Stormwater Facility/Green Infrastructure Strategy

The City is developing a new stormwater strategy to prioritize green infrastructure for managing stormwater from all development and redevelopment projects in the city. This strategy is required by the Department of Environmental Quality (DEQ). 

The City’s strategy was available for public comment for 30 days from October 15 to November 15, 2023. Thank you for your comments. 

What is Green Infrastructure?

Green infrastructure – stormwater rain gardens, planters, swales, and permeable pavers/pavement – play an important role in collecting stormwater, providing flood protection, and filtering pollutants to protect our rivers, lakes and streams.

Directing runoff to a natural vegetated system reduces the amount of runoff in our storm system, allows water to slowly soak into the ground, and removes sediments and pollutants to improve water quality.

Examples of green infrastructure are shown below:

Why is the City creating this strategy and what is changing?

Since 2003, stormwater has been required to be managed under the City’s Stormwater Management Manual, which provides guidance on the siting, design, and maintenance of structures used to manage stormwater from impervious surfaces in the city. The intent of the manual is to reduce the impacts of stormwater and keep the lake, rivers and streams in Lake Oswego clean and healthy. The 2020 Stormwater Manual is required by the City’s MS4 permit which was reissued in 2021.

Depending on the size of a project’s impervious areas, it must meet different requirements for managing stormwater. Currently, properties can choose several options to retain stormwater on site, but this strategy will now require onsite retention or extended filtration prior to discharge offsite. The inability to provide onsite retention must be backed by a technical analysis and the results from an infiltration test. If onsite retention cannot be done, then projects must treat stormwater with a filtration/flow-through planter or raingarden before discharging offsite. Projects not involving a single-family resident may still use water-quality vaults, such as a StormFilter, as a method for extended filtration.

Why does stormwater matter?

Stormwater is the water that originates from rainfall and snowmelt. In urban areas, precipitation lands on impervious surfaces (that do not allow water to infiltrate into the ground) such as roads, parking lots, and rooftops. When rain or snow lands on an impervious surface, it cannot enter the soil through infiltration, so it flows overland as surface runoff. Pervious surfaces, such as natural areas and lawns, allow water to infiltrate into the soil.

All surface runoff in Lake Oswego eventually finds its way to the Willamette River, Tualatin River, or Oswego Lake. As a steward of these resources, the City can play an important role in protecting them and the human users and animal species that rely on them.

What is the timeline for implementing this strategy?

The City is updating its Stormwater Manual which will incorporate this new green infrastructure strategy. Implementation is expected in 2024. The requirements of the 2024 Stormwater Manual will be implemented on all new projects submitted after the revised manual has been posted to the website.

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