How to read a flood map

Since the 1970s, the Federal Emergency Management Agency (FEMA)
has been creating, storing, and updating flood hazard maps for National
Flood Insurance Program (NFIP) communities across the United States.
Flood Insurance Rate Maps, known as FIRMs, are the primary tool for
state and local governments to mitigate the effects of flooding in their
communities.
We are pleased to present this guide to Flood Insurance Rate Maps.
Throughout this tutorial the common term Flood Map is used to refer to
these documents.
Using this tutorial in conjunction with the Flood Maps themselves can
assist you in deciding whether the potential flood risk for a specific
property should be insured against flood loss.
The glossary terms used in this tutorial will be listed at the end.
FEMA and Flood Insurance
In 1968, the US Congress created the National Flood Insurance Program
(NFlP). Their intent was to reduce future damage and to provide
protection for property owners from potential loses through an insurance
mechanism that allows a premium to be paid by those most in need of
the protection. FEMA produces Flood Insurance Rate Maps that show
areas subject to flooding. The flood risk information presented on the
Flood Insurance Rate Map is based on historic, meteorological,
hydrologic, and hydraulic data, as well as open-space conditions, floodcontrol
works, and development.
A variety of information can be found on a Flood Map, including:
• Common physical features, such as major highways, secondary
roads, lakes, railroads, streams, and other waterways.
• Special Flood Hazard Areas (SFHA)
• Base flood elevation (BFE) or depths
• Flood insurance risk zones
• Areas subject to inundation by the SOO-year flood
What Flood Maps Can Help You Do?
The Flood Map provides information that allows you:
• To identitY Special Flood Hazard Areas. (SFHA)
• To identitY the location of a specific property in relation to the
Special Flood Hazard Areas.
• To identitY the base (IOO-year) flood elevation at a specific site.
• To identitY the magnitude of flood hazard in a specific area.
• To locate regulatory floodways.
• To identify undeveloped coastal barriers, where flood insurance in
not available.
Who Uses Flood Maps?
FEMA distributes Flood Maps to a wide range of users. Private citizens,
insurance agents, real estate brokers, community officials, lending
institutions, and Federal agencies all use the Flood Maps to assist them
in understanding flood hazards. Private citizens, insurance agents, and
real estate brokers use the Flood Maps to locate properties, buildings and
corresponding flood insurance risk zones. Community officials use the
Flood Maps to administer floodplain management regulations and
mitigate flood damage. Lending institutions and Federal agencies use the
Flood Maps to locate properties, buildings and determine whether flood
insurance is required when making loans or providing grants for the
purpose or construction of buildings.
What Are Flood Maps?
To prepare the Flood Maps that illustrate the extent of flood hazard in a
flood prone community, FEMA generally conducts engineering studies
referred to as Flood Insurance Studies (FISs). Using the information
gathered in these studies, FEMA engineers and cartographers delineate
Special Flood Hazard Areas (SFHAs) on Flood Maps. Special Flood
Hazard Areas are subject to inundation by a flood that has a I-percent or
greater chance of being equaled or exceeded during any given year. This
type of flood commonly is referred to as the IDO-year flood or base
flood.
A 100-year flood is not a flood that occurs every 100 years!
In fact, the IDO-year flood has a 26 percent chance of occurring during a
3D-year period, the length of many mortgages. The IDO-year flood is a
regulatory standard used by Federal agencies and most states, to
administer floodplain management programs. The IDO-year flood is used
by the National Flood Insurance Program (NFIP) as the basis for
insurance requirements nationwide.
Documents Related to Flood Maps are Available
The results of the Flood Insurance Study are also available in a technical
document that provides information used for floodplain management.
This is known as the Flood Insurance Study Report (FIS).
Regulatory floodways and other floodplain management information
may be shown on a separate Flood Map. This is known as a Flood
Boundary and Floodway Map (FBFM). It is typically distributed with
the Flood Insurance Study report, but is not distributed as a part of a
standard map request. On many newer Flood Insurance Studies,
floodways are included on the FIRM so FBFMs are not included. The
Flood Insurance Study report and maps can be reviewed at the
community office for floodplain management activities in your
community.
Copies of the report and Flood Maps may also be obtained from FEMA’s
Map Service Center.
http://www.msc.fema.gov/
How to Obtain Flood Maps
The Flood Maps for your community should be available for review at
your local Community Map Repository site. Typically, this is your local
planning, zoning, or engineering office.
If you would like copies of Flood Maps, they may be obtained from
FEMA’s Map Service Center (http://www.msc.fema.govD. Because the
Map Service Center maintains thousands of Flood Maps, you will need
to provide specific information about the map(s) you want to order, such
as the name of the community and the county it lies in, or the six-digit
community number.
Federal Emergency Management Agency
Map Service Center
P.O. Box 1038
Jessup, Maryland 20794-1038
Telephone: (800) 358-9616
Fax: (800) 358-9620
http://www.fisc.fema.gov/
Flood Map Overview
Since the National Flood Insurance Program began over 25 years ago,
many improvements have been made to the design of Flood Maps, so
that they are easier to use and better meet users’ needs. To control costs,
FEMA includes design improvements in Flood Maps as new maps are
produced. These improvements occur on a community-by-community
basis. As a result, all Flood Maps are not exactly the same. They may
differ in format and content. The two basic formats used for Flood Maps
are a “Flat Flood Map” and a “Z-Fold Flood Map.” In addition, every
flood map has several basic elements, such as an associated index map,
panel, title box, legend, and standard symbols.
Flood Map Formats
A Flat Flood Map consists of one or more II “x 17” pages and a cover
sheet that includes an index and a legend.
A Z-Fold Flood Map is a larger map that is folded like a highway map.
Z-Folds consist of one or more panels, which have a legend printed on it.
Z-fold Flood Maps involving more than on panel also have an index.
When discussing Flood Maps, it is common to refer to the set of panels
that make up a geographic area as one Flood Map.
In order to determine your community’s flood map format or geographic
coverage, you can contact FEMA’s Map Service Center, to consult a
Flood Map Specialist.
Flood Map Coverage
The coverage of Flood Maps can include all of these jurisdictions:
Counties/Parishes, Towns, Townships, Cities. For standard identification
purposes, FEMA assigns a six-digit Community Identifier (CID) to all
active participants in the NFIP. Flood Maps contain flood hazard
information on one or multiple communities, identified by CID on the
map panels. When a Flood Map includes multiple communities, a list of
the affected communities, by name and CID, is listed on the title block
of each panel. To determine the geographic coverage of your
community’s Flood Map, contact the Map Service Center.
Most Flood Maps cover only one jurisdiction. If that jurisdiction is the
unincorporated part of a county, flooding information is shown only for
the areas under the jurisdiction of the county government. This means
you will not find flooding information for incorporated areas (e.g. town
and cities) on the Flood Maps produced for the unincorporated areas of a
county.
Separate Flood Maps are prepared for incorporated areas. More recently,
however FEMA has produced countywide Flood Maps. These Flood
Maps usually show flooding information for all of the geographic areas
of a county, including towns and cities. To determine the geographic
coverage of your community’s Flood Map, contact the Map Service
Center.
The Index
The index serves as a guide to map sheets found on a Flood Map. Since
the geographic coverage of a Flood Map may be quite large, FEMA
divides the area into sections called panels. The index is provided to
indicate what areas are shown on each map.
Due to variations in content found on Flood Maps, separate fonnats exist
for both Flat Flood and Z-Fold Maps.
See the section on How to Read a Flood Map Index for further
information on the variations of these maps. When detennining flood
hazard potential for an area, you should first consult the index to
detennine which flood map panel illustrates your community.
The Panel
The Flood Map for your Community may include one or more
individual maps. Each map is called a panel. When a Flood Map
contains multiple panels, that simply means that the Flood Map for your
community will not fit on one page. The number of panels depends on
the community size and the scale(s) of the Panels. The Index is used to
determine which panel should be utilized to obtain flood hazard
information for a specific location.
Flood Map Basics
In addition to the two basic [onnats, Flood Maps also have several basic
elements. Every flood map will include a title box, legend, body, and
basic symbols.
Due to variations in [onnat and content, not all elements appear on every
flood map.
The Title Block:
Found on each Panel (or page), the title box contains the community
name, the panel number (page number), date, and other information
necessary to identify the Flood Map panel.
The Legend:
Found on Z-fold Flood Maps, the Legend provides information for
identifying the risk zones and/or floodway on the Flood Map.
The Body:
The body of a flood map displays the map contents. On an index, the
body will usually only display primary features like major roads,
corporate limits and other general landmarks that help to identify
location. On a panel, the body will show more detail than what is shown
on the index, including secondary roads, bridges, and flood hazard
information.
In addition, every map regardless of format will contain a typical set of
symbology as noted on the next page.
Typical Flood Map Elements
These seven items are typically found on all FEMA Flood Maps
regardless of the format or the area covered by the Flood Map. Other
items may be found on specific Flood Maps depending upon the formats
and geographic area. These will be discussed later in this tutorial.
Community Name:
The title block displays name of the mapped community, the community
type (e.g. town, city, county), the county, and the state. When the
mapped community is a county, the words “Unincorporated Areas” often
appear below the county name. This indicates that the incorporated areas
in the county area not covered by the Flood Map. When the Flood Map
covers the entire geographic area of the county, the words “and
Incorporated Areas” appear after the county name.
Community Number:
The Community Number is a six-digit identification number assigned to
the mapped community. This number is also referred to as the
community identification number (eID). You need to use the
community identification number when you ask FEMA staff questions
about a Flat Flood Map or a Z-fold Flood Map for an individual
community and when your order a Flood Map from the Map Service
Center.
Typical Flood Map Elements (continued)
Corporate Limit or County Line:
The Corporate Limit or County Line identifies the jurisdictional limits of
the community’s regulatory authority over land development and
building construction. In some states, an incorporated community may
exercise extraterritorial jurisdiction over land development and building
construction in areas beyond its corporate limits. Where appropriate,
these limits are shown and labeled on the index.
Inset Note:
Inset Notes identify what panel includes special areas on the map.
North Arrow:
This arrow orients the Flood Map.
Panel Limit Line:
This line reveals the extent ofthe area covered by each panel shown on
the index.
Panel-not-Printed Notes:
These notes identify the panels included in the Flood Map layout that are
not printed and explain why they are not printed. For instance, when a
panel covers an area of the community that falls entirely in one flood
insurance risk zone, that panel may not be printed, and an explanatory
note is added to the index. If all panels are printed this annotation is
omitted.
How to Read a Flood Map Index
Flood Map Indexes
If a Flood Map is composed of more than one panel, an index is usually
provided that serves as a guide to the information found on each map.
FEMA produces indexes for both Flat Flood and Z-Fold Flood Maps.
Regardless of the format, each index is divided into sections that
correspond to individual flood map panels.
When determining flood hazard potential for an area, you should first
consult the index to determine which flood map panel illustrates the
area.
In order to determine your community’s flood map format or geographic
coverage, you can contact FEMA’s Map Service Center to consult a
Flood Map Specialist.
Types of Flood Map Indexes
The index shows the outline of the mapped community and the numbers
and positions of the individual panels. There are three different types of
indexes depending upon the community you are interested in, Flat Flood
Maps, Community Flood Maps, and Countywide Flood Maps.
Types of Flood Map Indexes
Flat Flood Map Index:
Flat Flood Map indexes cover communities that are mapped on 11″ x
17″ sheets. This index shows the outline of the community and the
numbers and positions of the individual panels (sheets). The flat map
index contains a key to the various flood insurance risk zone
designations and important notes to the user. Several important dates are
also shown on the index. These dates include the flood insurance rate
map effective dates, and dates of revision to the Flood Map. The
community name and number will be shown in the title block.
Individual Community Flood Map Index:
Individual Community indexes range in size. The smallest being 8Yz” x
II” to the largest 37Yz”x 25 Yz”. These index maps show the community
boundary and the number and position of the individual panels.
Individual Community Index Maps do not have a legend and general
panel notes to the user.
The title block contains the community name, community number and
the lowest and highest panels included in the layout of the Flood Map.
The community number and panel number are combined into a IO-digit
number for each panel. A map repository listing is contained on the
index.
Types of Flood Map Indexes (continued)
Countywide Flood Map Index:
A Countywide Flood Map displays an entire county and any
incorporated areas. Countywide Flood Maps may include more than one
community. As with all indexes the community boundaries are shown
along with the numbers and position ofthe individual panels. All panels
may not be printed so “Panels Printed” are listed in the title block.
Some indexes may contain a generalized depiction of SFHA shown on
each panel, this is for reference purposes only. On selected index maps,
a flood-prone streets index may be included that lists flood-prone streets
on the Flood Map. Occasionally, there are exceptions where a
community that falls within the area of a countywide Flood Map may
continue to be shown on a separate Flood map.
Elements Found Only on a Flat Flood Map Index
Legend:
This is the Flood Map Legend that identifies the symbols on the Flood
Map and provides information to assist users of the Flood Map,
including explanations of the various flood insurance risk zone
designations.
Notes to User:
These notes provide important additional information about the flood
map.
Elements Found Only on a Z-Fold Flood Map Index
Effective or Revised Date:
This is the date the new or revised Flood Map becomes effective for
flood insurance and floodplain management purposes. Partial revisions
are common and the inside of the index will show the suffix of each
panel to determine what has been revised. New countywide indexes will
have effective dates printed on each panel.
List of Printed Panels:
This list identifies those panels that are printed, but not necessarily
revised. Partial revisions are common and the inside of the index will
show the suffix of each panel to determine what has been revised. New
countywide indexes will have effective dates printed on each panel.
Elements Found Only on a Countywide Flood Map
Index
List of Floodprone Communities:
• Floodprone communities covered by the Flood Map. (Note: All
communities are listed; the non-floodprone communities are
footnoted to indicate they are non-floodprone.)
• The community identification number for each community
• The panel on which each community is shown
• The initial flood identification date (the date that flood hazards
were first identified)
• Post FIRM date for each community (the date of the first FEMA
Flood Map for the community
Elements Found Only on a Z-Fold Flood Map Index
for Individual Communities
Community Panel Number Range:
This range, presented in the title box of the index, gives the lowest and
highest panels in the layout of the Flood Map. The Panels are identified
by IO-digit community-panel numbers.
Elements Found on Some Z-Fold Flood Map Indexes
Due to the dynamic nature of flood hazard mapping these elements are
included on some versions of Z-Fold Flood Maps.
Floodprone Area Overview:
Floodprone Area Overviews are shown on some Flood Maps. These
areas on the map provide a generalized depiction of the Special Flood
Hazard Areas shown on each panel. It helps you find the appropriate
panel and orients you to the Flood Map. It should not be used in place of
the more detailed delineations on the panels.
Floodprone Street Index:
Shown on selected Flood Maps, this index lists the streets in the mapped
community that are partially or completely in the special Flood Hazard
Area. It also indicates the panel(s) on which each street is shown. Grid
coordinates that allow you to locate the street on the panel are listed for
each street. The Floodprone Street Index may be shown either on the
index or on a separate panel.
Map Repository Address:
This is the address of the official community office where reference
copies of the Flood Map and Flood Insurance Study report are stored
and made available.
How to Read a Flood Map Panel
When a Flood Map cannot be presented on one page, it is produced on
several pages known as panels. Flood Map Panels depict the flood
hazards in different parts of a community.
Elements Found on all Panels
As noted in the Map Overview Section, there are seven basic elements
common to all indexes and panels. In addition to these items, there are
14 more Elements that may be found on Flood Map panels.
• Area Not Included Label
• Base Flood Elevation
• Coastal Barrier Area
• Elevation Reference Marks
• Floodplain Boundary
• Hazard Area Designation
• Map Scale
• Panel Number
• Notes to User
• River Mile Marker
• Stream Line
• Zone Designation
• Zone Division Line
• Flood Insurance Risk Zone Label
Elements Found on all Panels
Floodplain Boundary:
These boundaries show the 100-year and SOO-year floodplains.
Hazard Area Designation:
These areas appear as dark and light tints. Dark tints indicate areas of
greater flood hazard; light indicates areas of lesser flood hazard.
Base Flood Elevation (BFE)
For detailed study areas this line and label indicates the water surface
elevation of the base flood in relation to a standard set of geographic
data in Special Flood Hazard Areas. A wavy line is used to indicate
when the base flood elevation varies along a watercourse. When the base
flood elevation is uniform across a large area, a label is used.
Zone Division Line
This line separates Special Flood Hazard Areas with different zone
designations and separates Special Flood Hazard Areas with differing
Base Flood Elevation in coastal hazard areas.
Elements Found on all Panels (continued)
Flood Insurance Risk Zone Designations:
The zone designations indicate the magnitude of the flood hazard in
specific areas of a community.
River Mile Marker:
This marker identifies the distance in miles from a reference point on a
river or other major watercourse.
Stream Line:
This line identifies the location of a watercourse. Narrower streams are
usually shown by a single line, representing the approximate location of
the stream centerline. Wider streams are often shown by double lines,
representing stream bank locations.
Coastal Barrier Symbol:
Coastal barrier symbols appear only on Flood Maps that contain coastal
communities. These symbols identitY undeveloped coastal barriers in
the Coastal Barrier Resources System. No new flood insurance coverage
may be provided after specified dates for new or substantially improved
structures on any Coastal Flood Hazard Area in the System.
Elements Found on all Panels (continued)
Area Not Included Label:
This label identifies areas that are in the panel area, but are not in the
jurisdiction of that community; thus, no flood hazard information is
shown in that area.
Map Scale:
This allows you to relate distances measured on the Flood Map to actual
distances on the ground. The scale shown on a panel applies only to that
panel. Most commonly used scales are one inch on the map equals 500,
1000, or 2000 feet on the ground
Panel Number:
This number distinguishes each panel from others in the same
community; each page is given a unique panel number. On Flat Flood
Maps, the panel number is a two-digit number. On Z-Fold Maps, the
panel number matches the last four digits of the community panel
number. On Countywide Z-fold Flood Maps, the panel number matches
the last four digits of the map number.
Notes to User:
These notes provide additional information to clarify zone designations
or special notes on the use of the Flood Map.
Elevation Reference Marks (ERMs):
These marks identify points where a ground elevation is established by
survey. Descriptions of reference marks appear on different places,
depending on the map format.
Elements Found Only on Some Z-Fold Panels
Alpha-numeric Grid:
This is the basis of the coordinate systems established for the
Floodprone Street Index.
Cross Section Symbol:
This symbol shows locations of floodplain cross sections used for
computing detailed flood information. These locations can be correlated
to the flood profiles refer to in the Flood Insurance Study.
Floodway Boundaries:
These boundaries show the limits of the floodway.
Floodway Designation:
This designation identifies floodway areas. The floodway is the channel
of a river or other watercourse plus any adjacent floodplain areas that are
reserved so that the IOO-yr flood discharge can be conveyed without
increasing the elevation of the IOO-yr flood more than a specified
amount.
Elements Found Only on Some Z-Fold Panels
(continued)
Map Repository Address:
This is the address ofthe official community office where reference
copies of the Flood Map and Flood Insurance Study report are stored
and made available for viewing.
Panel Locator Diagram:
This diagram shows the area covered by the panel in relation to the
outline of the mapped community.
Elements Found Only on Some Z-Fold Panels
Individual Communities
Community Panel Number:
This number identifies the community and panel; it corresponds to a
community panel number shown on the index. On the FIRM, a letter
suffix follows the number and usually indicates the number of times a
particular panel has been revised. Initial Identification Date:
This date appears in the legend and indicates when the flood hazards in
the mapped community were first identified.
Flood Hazard Boundary Map Revisions Chronology:
This chronology lists dates of revised versions of the Flood Hazard
Boundary Map (FHBM) for the community, if one was printed.
Map Effective Date:
The date, appearing in the legend, indicates when the Flood Insurance
Rate Map was first printed and became effective for the Regular Phase
of the National Flood Insurance Program.
Map Revisions Chronology:
This chronology lists the effective dates of revised versions of the Flood
Map, if any, and briefly describes the reasons for the revisions.
Elements Found Only on Countywide Panels
Effective Date on Countywide Map:
The date on which the Countywide Flood Map first became effective.
Flood Maps may have been in effect for one or more of the individual
communities in the county before the countywide Flood Map was
published. If so, the initial Flood Map Effective dates for each
community will be shown as “Post FIRM Dates” in the List of Flood
Prone Communities.
Map Number:
This number identifies the panel. It is composed of a five-digit code that
identifies the county; a letter “C” that indicates countywide mapping,
and a four-digit number corresponding to the panel number and a letter
suffix that indicates the number of times a panel has been revised.
Getting Specific Information
In this section, we present a step-by-step process that you may follow to
obtain this information when you locate a property on a Flood Map.
Background:
You are considering buying a new home on Water Street in Sampleville.
Before you make any final decisions, you want to know the potential
risk that this house could be flooded. To help you do this, you need to
know if the property is in a Special Flood Hazard Area. If it is in the
SFHA, you may also will want to find the flood insurance risk zone
designation and Base Flood Elevation that apply to the property to help
you determine the appropriate insurance rate.
Step 1: Finding the Correct Panel:
To find the panel that covers the property, you will first refer to the
index for the Sampleville Flood Map. By reviewing the index, you learn
that the Flood Map you need was prepared in a Z-Fold format composed
offour panels. The index title box indicates that all four panels (000 I,
0002,0003, and 0004) are printed. You then check the title boxes on
individual panels to find Panel 0004
Background: You know that the property is on Water Street, which lies
in the southeastern portion ofthe town, south oflnterstate Highway 32.
Although Water Street is not on the index, you may have determined
from the north arrow and the major roads shown on the index that Water
Street is on panel 0004.
Step 2: Finding the General Location:
Using the panel number determined from the index, you obtained Panel
0004 that shows the property. Because the individual panel shows more
physical landmarks than the index, you can find the general location by
referring to features such as major roads or city limits.
Your property is located east of the main intersection of Oak Avenue,
and Water Street. To find the specific property location, you will use
known property dimensions and the Flood Map scale.
In addition, to find the specific location of the property, you may refer to
a plat map of the property, tax assessor’s map, or the property
description found on the deed. The next page explains how to determine
the specific location of the property.
Step 3: Finding the Specific Location of the Property
You know the property lies on the northern side of Water Street, east of
the intersection of Oak Avenue and Water Street.
First, you find the dimensions of the property on a tax assessor’s map, a
plat map, or in a legal description.
Then, you convert the known dimensions to inches using the Flood Map
scale and measurements on the Flood Map panel.
For example, in a map scale where 1 inch=500 feet, you will find that
250 feet on the ground is equal to 1/2 inch on the Flood Map, and 50 feet
on the ground is equal to 1/1 0 inch on the Flood Map. Continue to the
next page for a description ofhow to determine the specific location
using the map scale.
Step 4: Identify the Flood Insurance Risk Zone of the
Property
The property you are considering buying is partially in the Special Flood
Hazard Area. In addition, the building on the property is partially in the
SFHA. (Note: Current NFIP regulations indicate that if any portion of a
building is in the SFHA, the entire building is considered to be in the
SFHA.) You can now identify the flood insurance risk zone designation
for the property by finding the zone label. The SFHA near the property
is labeled Zone AE. The zone designation applies both to the portion of
the property in the SFHA and to any building on that portion.
Step 4: Identify the Flood Insurance Risk Zone of the
Property (continued)
Occasionally, a property or building is shown in the SFHA on the map,
but the property or building is higher than the BFE. This is often due to
the limitations of the mapping.
To be sure whether a property or building should be in the SFHA, a
comparison between the elevation of the property or building and the
BFE should be preformed. lfthe lowest elevation ofthe property or the
lowest grade adjacent to the building (and, in some instances, the lowest
floor including the basement ofthe building) are below the BFE, then
the property or building are in the SFHA. If a property or building is
inadvertently shown within the SFHA or if a property or building has
been raised above the BFE by the placement of fill; FEMA can amend or
revised the maps with a letter that states the property or building is not
within the SFHA. These letters are called Letter of Map Amendment
(LOMA) or Letter of Map Revision based on Fill (LOMR-F).
To make this determination, you need to know the lowest lot elevation,
or the lowest adjacent grade at the building and in some instance the
lowest floor (including basement) elevation. This usually will require an
elevation survey of the property or building. Also, you will need to
know the BFE at the site.
Step 5: Identify the Base Flood Elevation at the Property
To estimate a Base Flood Elevation (BFE) at a property, the BFE lines
(labels) shown near the property on the Flood Map can be used. For
example, ifBFE lines 109 and 110 are near the property, and 110 is the
nearer of the two, you may estimate the BFE at the property by
interpolating between the two base flood elevations.For an accurate
elevation number, you refer to the flood profiles for the flooding source
that appear in the Flood Insurance Study (FIS) report for the associated
stream. However, the BFEs shown on the Flood Map are only accurate
to plus or minus a halffoot. For an accurate BFE, you should refer to the
Flood Profiles or Flood Elevation Tables in the FIS report.
Step 5: Identify the Base Flood Elevation at the Property
(continued)
Some SFHAs do not have BFEs shown on the Flood Maps. These
SFHAs are shown as zone A or zone V and where determined by
approximate methods.
Possible sources for obtaining a BFE in these areas may be the
community’s planning, public works, engineering, environmental or
transportation departments; the local district of the U.S. Army Corps of
Engineers; or the State’s Environmental, Natural Resources, or the
Transportation Departments. Also a registered professional engineer
could conduct an engineering analysis to determine the BFE at the site.
Additional information regarding BFEs in these areas is available in the
FEMA publication Managing Floodplain Development in Approximate
Zone A Areas.
The Future of Flood Insurance Rate Maps
DFIRM: The Next Generation
The new Digital Flood Insurance Rate Map (DFIRM) product involves
converting the existing inventory of manually produced Flood Maps into
digital format. This conversion is designed to be the building block for a
more robust, interactive, digital map, which may potentially include
multiple hazards.
Linkages will be built into a database to allow access to the engineering
back-up material used to develop the map (e.g., hydrologic and
hydraulic models, flood profiles, floodway data tables, digital elevation
models, and structure-specific data, such as digital elevation certificates
and digital photographs of bridges and culverts).
DFIRM: The Next Generation (continued)
The contents of the digital Flood Insurance Rate Maps will vary. Where
accurate community vector data is available, community vector data will
be utilized to show base map data as backdrops to flood data layers. If
accurate community data is not available, digital aerial photography
(Digital Ortho Quads (DOQs)) will be utilized as the base map layer.
Glossary Terms
I-percent annual chance floodplain
This is the boundary ofthe flood that has a I-percent chance of being
equaled or exceeded in any given year. Also known as the IOO-year
floodplain.
I-percent annual chance water-surface elevation
The height, in relation to the National Geodetic Vertical Datum of 1929
(or other datum, where specified), of the flood having a I-percent chance
of being equaled or exceeded in any given flood year (also known as the
IOO-year flood or the base flood).
IOO-year flood
The flood having a I-percent chance of being equaled or exceeded in
any given year; also known as the base flood. The I-percent annual
chance flood, which is the standard used by most Federal and state
agencies, is used by the National Flood Insurance Program (NFIP) as the
standard for floodplain management and to determine the need for flood
insurance. A structure located within a special flood hazard area shown
on an NFIP map has a 26 percent chance of suffering flood damage
during the term of a 30-year mortgage.
IOO-year floodplain
This is the boundary of the flood that has a I-percent chance of being
equaled or exceeded in any given year. Officially termed the I-percent
annual chance floodplain.
500-year floodplain
This is the boundary ofthe flood that has a O.2-percent chance of being
equaled or exceeded in any given year. Officially termed the O.2-percent
annual chance floodplain.
Backwater
The effect of downstream flow on the water-surface profile.
Base Flood
The flood having a I-percent chance of being equaled or exceeded in
any given year; also known as the IaO-year flood. The base flood,
which is the standard used by most Federal and state agencies, is used by
the National Flood Insurance Program (NFIP) as the standard for
floodplain management and to determine the need for flood insurance. A
structure located within a special flood hazard area shown on an NFIP
map has a 26 percent chance of suffering flood damage during the term
of a 30-year mortgage.
Base Flood Elevation (BFE)
The height of the base flood, usually in feet, in relation to the National
Geodetic Vertical Datum of 1929, the North American Vertical Datum
of 1988, or other datum referenced in the Flood Insurance Study report,
or depth of the base flood, usually in feet, above the ground surface.
Channel Bank Stations
Points that identify the extreme limits ofthe natural stream channel.
These stations are typically assigned at locations along a cross section
where a relatively flat area exists outside of the channel.
Conflnence
A location where two streams or rivers meet.
Contonr
A line on a map joining points of equal altitude.
Critical Depth
The depth of flow at which, for a given discharge at a given location, the
total energy is the minimum value possible for flow to occur.
Cross Section
A line developed from topographic information across a floodplain at
which a computation of flood flow has been made to establish a
potential flood elevation. Cross sections are shown on the Flood
Boundary Floodway Map, Flood Insurance Rate Map, and/or Flood
Profiles of a Flood Insurance Study.
Cubic feet per second (cfs)
Typical units used to express the rate of flow of surface water in open
channels. One cubic foot is approximately equal to 7.5 gallons per
second.
Datum
A fixed starting point of a scale.
Depth
Maximum depth of water in the cross section as measured below the
water-surface elevation.
Digital Flood Insurance Rate Map (DFIRM)
As part ofFEMA’s Map Modernization Objectives, a new Digital Flood
Insurance Rate Map (DFIRM) product is being developed. The new
DFIRM product will include a spatial database with options that can be
invoked depending on the available data. The DFTRM spatial database
will include certain standard features and meet minimum mapping
requirements. Additional enhancements will be included depending on
community needs, available data, and funding. A review of needs and
available data will lead to recommendations concerning which options to
exercise.
Discharge
The volume of water that passes a given location within a given period
of time. Usually expressed in cubic feet per second (cfs).
Federal Emergency Management Agency (FEMA)
An independent agency of the Federal government, founded in 1979,
which reports directly to the President. FEMA is responsible for
identifying and mitigating natural and man-made hazards. The agency’s
.. .
mISSIon IS:
to reduce loss oflife and property and protect our nation’s critical
infrastructure from all types ofhazards through a comprehensive, riskbased,
emergency management program ofmitigation, preparedness,
response and recovery.
Flood (also Flooding)
A general and temporary condition of partial or complete inundation of
normally dry land areas. For flood insurance claim purposes, two or
more structures must be inundated before flood damage will be covered.
Flood Boundary Floodway Map (FBFM)
A pre-Map Initiatives floodplain management map that delineates the
IDO-year (I% annual chance) and SOD-year (0.2% annual chance)
floodplains, floodway, and cross sections.
Flood Hazard Boundary Map (FHBM)
Initial map issued by FEMA to identify approximate Special Flood
Hazard Areas (SFHAs) within a community.
Flood Insurance Rate Map (FIRM)
A map on which the IDO-year (I% annual chance) and SOD-year (0.2%
annual chance) floodplains, Base Flood Elevations, and risk premium
zones (and floodway information on Map Initiatives FIRMs) are
delineated to enable insurance agents to issue accurate flood insurance
policies to homeowners in communities participating in the National
Flood Insurance Program.
Flood Insurance Study (FIS)
An examination, evaluation, and determination of flood hazards and, if
appropriate corresponding water-surface elevations. The resulting
reports are used to develop Flood Insurance Rate Maps. Also know as a
flood elevation study.
Flood Profile
A cross-sectional drawing showing the contiguous cross sections along a
stream, with ground elevations and potential flood elevations plotted.
Floodplain or Flood-Prone Area
Any land area susceptible to inundation by water from any source.
Floodplain Management
The operation of a program of corrective and preventative measures for
mitigating flood damage, including, but not limited to, emergency
preparedness plans, flood-control works, and floodplain management
regulations.
Floodway
Channel of a stream plus any adjacent floodplain areas that must be kept
free of encroachment so that the IOO-year flood discharge can be
conveyed without increasing the elevation of the IOO-year flood by more
than a specified amount (I foot in most states).
HEC-2
A step backwater program developed by the US Army Corps of
Engineers Hydrologic Engineering Center for use in calculating watersurface
profiles for steady, gradually varied flow in natural or man-made
channels.
Hydraulic Radius
Equal to (Area of cross section / Wetted Perimeter)
Mannings “n” Roughness Coefficient
Coefficient used to account for the friction caused by earthen,
vegetative, and/or man-made surfaces within a floodplain cross-section.
The coefficient, n, is commonly used to represent flow resistance for
hydraulic computations of flow in open channels. The procedure for
selecting n values is subjective and requires judgment and skill that is
developed primarily through experience. The expertise necessary for
proper selection ofn values can be obtained in part by examining
characteristics of channels that have known or verified roughness
coefficients. A table of Manning’ values is available from the pull-down
menu in the Quick-2 program.
Map Initiatives
A Flood Insurance Rate Map format developed in 1985 that incorporates
information formerly shown on the separately printed Flood Boundary
Floodway Map (FBFM) (e.g., floodways and cross-sections). Also
referred to as combined Flood Insurance Rate Map/FBFM.
Map Repository
The location where a community’s flood maps are kept; usually the local
zoning and planning office.
Map Service Center (MSC)
The Map Service Center (MSC) distributes National Flood Insurance
Program (NFIP) products including: Digital Flood Insurance Rate Maps
(DFIRM), Flood Insurance Rate Maps (FIRM), Flood Insurance Studies
(FIS), Digital Q3 flood data, Community Status Book, Flood Map Status
Information Service (FMSIS), and Letters of Map Change (LOMC).
National Flood Insurance Program (NFIP)
Federal insurance program under which flood-prone areas are identified
and flood insurance is made available to residents of participating
communities that agree to adopt and enforce floodplain management
ordinances to reduce future flood damage.
Normal Depth
The depth expected for a stream when the flow is uniform, steady, onedimensional,
and is not affected by downstream obstructions or flow
changes. This is the usual calculation that is utilized to determine Base
Flood Elevations for property or structures in approximate Zone A)
areas.
Overbank
The area of a cross section that is found outside of the channel bank
stations on either side of the stream channel.
Peak Discharge
The peak volume of water that passes a given location within a given
period of time. Usually expressed in cubic feet per second.
Rating Curve
A curve showing the relationship between depth of flow and the
discharge of a stream at a given location.
Scale
A representative fraction of map distance to ground distance. Example:
I: 12,000 is the representative fraction in which one unit of measure on
the map is equal to 12,000 of the same units of measure on the ground.
Federal Emergency Management Agency map scales are expressed in a
ratio of map distance equal to a given number of feet on the ground.
Special Flood Hazard Area (SFHA)
Area inundated by the base (I-percent annual chance) flood, identified
on the Flood Insurance Rate Map as Zones A, AE, AH, AG, AR, V, VE,
or A99.
Step-Backwater Analysis
Method used in Quick-2 (and other modeling programs) to analyze
multiple cross-sections. Water-surface elevations are determined for all
sections based on a given discharge. The initial water-surface elevation
is automatically determined by the normal depth method or by a direct
input of a water-surface elevation or depth.
Water-Surface Elevation
The height, in relation to the National Geodetic Vertical Datum of 1929
(or other datum, where specified) of floods of various magnitudes and
frequencies in the identified floodplains of coastal or riverine areas.
ZONE A
The flood insurance rate zone that corresponds to the 100-year
floodplains that is determined in the Flood Insurance Study by
approximate methods. Because detailed hydraulic analyses are not
performed for such areas, no Base Flood Elevations or depths are shown
within this zone. Mandatory flood insurance purchase requirements
apply.
Zone A99
The flood insurance rate zone that corresponds to areas of the 100-year
floodplains that will be protected by a Federal flood protection system
where construction has reached specified statutory milestones. No Base
Flood Elevations or depths are shown within this zone. Mandatory flood
insurance purchase requirements apply.
Zone AE and AI-A30
The flood insurance rate zone that corresponds to the 100-year
floodplains that is determined in the Flood Insurance Study by detailed
methods. In most instances, Base Flood Elevations derived from the
detailed hydraulic analyses are shown at selected intervals within this
zone. Mandatory flood insurance purchase requirements apply.
ZoneAH
The flood insurance rate zone that corresponds to the areas of the 100year
shallow flooding with a constant water-surface elevation (usually
areas of ponding) where average depths are between I and 3 feet. The
Base Flood Elevations derived from the detailed hydraulic analyses are
shown at selected intervals within this zone. Mandatory flood insurance
purchase requirements apply.
ZoneAO
The flood insurance rate zone that corresponds to the areas of 100-year
shallow flooding (usually sheet flow on sloping terrain) where average
depths are between I and 3 feet. The depth should be averaged along the
cross section and then along the direction of flow to determine the extent
of the zone. Average flood depths derived from the detailed hydraulic
analyses are shown within this zone. In addition, alluvial fan flood
hazards are shown as Zone AO on the Flood Insurance Rate Map.
Mandatory flood insurance purchase requirements apply.
ZoneAR
The flood insurance rate zone that results from the decertification of a
previously accepted flood protection system that is being restored to
provide protection from the IOO-year or greater flood event.
ZoneD
Designation on National Flood Insurance Program maps used for areas
where there are possible, but undetermined, flood hazards. In areas
designated as Zone D, no analysis of flood hazards has been conducted.
Mandatory flood insurance purchase requirements do not apply, but
coverage is available. The flood insurance rates for properties in Zone D
are commensurate with the uncertainty ofthe flood risk.
Zone V
The flood insurance rate zone that corresponds to the IOO-year coastal
floodplains that have additional hazards associated with storm waves.
Because approximate hydraulic analyses are performed for such areas,
no Base Flood Elevations are shown within this zone. Mandatory flood
insurance purchase requirements apply.
ZoneVE
The flood insurance rate zone that corresponds to the IOO-year coastal
floodplains that have additional hazards associated with storm waves.
Base Flood Elevations derived from the detailed hydraulic analyses are
shown at selected intervals within this zone. Mandatory flood insurance
purchase requirements apply.
Zone B, C, and X
Zones B, C, and X are the flood insurance rate zones that correspond to
areas outside the IOO-year floodplains, areas of IOO-year sheet flow
flooding where average depths are less than I foot, areas of IOO-year
stream flooding where the contributing drainage area is less than I
square mile, or areas protected from the IOO-year flood by levees. No
Base Flood Elevations or depths are shown within this zone.

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