and enhancement, urban wildlife management, and stormwater treatment. With its adjacent public park and trail system, the site is one of the largest man-made, multi-use wildlife refuges in the United States.

 

Background

GRNRA-Aerial 

The project area is in the northwestern portion of the Kent Valley and is bounded generally by the Green River to the west, South 212th Street to the north, 64th Avenue to the east, and the Puget Power pedestrian/bike trail to the south. The five original lagoon cells were used by the City of Kent for sewage treatment from 1969 to 1973, when construction of Metro's Cross-Valley Interceptor diverted all sewage flows to the Renton treatment plant. The lagoons then became a fairly stable pond system, maintained by a relatively impermeable clay liner. The northern cell was overgrown with vegetation; the two central cells were normally filled with water but became partially dry in the fall, creating mudflat habitat; and the two large southern cells (each 16.5 acres in size) contained up to 3 feet of water.

Before construction, the site contained a number of wetland and upland habitat types. Over 200 species of mammals and birds were observed at or near the lagoons, which served as a nesting, feeding, and brooding area for many species that use the Green River corridor and the nearby east-west powerline corridor as travel routes.

Planning for the new facility began in 1979 and focused on the site's potential as a regional stormwater detention facility that could help to alleviate increases in stormwater flows in Mill Creek. The City of Kent Public Works Department has funded the construction and continued operation of the facility since 1996. Grants from the King County 1993 Regional Conservation Futures Acquisition Program and Metro's Regional Shoreline Improvement Fund for $500,000 each helped purchase additional land bringing the project's total size to 304 acres.

Benefits of the project

Waterfall 

Flood control

The expected storm flows in the Mill Creek drainage basin in a 100-year flood event are 560 cubic feet per second (cfs). The GRNRA provides sufficient storage to reduce flows in the lower portion of the Mill Creek to 180 cfs - a 68 percent reduction. The project is therefore a critical element in preventing flooding in downtown Kent and nearby industrial areas, where certain low-lying streets, parking lots, and warehouses used to be inundated nearly every year during high-flow events in the spring and winter months.

Water quality improvement

The water quality of Mill Creek, which lies in a highly urbanized area, was heavily impacted by untreated stormwater. The GRNRA was configured to protect high-quality habitat at the site while substantially improving water quality in the lower reaches of Mill Creek. Stormwater runoff entering the site passes through an extensive treatment system, including two presettling ponds and a 20-acre constructed wetland, reducing sediment leads and urban pollutants from upstream. Water then enters the main lagoon for additional treatment prior to draining back into Mill Creek. In addition, provisions have been made to supplement flows during the critical low-flow periods in summer using three groundwater extraction wells. This flow augmentation has the potential to  improve oxygen levels and temperature conditions in lower Mill Creek.

Wildlife habitat restoration

The project's 304 acres provide habitat to an estimated 165 bird and 53 mammal species, and serves as a nesting, feeding, and brooding area for many species that use the Green River corridor as a travel route. The creation of a large emergent marsh/openwater wetland, in addition to increases in scrub-shrub, forested wetland, and improved upland habitats, is expected to increase the diversity of wildlife habitat, particularly for waterfowl. Upon completion, the area will not only have much greater value as habitat, but will also be maintained in a more stable condition through formal preservation status, habitat management, and specific limits on public access.

Workers 

Fisheries enhancement

The design of the GRNRA specifically addressed elements identified as critical limiting factors in a comprehensive fisheries evaluation of the entire watershed. For example, the stormwater detention pond was designed to provide critical habitat where coho fingerlings can overwinter without being swept out of the watershed in a storm event, as could happen in the main channel of Mill Creek. In addition, the detention pond was sized to accommodate the full fisheries potential of the watershed.

Preservation of open space

The Green River Valley is a highly urbanized area, with much of the land surface covered by asphalt or rooftops. Through funding obtained from King County, the City was able to purchase over 200 acres of fallow farmland adjacent to the abandoned sewage lagoons and incorporate this land into the GRNRA project site, thereby saving one of the last open areas in the region from development as an industrial park.

Reuse of existing facilities and excavated materials

A main feature of the project, and an impetus behind its development, was the reuse of a 65-acre sewage lagoon that was abandoned in 1973. Since that time, the five-cell lagoon had become a fairly stable pond system, with the two largest cells containing up to 3 feet of water year-round and two other cells shifting seasonally from water to mud flat habitat. The final design reused the existing embankments and followed existing land shapes to reduce project costs and minimize construction time, thereby limiting disruption to the species already in residence. A total of 600,000 cubic yards of material were removed to create the current wetland configuration and then reused to create on-site uplands.

Greenhouse 

Public education and recreation

With its nature walks, wildlife viewing towers, and a bike path along the west and south sides of the site, the facility provides extensive recreational as well as educational opportunities. The facility also provides ongoing opportunities for hands-on involvement in the management and maintenance of the facility - for example, in constructing habitat, maintaining vegetation, and monitoring water quality and wildlife populations. Volunteers have built and installed bird boxes, planted native plant species, and conducted bird counts. A five-acre native plant nursery, constructed on the southwestern corner of the site, will propagate the thousands of native plants for placement within the GRNRA and other natural areas within the City, and will provide a home for plant salvaging projects. The nursery will also serve as an education and training facility for youth and volunteers in the City of Kent. In addition, the site's Master Plan allows for construction of an Environmental Interpretive Center, which will be a focal point for citizens' involvement and education on environmental issues throughout the Green-Duwamish watershed.