In the summer of 2011, the City proposes to construct its second stormwater treatment wetland. The City is seeking grant funding through the Washington State Department of Ecology for construction of this facility.
Once constructed, this small facility will collect stormwater from 6.7 acres of urban commercial development that currently discharges to Bear Creek without any treatment. In the future, part of this drainage area will be diverted to a larger stormwater treatment facility proposed to be constructed near Redmond Way and the Sammamish River.
This will be the second of the City's stormwater treatment wetlands to be constructed, similar in function to the recently constructed Leary Stormwater Treatment Wetland . The proposed water quality facility will be located in Bear Creek Park, adjacent to and east of the Bear Creek Shopping Center between Redmond Way and NE Union Hill Road. An existing 30-inch-diameter storm drain will be rerouted through a constructed stormwater wetland set east of Safeway and west of the Bear Creek Regional Trail. The outlet from the proposed facility will tie back into the existing 30-inch-diameter storm drain just west of the trail.
What is a Stormwater Treatment Wetland?
Stormwater treatment wetlands (a.k.a. constructed wetlands) are structural practices similar to wet ponds that incorporate wetland plants in a shallow pool. As stormwater runoff flows through the wetland, pollutant removal is achieved by settling and biological uptake within the practice. Wetlands are among the most effective stormwater practices in terms of pollutant removal, and also offer aesthetic value.
While natural wetlands can sometimes be used to treat stormwater runoff that has been properly pretreated, stormwater wetlands are fundamentally different from natural wetland systems. Stormwater wetlands are designed specifically for the purpose of treating stormwater runoff, and typically have less biodiversity than natural wetlands both in terms of plant and animal life. There are several design variations of the stormwater wetland, each design differing in the relative amounts of shallow and deep water, and dry storage above the wetland. Since stormwater treatment wetlands must be maintained, they are not subject to environmental protection regulations typical of natural wetlands, but when they are not being maintained they provide similar habitat benefits.
Several Ecology-approved enhanced treatment best management practices (BMPs) were considered for this location. A constructed stormwater wetland was selected as the preferred enhanced treatment BMP for the following reasons:
- No head drop required across the facility (from inlet to outlet).
- Planted with native species that complement the naturalistic setting of the park environment.
- Appropriate for treating concentrated flows.
- Suitable for use with a liner to retain water as well as to prevent any potential for groundwater contamination.
- Minimal risk of damage to facility if slow-moving floodwaters from Bear Creek enter the wetland.
- Primary maintenance activities consist of vegetation management and catch basin cleaning.
- Consistent with existing park uses.
The design of this project is documented in RW Beck’s Design Report and Design Drawings.