Layer: FFRMS Riverine 2FVA (ID: 1)

Name: FFRMS Riverine 2FVA

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Type: Raster Layer

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Description: The Federal Flood Risk Management Standard (FFRMS) floodplain was developed by the Federal Emergency Management Agency (FEMA) to improve the resilience of communities and federal assets against the impacts of flooding. Through a series of Federal executive orders, the FFRMS was established (EOs 11988 and 13690); cancelled in 2017 (EO 13807); re-established in 2021 (EO 14030) and also mapped as the FFRMS floodplain; and then cancelled again in 2025 (EO 14148). This FFRMS floodplain data set was provided by FEMA at the request of Maryland Department of the Environment and is the same data available through the Flood Standard Support Tool (https://floodstandard.climate.gov/).3-meter DEM resolution terrain resolution used for this mapping. In some cases, 1-meter DEM data may have been re-sampled to a 3-meter DEM resolution. FEMA did this to create a consistent FFRMS data set nationwide. The primary sources of data used by FEMA to develop the FFRMS floodplain were USGS’s The National Map for terrain and FEMA’s National Flood Hazard Layer for regulatory water surface elevations.In Riverine areas, the FFRMS floodplain is derived from the National Flood Hazard Layer using the methodology developed by the Maryland Department of the Environment for production of the Riverine CRAB using a linear interpolation applied between two riverine cross sections from the National Flood Hazard Layer (FEMA 100-year floodplain). A freeboard (2-feet for the 2-foot freeboard map, and 3-feet for the 3-foot freeboard map) was added to the 100-year floodplain water surface elevation at each riverine cross section in the National Flood Hazard Layer data set. The new elevations were then interpolated to create a freeboard elevation gradient. This elevation gradient was used to map the horizontal extents of the FFRMS floodplain boundary based on topographic data. Additional details can be found here: https://www.fema.gov/sites/default/files/documents/fema_ffrms-data-methodology.pdfThe methodology to develop the coastal FFRMS floodplain (water surface) raster utilized the coastal static Base Flood Elevation values from the National Flood Hazard Layer as the water surface. To create an expanded surface, these water surface values were collected at uniformly spaced points along the boundary of the effective coastal 1%-annual-chance flood mapping and expanded inland utilizing Thiessen polygons. The Thiessen polygons were converted to a raster matching the spatial resolution of the terrain DEM, and the difference between the water surface elevation raster and the terrain was computed. Areas where the water surface raster value was higher than the terrain raster value indicate the area of flooding. Each freeboard value (2-foot and 3-foot) was mapped independently in whole foot increments utilizing the prior, lower freeboard value boundary of flooding to establish an expanded surface. One limitation of this methodology is that wave runup was modeled for the 100-year floodplain but not remodeled at the higher water surface elevation conditions.Please note that the CRAB data set shows inundation depths derived as the height difference between the DEM and the water surface elevation, whereas the FFRMS shows water surface elevations only. Their horizontal extents may be compared, but users should be careful comparing any color coding or depth values provided in the two applications.

Copyright Text: MD iMAP, MDE

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