creating depth grid from a dfirm fema region viii mitigation gis team wednesday, february 13, 2013
TRANSCRIPT
Creating Depth Grid from a DFIRM
FEMA Region VIII Mitigation GIS Team
Wednesday, February 13, 2013
Overview
• Floodplain and flood zone overview• Where to find DFIRM data• Depth grid basics• Topography• Water surface
• Creating a depth grid• Review and QC• Exercise – Creating a depth grid
At a minimum, the process determines the Base Flood.
Base Flood: NFIP flood of regulation for floodplain management. Defined by law as a flood that has a 1% chance of being equaled or exceeded in a given year.
Special Flood Hazard Area (SFHA): Areas subject to the Base Flood (1% chance) or greater, annual change of flooding in any year (aka the “100-year floodplain).
Base Flood vs. “100 year” Flood: The base flood is often referred to as the “100 year” flood or a “100 year” event. This is a misnomer because it doesn’t occur only once in 100 years.
Floodplain and Mapping Terms
• 100-year Floodplain, 100-year Floodway
1% Annual Chance (100-yr) FP
Zone A (Floodplain only) Zone AE (Floodplain and Floodway)Zone AH/AO (Floodplain only)
• 500-year Floodplain/Shaded Zone X (not always 500-year floodplain)/Zone B• <=1 foot of depth
0.2% Annual Chance (500-yr) FP
Zone A
Zone A (100-year Floodplain)Approximate model (automated or manual)No road crossings included in modelTopography: 4-ft or greater contour intervalsNo depths or BFEs mapped
Zone AE
Zone AE (100-year Floodplain)Detailed (manual)Road crossings included in modelTopography: 2-ft or less contour intervalsBFEs, Cross sections mappedIn the FIS profilesCan have a floodway
Zone AO
Zone AO (100-year Floodplain)Detailed (manual)Ponding with 1-3 feet of depthTopography: 2-ft or less contour intervalsElevations stated on map
Zone AH
Zone AH (100-year Floodplain)Detailed (manual)Ponding with 1-3 feet of depth
Zone V/VE
Zone V and VE (100-year Coastal Floodplain)Similar to Zone A and AE, but Coastal (Approximate
and Detailed Respectively)
Zone C and Unshaded Zone XFlood hazards larger event than a 500-year flood (less probability)
DFIRM Database/NFHL Data
• Available from the FEMA Maps Service Center• https://msc.fema.gov/portal• Click search all products
Topographic Data
• Best available data and best fit• USGS National Map (NED) http://nationalmap.gov/• Available nationally at resolutions of 1 arc-second
(about 30 meters), 1/3 arc-second (about 10 meters)
• 1/9 arc-second resolution data is very limited in coverage for most areas of the U.S.
• LiDAR available through local municipalities• http://coast.noaa.gov/inventory/
~90-meter ~30-meter ~10-meter ~1-meter
Raster Resolution Equivalence
Less information More information
Raster Resolution
• Site-Specific – LiDAR or < 3 meter resolution• County Level Analysis – 10 or 30 meter NED
Time and level of analysis are important when determining the topographic resolution
Water Surface Data
• Cross Sections (Xs) of riverine flooding sources are taken by surveyors and engineers to gather information about the size and geometry of the stream channel and the elevation of the ground. This information is used to accurately model the flood hazards for the stream. Use Cross Sections when available.
• Base Flood Elevations (BFEs) are the elevation shown on the Flood Insurance Rate Map (FIRM) for Zones AE, AH, A1-30, or VE that indicates the water surface elevation resulting from a flood that has a 1-percent chance of occurring in any given year. In riverine areas, BFEs are plotted as lines at different points along the stream on the FIRM which correspond to elevations in the flood profile in the Flood Insurance Study (FIS) report
DFIRM to Depth Grid
• ONLY can be done for areas of detailed study (surveyed zone As – AE, AH, and AO only)
• Use the available cross sections to interpolate the water surface for AE Zones
• AO Zones – Constant water depth• Create unique depth grid for 1, 2, or 3 ft
• AH Zones – Static BFE• Create unique depth grid using a constant WSEL
• Fill remaining areas with 0.1ft
Exercise
Create a depth grid based off of the City and County of Denver DFIRM for an area along the South Platte
River near Evans Ave and I-25. To do this, the DFIRM Database will need to be downloaded from
the FEMA Map Service Center, and the DEM and AOI can be found the FEMA MOTF FTP site. The DEM
has been clipped to the area of interest.
GO!!!!
Useful Links
• USGS NED - http://nationalmap.gov/• Vertcon Conversion -
http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl • FEMA Maps Service Center – https://msc.fema.gov/portal • MOTF SOPs – https://content.femadata.com/MOTF/SOPs/ • US Interagency Elevation Invnetory -
http://coast.noaa.gov/inventory/