How to Read and Interpret Flood Insurance Rate Maps
Brent Gotsch, CFM, MPA
Watershed Educator
Cornell Cooperative Extension of Ulster County
5th Annual Ashokan Watershed Conference
April 5, 2014
What Exactly is a Flood Map?
A special type of map using engineering studies (referred to as Flood Insurance Studies) to
designate the flood risk for a particular area (usually a county). The maps are divided up into
detailed areas known as Panels.
They have legends and indexes that are similar to more traditional maps.
There are studies that are specifically designed for riverine (stream) systems and coastal
areas. We will concentrate only on riverine systems today. Be aware that depending on
where the community is located it very well may have both riverine and coastal flood threats.
Why Do We Have Flood Maps?
Prior to 1968 US dealt with floods usually by hard measures (dams, levees, etc.) and paid out
money in disaster assistance. This began to cost taxpayers a lot of money.
National Flood Insurance Act of 1968
• Required the federal government to identify and map flood hazards across the nation
• Made flood insurance available to communities who participate in the NFIP
• Provided data to communities for planning purposes
National Flood Insurance Act (and future legislation) enforced this by putting the burden on
those who live in the floodplain (and pay flood insurance rates) rather than those who do
not. A community who does not participate in NFIP generally does not get disaster
assistance.
Many changes over the years, the most recent being Biggert-Waters Flood Insurance Reform
Act of 2012 which moves flood insurance rates to actuarial levels (previously were
subsidized).
Why Should I Care?
Chance of a building fire over the life of a 30-year mortgage
is approximately 5%.
Chance of flooding over the same 30-year mortgage for a building in the Special Flood
Hazard Area (SFHA) is approximately 26%.
2003-2012 there were over $4 billion in flood insurance claims.
Nationwide: Over 8.6 million housing units in the SFHA.
New York: Over 319,000 housing units in the SFHA.
Where Can I Get/Find Flood Maps?
• Town Halls
 Code Enforcement/Building Inspectors Office
 Zoning Secretary
• Banks
• Insurance agents
• ONLINE!
 (www.ashokanstreams.org) for Ashokan Watershed area.
• FEMA Map Service Center (www.msc.fema.gov)
Federal Emergency Management Agency
Map Service Center
PO Box 1038
Jessup, MD 20794-1038
Terminology
Base Flood: The 1% annual chance flood. It has a 1% chance (out of 100) of occurring in any
given year.
100-Year Flood: Same as the Base Flood (more informal term).
Special Flood Hazard Area (SFHA): The land area that is covered by floodwaters of the base
flood. NFIP communities must enforce floodplain regulations. Mandatory flood insurance
purchase requirement (for federally backed mortgage holders).
Base Flood Elevation (BFE): The elevation to which floodwaters are anticipated to rise during
the base flood.
Flood Insurance Study (FIS): A compilation and presentation of flood risk data for specific
watercourses. The FIS and associated maps delineate the SFHA, designate flood risk zones
and establish BFEs. They also include Floodway Data Tables.
A Few More Terms to Know
Floodway: The channel of a river or other watercourse and the adjacent land areas that
must be reserved in order to discharge the base flood without cumulatively increasing the
water surface elevation more than a designated height. Communities must regulate
development in floodways to ensure that there are no increases in upstream flood
elevations. It is usually the deepest, swiftest moving, and most dangerous part of a flood
flow.
Flood Fringe: areas outside the regulated floodway but still inundated by designated 1%
annual chance flood (also known as the floodway fringe).
500-Year Floodplain: Also known as the .2% chance annual flood. It is not regulated by FEMA
but it is usually mapped because it still provides a significant threat to life and property.
Flood Zones
A…B…C…D…V…X….or the Twilight Zone!
What does it all mean?!
A Zones: The 100-Year floodplain for riverine
settings. Most modern maps are designated AE
Zones. Some maps (usually older maps) have other
designations (e.g. A1-30, A99, etc.). There are a
number of different types of “A Zones.”
D Zones: Area of undetermined hazard.
V Zones: The 100-Year floodplain in coastal zones.
B, C, & X Zones: Correspond to areas outside of the
100-Year Floodplain (usually in reference to the 500Year Floodplain). Has no BFE or depths.
Flood Zones and Base Flood Elevations
SFHA Flood Zones
•
•
•
•
•
Zone AE (formerly Zones A1-A30): Include Base Flood Elevations (BFEs)
Zone A: No BFEs established (aka ‘approximate’ A Zone)
Zone AO/AH: Shallow flooding with depths (AO) or BFEs (AH) established
AO: generally sheet flow
AH: generally ponding
Other Flood Zones
• Shaded X Zones (formerly Zone B): The 0.2% annual chance (500-year) floodplain. No
BFE.
• Unshaded X Zones (formerly Zone C): Low risk area outside the 0.2% annual chance
(500-year) floodplain. No BFE.
In-Depth: Regulatory Floodway
The channel of a river or stream and adjacent land areas that must be reserved (no
obstructions including fill) to pass the base flood without increasing water surface elevation
more than 1 foot (Federal minimum).
Computer models simulate “encroachment” to predict where/how much the BFE would
increase if the floodplain is developed.
States and localities may use a more restrictive standard.
What Goes Into a Flood Map/Flood Insurance Study?
Hydrology (in the form of a hydrologic analysis): Determines the amount of rainfall that will
stay within a watershed and the rate at which the remaining rainfall will reach a stream.
Hydraulics (in the form of a hydraulic analysis): Determines how the floodwaters move
through the floodplain.
Cross Sections: A graphical depiction of the stream and the floodplain at a particular point
along the stream.
Elevation Reference Markers: Designated by a small “x” symbol. They provide ground elevation
references. It is a physical monument.
Flood Profile: A graph that allows you to determine the BFE in-between cross sections.
Flood Data Table: Contains cross section, floodway, and base flood elevation information for a
particular reach of stream.
What Goes Into a Flood Map/Flood Insurance Study?
Historic Information: River flow, storm tide, rainfall data, etc.
Metrologic Data: How much precipitation does the area get?
Topographic Information: Is the area steep/mountainous or flat?
Open Space Conditions: How much of the watershed is forested? How much impervious
surface? Is the riverine system natural or developed?
Flood Control Works: Levees, berms, etc.
Development: How much of the area is urban?
Cross Section
Elevation Reference Mark
Floodway Data Table
Flood Profile
Elements of the Flood Maps
• Index
• Panel
• Legend
• Title Block
• Body
Index
• If a Flood Map has more than one panel then it usually has an index.
• Each index is divided into sections that correspond to individual map panels.
Index—Old Version
Index—New Version
Panel
• Each individual map is called a panel.
• Essentially, it is blown-up, more detailed section of your county-wide map.
• Use the Index to determine which panel you need to reference
Panel
Title Block
• Found on each Panel. It includes the community name, panel number, date and other
information. Usually found in lower right hand corner.
The Legend
• Provides information
on how to understand
the various symbols,
features, and scale of a
map. Usually found in
upper right hand
portion above the
Title Block.
Body
• The body is the “meat” of the map. It displays all of the map contents. Basically it is
everything except the Title Block and Legend.
• The Index has a body that shows primary features like major roads, corporate limits, etc.
• Panels show these things plus more detail (e.g. secondary streets, lesser streams, etc.). It
will also show bridges and provide specific flood hazard information.
Floodway
The area within
the red lines
(blue on real
the map) with
the white
diagonal lines is
the floodway.
Floodway Fringe
The area that is
outside the
white diagonal
lines but still in
the blue lines is
the floodway
fringe.
Taken together
everything within
the blue lines is
considered the
100 year
floodplain (or 1%
chance annual
flood).
The area with the
black dotting is the
500-year or (.2%
annual chance)
floodplain. It includes
everything in the 100Year Floodplain and
then some.
Another Way of Looking at Things
Another Way of Looking at Things
In-Depth: Flood Insurance Study
Accompanies the FIRM
Should be used in conjunction with the FIRM for BFE and floodway determinations at specific
sites
Contains information on:
• Principal flood problems
• Mapping methods
• Hydrologic and Hydraulic analyses
• Summary of flood discharges
• Summary of Stillwater elevations
• Floodway Data Tables
• Flood Profiles for riverine flooding
FIS Layout
Introduction
Area Studied
Engineering Methods
Floodplain Management Applications
Insurance Applications
Flood Insurance Rate map
Other Studies
Location of Data
Bibliography and References
Flood Profiles
In-Depth: The Floodway Data Table (FDT)
Provides data from the hydraulic model for each stream studied by detailed methods with a
floodway
Information on lettered cross sections which are shown on the FIRM
Information on the Floodway
• Width
• Section Area
• Velocity
• Surcharge
Includes regulatory BFE at each cross section to the nearest 1/10 foot
Finding Elevations and Points on a Map: Things to Consider
• The most accurate BFEs are found in the floodway data table.
 Unfortunately, they are only good for sites on or next to a cross section.
• The next most accurate source of elevation data is the profile.
 Unfortunately, profiles are notoriously difficult to read.
• The least accurate source of elevation data is the FIRM since BFEs are rounded to the
nearest whole foot.
In-Depth: The Flood Profile
Graph created by plotting the flood elevations at each station in the hydraulic model.
Selected lettered stations shown on the map are called cross sections.
Flood Profiles include:
• Channel invert elevations
• Structures (bridges, culverts)
• Cross section locations
• Flood elevations at selected recurrent intervals
Used to determine the BFE between cross sections
In-Depth: Summary of Discharges
A summary of the
drainage areapeak discharge
relationships for
all the streams
studied by
detailed methods.
In-Depth: Summary of Stillwater Elevations
The stillwater
elevations are
determined to
1/10 of a foot (or
occasionally to
the 1/100 of a
foot).
These elevations
are usually more
detailed than
what is shown on
the FIRM (whole
foot).
Finding a Location on the Map
Sometimes it can be difficult to find a location on the map
Things to consider:
Use multiple resources to find a location. Decreases chances of making a mistake
• Tax assessment data
• Aerial images
• Latitude/Longitude
• Google Maps/Google Earth
• Local GIS system
Each one has its advantages and disadvantages so use a number of them together.
Finding Elevations and Points on a Map: Determining Stationing
In order to identify the BFE at a site, the stream stationing for the site must be determined.
Step 1: Locate a site on the FIRM that shows cross sections. Identify which labeled cross
sections are nearest to your site, both upstream and downstream.
Step 2: Check the map scale used for the panel. It is located in the map legend.
Step 3: Use a ruler, engineer’s scale, or similar to measure the distance along the stream
from the site to the nearest cross section, following all the bends and curves of the stream.
If stationing is based on mileage, convert the distances by dividing by 5,280.
REMEMBER: If the location is directly on a cross section use the floodway data table. If it is
not then you will have to use the flood profile.
Let’s Find A Spot on the Map and its BFE
For this exercise we’re going to be using the same map panel that we’ve been looking at
throughout this presentation. We will also be using the corresponding Flood Insurance
Study.
See if you can find the following:
A. What is the BFE for Esopus Creek Reach 2 at Cross Section AC?
956.8 feet
B. What is the drainage area for Esopus Creek Reach 2 above the confluence with
MicKinley Hollow?
16.14 square miles
C. What is the stillwater elevation for the .2% annual chance flood on the East basin of
the Ashokan Reservoir
594.4 feet
D. What is the BFE 1000 feet upstream of Cross Section AD on Esopus Creek Reach 2?
976.5 feet
E. What is the BFE 250 feet downstream of Cross Section AF on Esopus Creek Reach 2
1,002.5 feet
Questions?
Thank You!
Brent Gotsch, CFM, MPA
Watershed Educator
Cornell Cooperative Extension of Ulster County
Ashokan Watershed Stream Management Program
Tel. 845-688-3047 ext. 3
[email protected]
www.ashokanstreams.org