CHAPTER II – FLOODPLAIN DATA
2-5
Making Map Determinations
The Flood Insurance Rate Map (FIRM) is used to determine if proposed development, or any part of it, is located
within the Special Flood Hazard Area (SFHA). Development that falls within the SFHA is subject to the provisions
of the local floodplain management ordinance. In order to make a determination, use the FIRM or FHBM index to
locate the map panel for the development site. An asterisk on the panel number indicates that no flood hazard has
been identified, or the entire area is situated in one flood zone and the panel was not printed. Be sure the map
panel is the most recent one printed by comparing the panel suffix letter with the suffix letter shown on the current
map index. Panels for some communities show different effective dates because revisions did not affect the whole
community. The Community Status Book shows the most current effective map date.
It is most often helpful in doing flood hazard determinations to use a tax assessor’s map indicating property
boundaries. First, obtain the distance on the ground between the site and one or more identifiable points, i.e., the
centerline of a road or street, a bridge, or some other cultural feature on the map. Using the map scale and the
measured ground distance from these points, plot the site on the map. The degree of difficulty in locating the
project site will largely depend on familiarity with the properties in the community and with the scale of the flood
maps. Determinations are much more challenging when a large scale map is being used to identify property in a
rural area.
If the site is within the mapped flood boundary, it is subject to the provisions of the local floodplain ordinance. If the
site is clearly outside the flood boundary, no floodplain regulations apply unless the site adjoins the SFHA and
surveyed ground elevations at the project site are below BFE. In this instance, the site would be considered
floodprone.
Additional information and/or studies may be needed if it cannot be determined whether or not the site is located in
a floodplain. In this instance, ground elevation, lowest adjacent grade elevation, and lowest floor elevations of any
structures will be necessary for the site, therefore the applicant will have to hire a Professional Land Surveyor. A
field visit and measurements on the ground may also be required. In a riverine environment, the actual site
elevations should be compared to the base flood elevation, which is read from Flood Insurance Study profiles, if
available.
If the site is near the floodway boundary, the same process is used to determine if any portion of the site is in the
floodway. In this instance, measuring identifiable land features located on both the ground and the map will be required.
Scale these distances on the map to determine site location relative to the floodway boundary. Because the floodway
boundary is not based on a flood elevation, both field and map measurements are needed to assess whether the site is
located in a floodway.
If the site is at a surveyed cross section, floodway width data from the FIS floodway data table may be used to
supplement field and map measurements. Note that the width listed in the table is the distance from the floodway
line on one side of the stream to the floodway line on the other side of the stream. This is important if the stream
channel is the community boundary, so only about half of the floodway is shown on the community’s FIRM.
If the width measured on the map at that site is not comparable to that shown in the table, the table data are to be
used. If there is a significant difference, contact the FEMA regional office for an interpretation. The proportional
width of the floodway from the center of the stream can be used to determine the floodway boundary on the ground.
In Zones A1-30, AE, and A riverine areas for which no regulatory floodway is designated, the regulatory floodway is
determined to be the channel of the river or other water course and the adjacent land areas to a distance of one-
half the width of the floodplain as measured from the normal high water mark to the upland limit of the floodplain.
If any portion of the building site, proposed grading, filling, bridge or other development is determined to be within
the floodway, then the floodway provisions of the ordinance also apply. If the proposed development lies in more
than one zone, then the standards for the most restrictive zone will also apply to the development. For example, if
any portion of the proposed construction is located in an AE Zone and a VE Zone, then the entire building must
meet the VE Zone requirements.