Managing Stormwater

Guest author: Chris Bosley, City Engineer for the city Coeur d’Alene

For the sake of the lake’s health, the City of Coeur d’Alene has been working to reduce the amount of untreated stormwater runoff that enters Coeur d’Alene Lake and the Spokane River. Stormwater runoff in the southeastern area of the city is collected and conveyed through pipes to thirteen outfalls along Coeur d’Alene Lake and the Spokane River. Stormwater discharge into the water is permitted under the Idaho Pollutant Discharge Elimination System, but the city is then required to implement at least one activity designed to reduce pollutants such as lead and phosphorus entering each water body. However, Coeur d’Alene has begun or completed many projects to support the health of the water.

Due to the sand and gravel soils found in Coeur d’Alene, infiltration into the ground is a cost-effective strategy for treating stormwater. As part of the City’s Stormwater Outfall Volume Reduction Program, staff identified locations where the pipes could be diverted into swales or other treatment facilities, thereby reducing or eliminating the stormwater discharging from these outfalls.

In 2018, the City partnered with the University of Idaho (U of I) to divert a 70-acre drainage area from an outfall to the Spokane River. City crews constructed a large swale to contain the stormwater and assisted U of I with the construction of an outdoor classroom that is used for teaching stormwater education and hosting related meetings. Stormwater education signage has been installed at the outdoor classroom to illustrate the positive impacts of the project.

In 2020, the City completed the largest stormwater outfall reduction project, the US-95/Northwest Boulevard swale. A large pipe was installed beneath US-95 to redirect stormwater from the 210-acre drainage area to this area where a series of stepped swales filters the water using topsoil and vegetation.

The City’s latest stormwater construction project is being completed as part of the Lacrosse Avenue extension project. This 60-acre drainage area between Lakewood Drive and US-95 is conveyed to an outfall on the Spokane River. Because the Lacrosse Avenue construction project crossed the stormwater pipe near the outfall, it presented an opportunity to divert flows from the outfall. The swale, currently under construction, provides a more natural looking facility where stormwater will be diverted once vegetation is established.

Recently, the City was awarded $695,000 for three separate stormwater projects under Idaho Department of Environmental Quality’s Leading Idaho Initiative. The projects include stormwater treatment in the Sanders Beach, Mullan Ave. and Independence Point areas.

The Sanders Beach project consists of two outfalls; one at the 11th Street Marina; the other at the 12th Street public beach. Since little room is available to construct swales, the design includes underground basins. Stormwater will be filtered through a sand and compost mixture as it absorbs into the ground. The project is currently in design with construction anticipated for the Fall.

For the Mullan Ave outfall, the City has teamed with the North Idaho College Venture Center for this small 12-acre drainage area. They’re using filter socks filled with biochar, a lightweight black material made of carbon and ash, inserted into catch basins where stormwater is filtered as it passes through. Early test data from the deployment of the biochar socks at this outfall indicate they are highly effective in absorbing phosphorus as well as other contaminants found in stormwater. Testing will continue into 2023.

The latest stormwater design project funded under the Leading Idaho Initiative is the Independence Point drainage basin. The outfall located at Independence Point drains 130 acres extending north into Midtown. An engineering consultant will analyze the stormwater system and identify opportunities to remove or reduce stormwater flow to the outfall. Construction is anticipated for 2023.