Description: The baseline floodplain extent layer is a combination of three primary data sources including:NOAAFloodplain Mapping (2018):NOAA Fisheries developed this layer in 2018 to delineate river floodplain boundaries within the Puget Sound region to be used as part of salmon and steelhead habitat monitoring effort by Salmon Habitat Status and Trend Monitoring Program. This layer represents the geomorphic floodplain, which is the relatively level surface extending laterally from the edge of the bankfull channel. Floodplain boundaries were manually digitized from LiDAR where possible. This allowed the exclusion of low terraces that hold little potential for development of salmon habitat (often 5 m or more above the channel elevation). Where there was no LiDAR coverage, floodplain boundaries were based on the National Elevation Data 10-m DEM and aerial photography. This floodplain layers' extent is restricted to salmon and steelhead habitat upstream of delta boundaries and up to natural salmon barriers. It includes all river reaches with drainages greater than 50 square kilometers, which typically have a bankfull width of 15-20 meters.Delta Boundary Mapping (2018): This layer was developed by NOAA Fisheries to delineate delta boundaries within the Puget Sound region to be used as part of salmon and steelhead habitat monitoring effort by Salmon Habitat Status and Trend Monitoring Program. The landward boundaries were guided by the two-year or 50% tidal exceedance boundary and were digitized to capture all areas likely to have contained tidal marsh or tidal channel features historically, including areas that have been blocked or filled. Areas that appear to be filled were identified and digitized based on their appearance on topographic maps and/or aerial photographs as they were too high to be mapped within the current delta polygon based on LiDAR elevation. Areas of tidal flooding that were confined to distributary and main channels upstream of the delta were excluded. The seaward boundaries of the deltas were digitized at the edge of vegetated marshes within the geomorphic delta fan, or –where rivers flow into a bay –at the mouth of the bay. For deltas without bays, the seaward edge of the delta boundary polygon was extended to the seaward extent of densely vegetated marsh surfaces where vegetation covers nearly 100% of the surface.USGS Low Floodplain (2015)Low Floodplain, High Floodplain, Connectivity USGS used publicly available geospatial data to assess ecological functions and flood-related risks to people on major river floodplains in the Puget Sound basin. The project produced multiple assessment products (datasets) that can be used in a geographic information system to inform floodplain management about opportunities to protect and recover ecological functions and to reduce flood-related risks to people. These data are not intended to be used for site-specific evaluation or project design.A modified version of "height above water surface" (Jones, 2006) was used to delineate four land forms—valley bottoms, high floodplains, low floodplains, and river areas. Elevations from the NED for cells crossed by major rivers were extrapolated across each river basin. Areaswhere the land surface was less than 10 m above the extrapolated water surface were designated as valley bottoms. The other three land forms (river areas, low floodplains, and high floodplains) were delineated respectively as areas lower than a base water surface elevation plus the estimated changes in stage from median daily stage for a high flow, small flood, and large flood. The base water surface elevations (riv_z_ned) used to delineate high floodplains were extracted from the elevation values in NED for cells crossed by the NHD flowlines for the major rivers and extrapolated across the valley bottoms using inverse distance weighting. Because of the resolution of NED, cells used to extract river elevations may include river banks and hillslopes. In these cases, river elevations and the extrapolated water surface elevations will have an upward bias. The base water surface elevations used to delineate low floodplain and rivers were extracted from LiDAR, because of its higher vertical and horizontal resolution, and extrapolated across the valley bottoms. Because LiDAR data were not available for all river segments, low floodplain and river areas were not delineated comprehensively. Connectivity of floodplains was assessed using topography and infrastructure that arelikely to impede the movement of materials or biota. As the first step, any cells crossed by roads, railroads, and levees were removed from the floodplain. This step presumes these features have a nominal width of 10 m and provide no ecological function. The remaining floodplain cells were converted into polygons where each polygon had a border defined by a road, railroad, levee, or land surface higher than the high floodplain. The resulting floodplain polygons are referred to as “fragments.” Any floodplain fragment crossed by a river was designated as a “connected” floodplain. Fragments that were separated from a river by topography, roads, railroads, or levees were designated as a “reduced connectivity” floodplain indicating some type of disconnection is probable. Under this scheme, a floodplain fragment had to be completely surrounded by road, railroad, levee, or high elevation land to have reduced connectivity. For more information on these datasets, see: https://pubs.er.usgs.gov/publication/sir20155033. FEMA 500-Year Floodplain Mapping (Aggregate) This aggregate dataset includes the most-up-to-date FEMA 500-year floodplain boundaries for the 17 watersheds. The dataset includes the following county FEMA Digital Flood Insurance Rate Map (DFIRM) or flood maps: Whatcom, Skagit, Snohomish, King, Pierce, Thurston, Lewis, Mason, Jefferson and Clallam. The FEMA 500-year floodplain represents areas of a flood that has a 0.2-percent chance of being equaled or exceeded in any given year (officially termed the 0.2-percent annual chance floodplain).
Copyright Text: Puget Sound Partnership
Jennifer Burke – Information and Geospatial Systems Manager
P 360.999.3849
JENNIFER.BURKE@PSP.WA.GOV