Floodplain Cross Sections [Washington (State)--Pierce County]
Pierce County, Washington · 2025 Full Details
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Full Details
- Title
- Floodplain Cross Sections [Washington (State)--Pierce County]
- Description
- Data Disclaimer for Use of Pierce County GIS Data The purpose of this data is to show the base flood elevation at the modeled cross sections for identified areas that have a 1% chance flooding in any given year (i.e. 1 in 100 chance per year or base flood). The cross section represents the most accurate depiction of flood risk of all the mapping products as it is the foundational data of the hydraulic modeling. Most of the hydraulic models in the countywide flood insurance study also ran scenarios for the 0.2% annual chance flood (i.e. 1 in 500 chance per year), the 2% annual chance flood (i.e. 1 in 50 chance per year), and the 10% annual chance flood (i.e. 1 in 10 chance per year). These unique flood scenarios are published in the flood profile of the Flood Insurance Study (volumes two through five). The distance between cross sections can be a few feet near bridges and up to half a mile on large river systems. The most accurate determination of the base flood elevation at any site is to interpolate the flood elevation from the cross section or plot the location on the profile. In coastal Puget Sound the engineering model uses transects that are surveyed perpendicular to the shoreline. The transect data can be found in the flood insurance study volume one. Coastal flood elevations are shown to only the nearest integer because a single transect is used to charaterize long stretchs of shoreline. The calculated transect elevation is shown in the "WSEL_REG" field to the nearest tenth of a foot but this precise value should only be used for design or letters of map change for locations directly crossed by the transect. Generally, FEMA does not map special flood hazard areas that have a drainage basin less than one square mile. The County has identified several locations with small drainage basins that can experience severe localized flooding and these are shown on the map as X zones or 0.2% annual chance flood areas. FEMA (the Federal Emergency Management Agency) uses FIRM (Flood Insurance Rate Map) data for the National Flood Insurance Program (NFIP). Once aspect of the NFIP is that the flood insurance is required for any building in the 1% annual chance floodplain that has a mortgage backed by the Federal government. The 2017 DFIRM (Digital Flood Insurance Rate Map) replaced the retired 1987 FEMA FIRMs for Pierce County, Washington. The FIRM is the basis for floodplain management, mitigation, and insurance activities for the National Flood Insurance Program (NFIP). The DFIRM Database presents the flood risk information depicted on the FIRM in a digital format suitable for use in electronic mapping applications. The DFIRM database is a subset of the Digital FIS database that serves to archive the information collected during the FIS. Flood insurance applications include enforcement of the mandatory purchase requirement of the Flood Disaster Protection Act, which "... requires the purchase of flood insurance by property owners who are being assisted by Federal programs or by Federally supervised, regulated or insured agencies or institutions in the acquisition or improvement of land facilities located or to be located in identified areas having special flood hazards, " Section 2 (b) (4) of the Flood Disaster Protection Act of 1973. In addition to the identification of Special Flood Hazard Areas (SFHAs), the risk zones shown on the FIRMs are the basis for the establishment of premium rates for flood coverage offered through the NFIP. Description This is 2017 DFIRM Data for cross section lines. New to the DFIRM data is the coastal flood study, that adds Velocity Zones (VE) along Puget Sound and the Seclusion Boundary. Seclusion areas are located where floodplains are affected by non-accredited levees and retain 1970s modeled flood hazards. These lines show the locations of channel surveys used to calculate flood elevations in the hydraulic models. The Cross Sections are also shown on the Flood Profiles in the Flood Insurance Study (FIS) report and can be used to cross reference the Flood Profiles to the planimetric depiction of the flood hazards. All cross sections for which a spatial location is available should be included in the S_XS table. The Cross Sections are lines generally extending from outside the floodplain, across the entire floodplain and out the other side. In this data set the cross sections have been trimmed to include only the areas show to be in a flood hazard area. Each cross section is represented by a line feature. The line has been split where the modeling shows a different base flood elevation than the stream center on the left or right bank (reference 2017 FIS Volume 2). The Digital Flood Insurance Rate Map (DFIRM) Database depicts flood risk information and supporting data used to develop the risk data. The primary risk classifications used are the 1-percent-annual-chance flood event (Zones A & V), the 0.2-percent-annual- chance flood event (Zone X shaded), areas of undetermined flood risk (Zone X shaded) and areas of minimal flood risk (Zone X unshaded). The DFIRM Database is derived from Flood Insurance Studies (FISs), previously published Flood Insurance Rate Maps (FIRMs), flood hazard analyses performed in support of the FISs and FIRMs, and new mapping data, where available. The FISs and FIRMs are published by the Federal Emergency Management Agency (FEMA). The file is georeferenced to earth's surface using the UTM projection and coordinate system. The specifications for the horizontal control of DFIRM data files are consistent with those required for mapping at a scale of 1:12,000.
- Creator
- Pierce County, Washington
- Temporal Coverage
- Last Modified: 2025-05-13
- Date Issued
- 2017-05-25
- Rights
- Please see provided hyperlinks for metadata and Terms of Use.
- Access Rights
- Public
- Format
- ArcGIS FeatureLayer
- Language
- English
- Date Added
- August 10, 2025
- Provenance Statement
- The metadata for this resource was last retrieved from Pierce County Open GeoSpatial Data Portal on 2025-08-24.
Resource Class
Place
Local Collection
Cite and Reference
-
Citation
Pierce County, Washington (2017). Floodplain Cross Sections [Washington (State)--Pierce County]. . https://gisdata-piercecowa.opendata.arcgis.com/datasets/piercecowa::floodplain-cross-sections-1 (web service) -
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