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 500-Year 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.

The area within the red lines (blue on real

the map) with the white

diagonal lines is the floodway.

Floodway

The area that is outside the

white diagonal lines but still in the blue lines is

the floodway fringe.

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 100-

Year 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 area-peak 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 bwg37@cornell.edu www.ashokanstreams.org