Salem, OR Bioswale Demonstration Project
Status: Initiated in 2001
Kroger Park Bank Restoration and Bioswale Demonstration Project
Introduction: the Kroger Park Bioswale (formerly known as Cannery Park)
National and regional studies have found that larger concentrations of pollutants enter streams in urban areas than in undeveloped watersheds. A major reason for this is the high percentage of land covered with impervious surfaces, such as street pavement and roofs. Impervious surfaces are intended to prevent flooding by rapidly draining stormwater to streams via piped or ditched stormwater systems. As water moves across building tops or parking lots, a variety of pollutants—automobile oil, break liner metals, fertilizer, etc.—are picked up and discharged into the nearest stream.
To demonstrate a stormwater "best management practice (BMP)" technique that can reduce water pollution, the City of Salem constructed a bioswale in Kroger Park during the summer and fall of 2001. Kroger Park, a seven acre neighborhood park located in South Salem, borders both Pringle Creek and a facility called Shurgard Storage. Shurgard has multiple, enclosed storage units and spaces for such items as RV's, vehicles, boats, etc. The Shurgard site, typical of other commercial and industrial areas, is almost completely covered by roof tops and an asphalt parking lot. And like most other areas with large amounts of impervious surfaces, the stormwater from Shurgard discharges directly into the nearest stream (Pringle Creek).
The Kroger Park Bioswale Demonstration Project was designed and built to treat the stormwater runoff from Shurgard Storage's 4.4 acre property before the water runs into Pringle Creek. The bioswale, which is a shallow "stream" that receives the Shurgard stormwater runoff, uses natural filtration to remove contaminants in the water before it enters Pringle Creek. The bioswale began operating in the fall of 2002.
The Kroger Park Bioswale Demonstration Project was jointly undertaken by Salem Public Works, Community Services, and Community Development Departments; Shurgard Storage, Inc.; the Pringle Creek Watershed Council; Marion County Master Gardeners; and the Oregon Watershed Enhancement Board (OWEB). This project was partially funded by an OWEB grant received in 1998. OWEB provides financial support for projects that improve watershed conditions throughout Oregon. The 25 percent local match required by OWEB grants was provided by the City of Salem and the other project partners listed above. Bank restorationBank Restoration Project
During construction of the bioswale, a streambank restoration project was implemented along parts of Pringle Creek. As Pringle Creek flows through Kroger Park, the area nearest the bioswale has suffered from bank erosion, an incised channel, and a lack of an adequate vegetative canopy above the stream. To offset these conditions, the channel was regraded, erosion control matting was installed, and vegetation was planted to help stabilize soils and provide stream shading.
Bank restoration projects like these can help improve the biological function of local streams. By increasing the canopy cover over streams, shading of the water will help minimize the problem of high water temperatures for fish. Bank stabilization helps control erosion, thus decreasing the amount of sediments that may move into the stream. Sediments can choke spawning beds and make finding food difficult for young fish. The result will be cleaner, cooler water in Pringle Creek.
What is a "Bioswale"?
A bioswale is a shallow depression created in the earth to accept and convey stormwater runoff. A bioswale uses natural means, including vegetation and soil, to treat stormwater by filtering out contaminants being conveyed in the water. Stormwater Filtration Systems
A bioswale is a type of stormwater filtration system. However, there are many types of stormwater filtration and management systems that can be applied to urban environments to remove stormwater pollutants. Some are mechanical, natural, or a combination of the two. Some mechanical options use manhole type structures to allow sediments or pollutants to settle. Other mechanical methods direct water into a filtering media, such as sand, that filters out pollutants and allows cleaner water to continue on its course. Whatever method is used, the practice of removing stormwater pollutants is generally known as a "best management practice," or BMP, which may be a requirement of the U.S. Environmental Protection Agency or the Oregon Department of Environmental Quality. Bioswale Design Consideration and Performance Objectives
A Bioswale, like any other treatment option, will only be effective when it is designed correctly. Here are several important elements to consider when designing a bioswale:
- Slope. The slope should be steep enough to prevent ponding and shallow enough to slow water velocity. Soils must not readily drain water; the goal is to get cleaner water to flow downstream. Recommended slopes of 1.0% to 4.0% should be used.
- Channel Bottom Width. A wider channel allows for maximum filtering surface and for slower water spreading throughout the channel. A maximum width prevents shallow flows from concentrating and gullying, thereby maximizing the filtering by vegetation structure.
- Roughness coefficient. This varies with the type of vegetative cover and flow depth. There should be sufficient roughness to slow water velocity and to allow water to contact vegetation within its journey through the bioswale.
- Flow depth. This should not be taller than the vegetation, mainly grass. Maximum depth of 4" is recommended.
- Flow Velocity. Flow should be sufficiently low enough to provide adequate residence time within the channel. A maximum flow velocity of 1.0 feet per second for water quality treatment is required. 2-year storm events should be non-erosive, usually having not greater than a 4-5 feet per second velocity.
- Length of Channel. The length of the channel should be long enough to provide approximately 10 minutes of residency time.