About SPU
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Drainage & Sewer System
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Natural Drainage Systems
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Natural Drainage Overview
Frequently Asked Questions
Why is Seattle doing this program?
Urban activities such as landscaping, transportation, and business generate pollutants which are carried through creeks into nearby lakes and Puget Sound – impacting our local marine life food chain.
Traditional stormwater management, focusing on pipes to move stormwater from one area to another, serves worthy goals of flood reduction and public safety. Unfortunately, this type of management negatively impacts our creeks and the larger receiving water ecosystems they flow into.
Natural drainage systems (NDS) are a response to the public call to develop drainage projects that sustain high quality water bodies and species diversity. In addition, emerging federal and state environmental regulations clearly indicate that efforts to protect and enhance the state’s receiving waters will be mandatory.
How are these projects funded?
Seattle property owners pay a drainage fee based on impervious surface coverage. The drainage fee supports many different drainage projects and programs at Seattle Public Utilities.
What are the benefits of using natural drainage systems?
Natural drainage systems
- • Improve water quality through filtering out or chemically breaking down pollutants
- • Slow the flow and reduce the volume of stormwater
- • Incorporate more trees and green space into our neighborhoods
- • Involve neighbors in planning, implementation, and maintenance
- • Increase pedestrian and traffic safety by reducing average auto speeds
- • Become more effective as plants and trees grow
What are the drawbacks?
Because of the participatory and comprehensive nature of the projects, NDS can take more time than traditional systems in the planning and design phase. It may also be necessary, and difficult, to change street standards.
How much more do they cost than “traditional” systems?
NDS cost about 10 to 20 percent less than traditional street redevelopment with curb, gutter, catch basins, asphalt, and sidewalks.
How do they relate to work done in support of compliance with the Endangered Species Act?
NDS are an important element of Seattle’s Endangered Species Act response. They are a key strategy to protect water bodies with all types of salmon and other aquatic life.
How do these systems perform over time?
From October of 2000 through September of 2003, the University of Washington monitored the stormwater flow of the SEA Streets project and found that the project prevented 99 percent of the wet season runoff from flowing directly into Piper’s Creek.
SPU is monitoring the flow and water quality of the Broadview Green Grid and High Point.
How are sites selected?
Streets are selected based on the following criteria:
- • The proposed project streets are residential streets.
- • Each project street has or could have (by diverting flow from an adjacent ditch) a minimum of five acres of upstream drainage area directed to it.
- • The majority of the project streets have no existing local drainage infrastructure.
- • The potential project blocks have no existing formal street improvements, such as sidewalks and asphalt or concrete streets.
- • All potential project streets are a minimum of 300 feet from a critical slope area (as defined by the Department of Planning and Development) so that infiltration technologies can be used.
- • Existing soil maps do not indicate clay in the area.
- • The longitudinal slope of project streets is less than eight percent. Larger longitudinal slopes may limit infiltration capacity or drive up project costs by requiring earth stabilization walls and fences.
- • Potential project streets do not have a bus route on them.
If any potential funding partnering is desired from Seattle Public Utilities, the proposed project streets must be in an area prioritized for one of our business area needs. These areas include creek basins, combined sewer overflow problem areas, and prioritized water quality basin areas. No funding mechanism is currently in place for partnering on these types of projects; this process will be initiated in 2008. Due to intensive time commitment for City staff to evaluate and develop a partnering strategy, the minimum project area that SPU can consider for partnership is a 600-foot project length.
Related links
Be a Creek Steward
Creek Restoration Projects
Charting Common Ground for Salmon and Buildings
Links to other sites
Low Impact Development - Learn how Puget Sound planners, developers, engineers and others are transitioning to an innovative approach to land development and stormwater management
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