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Utilities’ Scientists Re-map Seattle’s Flood-prone Areas


Oct. 5, 2009
For Immediate Release:

Holly McCracken, 206-386-4195

With New Technology, Utilities Scientists Re-map Seattle’s Flood-prone Areas

Urban Flood-prone Areas Face Significant Risk to Personal Safety and Property

SEATTLE — Using new hydrologic models, optical remote sensing technology and data from the record-setting 2007 floods, Seattle Public Utilities scientists are re-mapping areas of the city that are prone to flooding.

Seattle’s flood-prone areas were last mapped in 1987. Since then, computer modeling methods have been developed that allow more precise predictions of areas that are likely to flood during big storms.

The City’s current flood mapping efforts began after record-setting rains and flooding in the winter of 2007 provided stormwater data that could be analyzed using the new modeling methods. The first results of that analysis are new flood maps for the Thornton Creek and Densmore Basin areas, in north Seattle.

Other areas of the city that will be mapped are floodplains along Longfellow Creek, Pipers Creek and the Duwamish River, over the next several years.

Urban areas designated as flood-prone face a significant chance of serious flooding, during heavy rains. Such floods are likely to cause property damage and be a risk to personal safety, including loss of life. Nationally, all flood-prone residences have a 26 percent chance of being damaged by a flood during a 30-year timeframe, compared to a 9 percent chance of damage due to fire.

SPU considers flood-prone areas to be critically important locations for emergency response. To reduce the risk of urban flooding, SPU has a year-round maintenance program that addresses the repair and maintenance of storm drains, culverts and other key drainage infrastructure to reduce flooding.

The Utility’s staff monitor real-time information about flooding conditions during severe rainstorms. SPU is also utilizing new stormwater flooding data in the investigation and prioritization of long-term capital improvement project options to further address flooding in vulnerable areas throughout the city.

Urban flood-prone areas are regulated under Seattle’s Environmentally Critical Areas code (SMC 25.09) and subject to different development standards and land use regulations. Residents of the newly designated flood-prone areas are being advised to purchase flood insurance, to refrain from living in or renting any below-grade dwellings on flood-prone property, and to stay out of low-lying areas or basements during times of heavy rains.

Owners of property in flood-prone areas should also consult with a real estate professional about disclosures required for flood-prone property.

“SPU conducted the flood mapping study in the Thornton Creek and Densmore Basin to provide our customers and staff with the most up to date understanding of potential flood risks during extreme rain events based on the best available science,” said Trish Rhay, Seattle’s director of drainage and wastewater.

The new maps removed 312 properties from the Thornton Creek floodplain and added 88 new ones. In the Densmore Basin, which had previously not been on the City’s flood plain maps, 28 properties were added. A total of 624 properties are on the new floodplain maps for both areas — a net decrease of 196 properties.

The updated mapping is being done for SPU with the assistance of Northwest Hydraulic Consultants (NHC) using the best available data, mapping, and water modeling techniques.

Federal flood insurance can be purchased anytime. There is a 30-day waiting period after you’ve applied and paid the premium before your policy is effective. However, if you purchase flood insurance in connection with making, increasing, extending or renewing a loan, there is no waiting period.

Flood damage is not covered by regular homeowner’s insurance policies.

To learn more about flood insurance, go to FEMA’s Web site at or call FEMA’s Disaster Assistance at (800) 621-FEMA / TTY (800) 462-7585.

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In addition to providing a reliable water supply to more than 1.3 million customers in the Seattle metropolitan area, SPU provides essential sewer, drainage, solid waste and engineering services that safeguard public health, maintain the city’s infrastructure and protect, conserve and enhance the region’s environmental resources.