Preserve natural drainage systems

Learn about natural drainage systems and how they are included in flood projections.


To restore and preserve natural drainage systems involves the promotion of vegetated and other “soft” ground covers over impervious surfaces in developed areas. These types of ground treatments provide drainage that more closely resembles the natural landscape prior to development. This is also an underlying principle of green infrastructure, which refers to man-made stormwater management infrastructure that relies on natural components such as plants, well-drained soils, and other “soft” materials to soak up runoff, thereby reducing flooding and improving surface water quality.


Stormwater infiltration with impervious vs. pervious groundcover.

Seattle's pilot Street Edge Alternatives Project (SEA Streets) was completed in 2001 and relies on the restoration and preservation of natural drainage systems. The street features a narrower design to reduce impervious surface area from pavement, and numerous bioswales, as well as over 100 evergreen trees, and 1,100 newly planted shrubs to help soak up stormwater. According to the City of Seattle’s website, two years of monitoring show that SEA Street has reduced the total volume of stormwater leaving the street by 99%.

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Courtesy Seattle Municipal Archives: 131697 and 155415

The installation of rain gardens is another way communities can restore and preserve natural drainage systems. For example, rain gardens such as those installed along Military Road in Auburn, WA, reduce frequent nuisance flooding from stormwater on nearby properties by soaking up excess runoff. In fact, rain gardens are so efficient at soaking up stormwater that they absorb as much as 30% - 40% more than a standard lawn of a similar area (source: USDA).

Flood risk reduction

Information on Flood Factor comes from the First Street Foundation Flood Model. Restoring and preserving natural drainage systems is a principle approximated by many of the 40 different types of flood risk reduction projects, known as adaptation, that this model considers when calculating and validating flood projections. 

The Adaptation Team continues to collect information on the flood infrastructure that exists across the country to make sure the Flood Model includes as many adaptation projects as possible. If you know of any projects that are not shown today, please help the team by submitting this flood protection project user input form. The adaptation database contains 23,000 features today. We know there are more projects to include and value your input!


Learn more

Learn about ways to improve runoff reduction

Visualize flood risks

What is urban flooding and how is it incorporated in the Flood Model? 

How can community science and local knowledge improve the Flood Model?

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