Preserve natural drainage systems

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

Unmanaged flooding can lead to a natural disaster event resulting in financial, social, and structural ruin. There are many different ways communities have adapted to flooding, however, there is not one central repository that holds the information on the types of adaptation structures, their costs, or benefits. In our efforts to create as accurate a flood model as possible, we decided to create the nation’s first flood adaptation database. Stormwater management plans often contain multiple of the 40 different types of flood risk reduction projects, known as adaptation, that this model considers when calculating and validating flood projections. 

Use Risk Factor™ to find the only property-specific flood risk assessments that consider flood mitigation structures.


Restoring and preserving 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. previous ground cover

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%


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 an issue here; select "Questions or comments about Flood Factor" and then "share home or area protections." The adaptation database contains 23,000 features today. We know there are more projects to include and value your input!

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