Natural stormwater management, or low impact development (LID) is based on the premise that nature knows how to manage stormwater runoff best. In contrast, traditional development practices involve removing all vegetation, compacting soil and putting in large areas of hard (impervious) surfaces like roads, parking lots and roofs. The compacted soil and impervious surfaces prevent stormwater from soaking into the ground (called infiltration). This results in a tremendous increase in surface runoff and pollution entering streams, wetlands and bays. Stormwater runoff overwhelms these natural water bodies causing flooding, damaging public and private property and destroying habitat for fish and wildlife. Traditional development practices also require large and costly pipes and stormwater ponds to remove pollutants and slow flow.

Seattle Green Factor

As of Jan. 20, 2007, developers must meet a new landscaping requirement for Seattle’s commercial areas that encourages planting layers of vegetation in areas visible to the public and along the street, using rain water harvesting and choosing low-water use plants, using larger trees, tree preservation, green roofs and even green walls. For more information and to view a worksheet to help you calculate your Green Factor score, go to: Seattle Green Factor.

LID Approaches:

These LID approaches are approved by the City of Seattle to comply with the flow control requirements prescribed in the City's Stormwater Code. The "Flow Control Technical Requirements Manual" will give you the size and design requirements you need for all the approved methods to meet Seattle's code.

• Soil amendment
  
• Pervious paving
• Rain gardens
• Green roofs
• Right of ways and public spaces
• Construction BMPs

Amend the top 8-12" of all disturbed soils with compost.

This simple, cost-effective technique improves the soil's ability to store water during a storm event, and supports plant growth that can provide additional water storage. Compost can also be used during construction to protect disturbed soils and in berms in place of silt fences. Look for compost that is "recycled" from yard waste or land clearing. For more information, visit the Soils for Salmon Web site.

Use pervious paving systems.

Using a pervious system for paving allows stormwater to infiltrate into the ground or sub base and be retained for a short period. A variety of systems, in a number of price ranges, and for all types of applications are available, including ground wood chips or rubber, pervious concrete and asphalt, grass paving systems, and hard pavers. Most Seattle construction projects have to provide flow control measures for stormwater running off all impervious area, including paved surfaces.

Incorporate "rain gardens."

Rain gardens are biological stormwater facilities engineered to control the flow of stormwater on a site, a strategy also referred to as biorention. The City of Seattle provides guidelines in their technical manuals for two types: stormwater planters and bioengineered planting strips. They are great for managing stormwater from parking lots or as a way to add landscaping to a multifamily project. For more information on rain gardens, read the detailed case study (PDF, 283 kb) of the City of Bellingham's use of rain gardens to manage stormwater runoff in parking lots. The case study includes instructions to create rain gardens, architectural drawings and cost comparisons between conventional in-ground water storage and treatment systems and rain gardens.

Consider a green roof.

A modern evolution of sod roofs, green roofs consist of a waterproof barrier, a drainage and retention system, soil, and plants installed on a roof. Thin, lightweight systems with just a few inches of soil support small plantings and are great for remodels. In new construction, you can design for deeper, heavier systems where shrubs and small trees can grow. Either type turns a roof's large impervious surface into an attractive, natural feature that retains and slowly releases stormwater. To see examples of green roofs on commercial buildings, visit Seattle's City Hall or the Ballard Public Library.

Use green techniques to manage stormwater in right of ways and public spaces.

Rain gardens and green roofs are valuable because they can be used directly on a building site. If your project involves any development, there are additional opportunities to use green strategies for managing and treating stormwater including: grass filter strips, bioswales, wet ponds and stormwater wetlands. On an urban project that won't have large public areas, right-of-ways represent the best opportunity to use these techniques. Go to Street Edge Alternatives (SEA Streets) project to learn more.

City of Seattle's SEA Streets Project

Promote construction stormwater best management practices (BMPs).

Be sure to address green stormwater management not only through design, but also in the plans and specifications for stormwater management during construction. The Construction Stormwater Control Technical Manual (PDF, 5,675 kb) identifies a series of best management practices for addressing erosion and sedimentation during construction. Work with the contractor to identify the most appropriate methods for the project and share with them the fact sheet Erosion and Sedimentation Control for LEED™ (PDF, 88 kb) on managing construction stormwater to meet LEED™ requirements.