Name: Countywide Base Floodplains 100 Year

Display Field: WATERSHED_NAME

Type: Feature Layer

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Description: To prepare this layer, 1-D base hydraulic models have been developed in HEC-RAS (version 6.2) for 30 watersheds within Fairfax County based on estimated ultimate condition flows computed using NRCS hydrologic methods as implemented in HEC-HMS (version 4.8). During the hydraulic modeling of each watershed, each stream is modeled independently using normal depth as the starting boundary condition in the HEC-RAS model. The terrain for the hydraulic modeling was derived from Quality Level 1 (QL1) LiDAR acquired in December 2018 with eight points per square meter density that meets vertical accuracy standards for 1' contours. The vertical datum is the North American Vertical Datum of 1998 (NAVD88), therefore all model water surface elevation outputs are referenced to this vertical datum. After the development of the base hydraulic models, the 100-year inundation boundary polygon layer was generated in RASMapper. The final floodplain shapefile i.e., “COUNTY_BASE_FLOODPLAINS_100YR” has been prepared by merging the inundation boundary layer for all 30 watersheds. The base model does not include crossings such as bridges and culverts. Additionally, in-line impoundments or other structures are not modeled in these base models.Contact: Fairfax County Department of Public Works and Environmental Services (DPWES)Data Accessibility: Publicly AvailableUpdate Frequency: As NeededLast Revision Date: 6/20/2025Creation Date: 6/20/2025Feature Dataset Name: STWMGR.COUNTYWIDE_BASE_FLOOD_STUDIESLayer Name: STWMGR.COUNTY_BASE_FLOODPLAINS_100YR

Copyright Text: Fairfax County, Virginia; DPWES; AtkinsRéalis

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Spatial Reference: 102746  (2283)

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Fields:

• OBJECTID ( type: esriFieldTypeOID, alias: OBJECTID )
• WATERSHED_NAME ( type: esriFieldTypeString, alias: Watershed Name, length: 50 )
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