USGS Washington Water Science Center
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9722-DSE00 - Assessment of Flooding in Western Washington, January 2009
Background - A major weather system delivering a stream of moisture-laden tropical air to the Pacific Northwest resulted in significant flooding throughout western Washington on January 7 and 8, 2009. Total rainfall for January 6-7 ranged from 6 to 15 inches with the highest amounts occurring in the Cascade Range. On January 30, 2009, President Obama declared eight Washington counties as disaster areas as a result of flooding and landslides caused by the severe weather. On February 6, 2009, seven counties were added to the Individual Assistance Declaration bringing the total to 15 (Benton, Clallam, Cowlitz, Grays Harbor, Kittitas, King, Lewis, Mason, Pacific, Pierce, Skagit, Snohomish, Thurston, Wahkiakum, and Whatcom Counties) (fig. 1). On February 11, 2009, a Public Assistance Declaration was made for 22 counties (fig. 1). Provisional data from U.S. Geological Survey (USGS) long-term streamgages indicate flood peaks with recurrence intervals equal to or greater than 100 years occurred in eight major river basins in western Washington; Cedar, Chehalis, Cowlitz, Hoko, Naselle, Puyallup, Snohomish, and Stillaguamish River Basins.
Problem - As part of its Hazard Mitigation effort, the Federal Emergency Management Agency (FEMA) Region X needs to document the extent of flooding that occurred in western Washington on January 7-8, 2009. FEMA needs this information for verifying Preliminary Digital Flood Insurance Rate Maps (DFIRMs) and corresponding Flood Insurance Studies (FIS) that have been recently completed, or are planned for some of the rivers affected by flooding on January 7-8. 2009. For this flood event, FEMA will be conducting a comprehensive post-flood risk analysis that ties together event characterization and resulting property damage. To help with this effort, FEMA has requested USGS assistance with documenting the extent and magnitude of flooding in selected river reaches and with providing a summary of the hydrologic and meteorological characteristics of the event.
Objectives - The general objectives of the study are to document the extent and magnitude of flooding in selected river reaches that flooded in western Washington on January 7-8, 2009 and to document the meteorological and hydrologic conditions associated with the flood event. Specific objectives of the study are to:
Relevance and Benefits - This study is consistent with the USGS science strategy of using the best science available to help decisionmakers and citizens respond to natural hazards such as floods as identified as part of one of six USGS strategic directions for 2007-2017 (U.S. Geological Survey, 2007). The study is also consistent with the Washington Water Science Center’s Strategic Plan and it will serve to assist resource managers in minimizing the impacts of flooding.
Approach - The objectives will be met by conducting a series of tasks that are described in detail below.
Task 1: The areal extent of flooding in selected river reaches (table 1, figs. 2-8) will be marked by securing pieces of flagging tape at high-water marks (HWMs). The objective will be to flag HWMs at about 500-ft intervals on both banks of the identified reaches; however, the intervals will vary with the size of the river, ability to find good HWMs, and ability to gain access to the banks. The location of each HWM will be determined with a hand-held global positioning system (GPS) and referenced to the North American Datum of 1983 (NAD83). A description of each HWM will be tabulated including quality of the mark, type of mark, and location information of the mark. Stream-reach surveys will be completed in the order listed in Table 1.
Traditional surveying techniques with a total station will be used to determine the elevations of all flagged HWMs. Established reference marks will be used to reference elevations to the North American Vertical Datum of 1988 (NAVD 88). If there are no nearby reference marks with third order or better vertical and horizontal accuracy, a survey-grade GPS will be used to acquire temporary benchmarks with accurate horizontal and vertical coordinates. Where existing or on-going FEMA flood studies coincide with the study reaches, FEMA will provide the USGS with the reference-mark coordinates and descriptions.
|Order of field Surveys||
Reach length (miles)
|South Prairie||Pierce||South Prairie Creek||
Task 2: Based on data collected from task 1 and digital elevation data, the USGS will produce maps showing the extent and depth of flooding in each surveyed river reach. The source of the digital elevation data will be a 10-meter digital elevation model (DEM) or, where available, existing digital elevations collected using LiDAR. The highest resolution DEM available will be used in the analysis. One approach to developing a depth of flooding map that may be used in this project is to digitize cross sections at each HWM perpendicular to the flow of the river. Using the known peak water-surface elevation at that point and the cross section computed from the existing DEM, a digital water surface can be interpolated between cross sections. The areal extent of inundation will be estimated where the interpolated water-surface intersects the DEM surface.
Task 3: The gage height and discharge of flood peaks recorded at USGS streamgages located in areas where flooding occurred will be tabulated. Recurrence intervals for the peaks will be determined from tables in the publication entitled “Magnitude and frequency of floods in Washington” (Sumioka and others, 1998), or computed using the regression techniques documented by Sumioka and others (1998). The recurrence intervals will be compared with preliminary or new FEMA Flood Insurance Study data.
Task 4: The meteorological conditions associated with the flood will be summarized using data from the National Weather Service and other available sources. The hydrologic conditions associated with the flood will be described using data collected from a network of 150 USGS streamgages in western Washington.
Task 5: The study methods and results will be published in a USGS Open-File Report. At the conclusion of the study, the USGS will provide FEMA with the results of the field surveys and mapping in a GIS-compatible format.