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Natural Drainage Systems
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Natural Drainage Overview
The Story of Natural Drainage Systems
The Problem
Prior to development, present-day Seattle was covered by forests and grasslands that acted like a sponge, absorbing rainfall into the ground and letting it seep into creeks and rivers. Today, the rooftops, streets, and parking lots that cover much of the land in Seattle prevent rain from moving through the soil. Instead, water is carried away from these hard surfaces through pipes and ditches to our waterways.
This can lead to two problems:
- • Runoff carries everyday contaminants, such as oil, paint, fertilizer, and heavy metals directly into creeks, lakes, and Puget sound, impacting the food chain that supports fish and other wildlife
- • The speed and volume of stormwater coming out of pipes erodes stream channels, impacting wildlife habitat
View the Seattle's Natural Drainage Systems Booklet (PDF) for a complete history of the development of which this page summarizes.
An Innovative Solution
In response to the threats to habitat, Seattle Public Utilities began programs such as the Urban Creeks Legacy projects to actively promote citizen stewardship and restoration of local creek habitat.
However, in-stream restorations alone are not enough. In 1999, SPU initiated the natural drainage systems (NDS) program, an innovative approach to stormwater management that protects the fragile ecosystems in Seattle’s water bodies.
In pristine forest conditions, creeks maintain a dynamic equilibrium with storm cycles, allowing for aquatic organisms to adjust to changing creek conditions. NDS mimics natural forested conditions to limit the impacts of urban development on Seattle’s water bodies.
SPU redesigns residential streets to include vegetated swales, stormwater cascades, and small wetland ponds. These features slow stormwater flows and allow soils to absorb water and filter out many contaminants. The system relies on plants, trees, and the deep, healthy soils that support them. If maintained properly, the plants and soils combine to form a living infrastructure that, unlike pipes and vaults, improves in function over time.
Achieving Positive Results
The first natural drainage project, Street Edge Alternatives (SEA Streets), was implemented in a single residential block in a low-density neighborhood in Piper’s Creek watershed in northwest Seattle. The project combined swales with traditional drainage infrastructure (such as catch basins and culverts) and decreased the watertight surface area of the site’s streets by 11 percent.
Two years of monitoring show that SEA Streets reduced the total volume of wet season runoff by 99 percent.
Natural Drainage System Evolution
The success of SEA Streets enabled the NDS projects to grow and diversify. The High Point project features the largest natural drainage project that the City has undertaken and protects 10 percent of the Longfellow Creek watershed. The Capitol Hill Water Quality Improvement project will redesign the drainage system in one of Seattle’s densest neighborhoods.
For more information about specific natural drainage projects, visit the Natural Drainage Systems Program Overview page.
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