- Water Resources
City of Tacoma
2020 – Ongoing
Benjamin Lee, PE, CWRE
Eric Weber, LHg, CWRE
Kaila Anderson, PE
Flett Creek Urban Streamflow Restoration
The City of Tacoma (City) averages 40 inches of rain annually. The City’s location on the shores of Puget Sound and relatively high proportion of impervious land coverage emphasizes the need for responsible and innovative stormwater management. In 2020, the City teamed with Landau Associates to apply for a competitive Streamflow Restoration Grant through the Washington State Department of Ecology (Ecology) to evaluate a three-phase streamflow benefit project in the Flett Creek Watershed. The Flett Creek Watershed is the second largest sub-watershed in Tacoma, encompassing 7,153 acres. Stormwater from this sub-watershed flows to the Flett Wetland and then into Flett Creek. Downstream, Flett Creek flows into Chambers Creek, a salmonid-bearing stream that discharges into Chambers Bay of the Puget Sound. The City was awarded the Streamflow Restoration Grant, funding a multi-year feasibility study to evaluate the effectiveness of aquifer recharge, stormwater management, streamflow restoration to improve Flett Creek stream function and support robust and healthy salmon populations.
This project was grant funded pursuant to the 2018 Streamflow Restoration Act that includes the intent to restore stream flows to support salmon populations.
The goal of the project is to develop a plan to improve summer streamflow and riparian habitat in this urban drainage. The project feasibility study involves consideration of multiple technical elements and coordination with multiple stakeholders. Flett Creek is a natural drainage that is now an integral part of the City’s stormwater system, receiving water from multiple watersheds. There are also constructed stormwater elements, wellhead protection areas, a groundwater protection district, and designated critical areas along the channel. Interested parties include the Puyallup Tribe of Indians, Clover Park Technical College, Metro Parks Tacoma, Tacoma Public Utilities, the City of Lakewood, and a network of private landowners and developers.
The feasibility study approach is to develop a plan to improve stormwater management and provide a quantitative assessment of potential streamflow enhancement. Long-term environmental investigations and monitoring programs are underway to assess whether conditions are favorable for infiltration facilities and streamflow enhancement. Landau has excavated geotechnical test pits to investigate subsurface conditions and measure in-situ infiltration rates, constructed a network of groundwater monitoring wells to evaluate hydrogeologic and hydrologic gradients, and installed several new stormwater flow meters to quantify flow conditions within the sub-watershed and guide the basis of design. Natural resources investigations are also underway to assess the existing level of habitat function within stream and wetland settings in the project area and to identify specific areas where potential restoration treatments would be most beneficial. Stormwater modeling is being conducted to optimize stormwater management in the context of the project goals. Groundwater modeling has also been performed to predict the effectiveness of up-gradient winter stormwater infiltration on down-gradient summer stream flow.
The goal of the project is to improve streamflow function and improve salmon habitat. A successful grant application was the first step. Landau is collecting the technical data and performing the modeling and analysis necessary to meet
the requirements for and guide full funding and project implementation. The stakeholder community is a full partner in this endeavor with the shared project goal of improving the stream function of Flett Creek.