The Cochran/Downriver Stormwater Management Facilities plan intends to establish the integration of stormwater management facilities into Downriver Park while enhancing and protecting the park’s natural features, shoreline environment, and current uses.
The Cochran Basin is a natural topographic basin located within the corporate limits of the City of Spokane where it is bounded on the west by Alberta Street, the north by Francis Avenue, the east by Market Street, and the south by Montgomery Avenue. The basin comprises approximately 60 percent of the City’s municipal separate stormwater system (MS4), and is the largest in the City at approximately 5,160 acres in size. It consists of approximately 26 percent hard ground cover (impervious surfaces), and the impermeable nature of the land in the basin results in significant amounts of stormwater runoff.
Stormwater runoff from the basin generally emanates from three different land uses: residential (85%), commercial (2%), and light industrial (3%), where it sweeps up typical urban pollutants such as vehicle fluids, landscaping chemicals, sediment, et cetera as it flows downhill over roads, parking areas, and other impervious surfaces. Where applicable, stormwater runoff is managed in the basin on location by the use of swales and drywells for treatment and subsequent infiltration. Runoff that is not infiltrated locally is conveyed overland via roads or through a series of catch basins and piping. The stormwater runoff that leaves the basin is ultimately discharged untreated into the Spokane River through an outfall located at the southwest corner of the basin near the TJ Meenach Drive Bridge.
The Spokane River downstream of the Cochran Basin outfall, including Long Lake, is considered impaired with respect to low dissolved oxygen levels. This impairment is directly caused by the introduction of the pollutants that are carried in the stormwater runoff (e.g. such as leaked motor fuels/oils, lawn chemicals). Hence, Cochran Basin runoff is monitored for constituents that contribute to low dissolved oxygen, specifically, phosphorous, ammonia, and chemical biological oxygen demand (CBOD). Runoff is additionally monitored for temperature, pH, total suspended solids (TSS), and polychlorinated biphenyls (PCBs). The Cochran/Downriver stormwater management facilities will work towards minimizing or eliminating discharge of untreated stormwater from a large portion of the City into the Spokane River, thus increasing pollutant mitigation efforts.
To mitigate the impairment to the river, stormwater facilities will be constructed to divert the majority of the natural flow of stormwater runoff from Cochran Basin for treatment, minimizing the pollutant loading in the river. This flow will be treated at the facilities utilizing natural bioretention subsequent infiltration into the ground. Four treatment areas will be constructed in the Downriver Park that will function as stormwater bioretention facilities. Flow will be diverted to the facilities as they are constructed. The facilities will be implemented in this order:
An emergency outlet will provide an overflow to the river in case of mechanical failure or large or intense storms. The outlet will use the existing outfall pipe to direct these flows to the river.
In addition to the implementation of the stormwater facilities, the City will use this project as an opportunity to enhance recreation experiences in the Downriver corridor. Current recreation access points will be improved using current land use best practices.
Project #2018058: Cochran Basin Disc Golf and Boat Launch Stormwater Treatment Facility
Project #2018060: Cochran Basin Stormwater Conveyance Pipe Downriver Disc Golf Course to T.J. Meenach Dr.
Project #2021067: Cochran Basin Stormwater Pipe - TJ Meenach Dr to Downriver Golf Course via Cleveland
Project #2014153: T.J. Meenach Pettet Dr to Northwest Blvd.
City of Spokane - Engineering Services
509.625.6700
engineeringadmin@spokanecity.org
