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Good practice Imported

Street Edge Alternatives (SEA Streets) — Seattle's Natural Drainage Retrofit Cutting Stormwater Volume by 97% (USA)

United States of America · Seattle · See the United States of America profile · See the Seattle profile

Evidence: Observational / pre–post Top 59% 40/100 · Ask Evidence Copilot about this practice

Seattle Public Utilities rebuilt a residential block (completed 2001) with vegetated swales instead of piped drainage. Two years of SPU/EPA-documented monitoring found the retrofit cut stormwater volume by 97% and eliminated 98% of wet-season runoff.

98 %
Wet-season runoff reduction
97 %
Total stormwater volume reduction
37 % above required
Detention volume vs ordinance requirement
11 %
Impervious surface reduction vs standard street

Details

Maturity
Established
Promoter
Seattle Public Utilities (SPU)
Period
2001-2003 (pilot construction and post-completion monitoring)
Keywords
sustainable drainage, urban greening, watershed and salmon-habitat protection, green infrastructure

Context

Seattle Public Utilities' Street Edge Alternatives (SEA Streets) pilot reconstructed a residential block of 2nd Avenue NW in the Broadview neighbourhood, completed spring 2001, using vegetated swales instead of a piped storm sewer to mimic pre-development hydrology and protect downstream salmon-bearing streams.

Activities

The design reduced impervious surface 11% below a standard street, added 100+ evergreen trees and 1,100 shrubs, and sized swales to detain the pre-development 2-year, 24-hour storm event (37% more capacity than the city's drainage ordinance required).

Results

Two years of post-construction monitoring found the site eliminated all dry-season discharge and 98% of wet-season runoff, and cut total stormwater volume leaving its 2.3-acre catchment by 97% relative to baseline.

Conclusions

The pilot detained rather than fully retained runoff onsite, so it fell short of the city's more stringent 'Healthy Waters' goal, and no pollutant-removal figure exists; it nonetheless became the template for SPU's wider Natural Drainage Systems programme.

Implementation
Indicative cost
Low (< €50k) — Single-block pilot retrofit, low relative cost compared with conventional piped infrastructure.
Time to results
Short (< 1 year)
Staffing & skills
Seattle Public Utilities engineers

Conditions for success

  • Suitable soil/slope for vegetated swales
  • Alignment with city drainage ordinance
  • Neighbourhood buy-in for streetscape change

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Data sources

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