New county-wide digital maps Effective November 4, 2016
See FEMA MAP Service Center (Washington County, Pending Maps) https://msc.fema.gov/portal/advanceSearch
Principal Flood Problems
The past history of flooding of the streams within Washington County indicates that, for a particular storm, flooding usually occurs on many streams throughout the study area. Flooding is caused by heavy rainfall augmented by snowmelt at a time when the soil is near saturation. Damaging floods may occur any time between late October and late April. The most severe floods occur in December, January, and February.
The largest flood recorded on the Tualatin River at West Linn since the U.S. Geological Survey (USGS) established a stream gage there in 1928 occurred in December 1933. The peak discharge at the gage was 23,300 cubic feet per second (cfs) with a 1.4-percent-annual-chance of flooding. The January 1974 flood on the Tualatin River also caused considerable damage to the study area. That flood had an estimated discharge of 22,300 cfs with a 4-percent-annual-chance of flooding. The flood occurred before the operation of the Henry Hagg Lake Project. The operation of the Henry Hagg Lake Project has decreased the frequency of a flood of 21,500 cfs to a 2-percent-annual-chance of flooding for the Tualatin River at West Linn.
Other major Tualatin River flood occurred in December 1937, December 1955, December 1964, January 1974, and February 1996. Before 1928, flooding was not well documented, but major floods occurred in February 1890, November 1896, February 1904, January 1905, and January 1914.The February 1996 flood on the Tualatin River was the largest flood flow ever recorded with an estimated 84-year return interval and an annual probability of recurrence of 1.2%. However, for almost all of the smaller urbanized Tualatin River tributaries that were studied, the November 1996 flood is thought to be the largest flood ever observed with an estimated 25-year return interval and an annual probability of recurrence of 4%. default Read more...
A "KML" is a file that can be downloaded and opened directly in Google Earth, ArcGIS Explorer and other viewing software available on the Internet. The KML below is for the Special Flood Hazard Area (SFHA), also known as the "100-year" flood.
- Download KML - Washington_Co_SFHA
A Flood Insurance Study (FIS) is the publication developed in conjunction with the Flood Insurance Rate Map (FIRM) and discusses the engineering methods used to develop the FIRMs. The study also contains flood profiles for studied flooding sources and can be used to determine Base Flood Elevations for some areas.
Note: Preliminary Flood Insurance Rate Maps (FIRMs) are currently being used by some local communities as best available and may be more conservative. These preliminary maps will be in effect November 4, 2016, and replace the current FIRMs. SFHA information must be obtained from the local community before developing in or around Special Flood Hazard Area.
Tualatin River Watershed Risk MAP project currently underway
Snapshots around the County
None at this time
Washington County is FEMA PRELIMINARY DATA - DO NOT use for regulatory purposes: see "description" for source information
(hint: once in viewer select "Basemap" in upper left corner for imagery background)