Lesson 6 Objectives

At the end of this lesson, you will be able to:

  • Identify the information contained in the Flood Profiles in a Flood Insurance Study (FIS)
  • Identify what floodplain determination must be made by using the flood profile
Flood Insurance Study (FIS) — Supporting Data - Flood Profiles

The Flood profiles are important supporting data in the FIS.

Other Supporting Data within the FIS sections and appendices includes:

  • Vicinity Map
  • Summary of Discharges Table
  • Floodway Data Table
  • Summary of Stillwater Elevations Table
  • Flood Boundary and Floodway Map (FBFM) or Flood Insurance Rate Map (FIRM)
  • Transect Data (in coastal and Great Lake communities)

Remember, this is a generalized structure. Individual studies may vary from the Flood County generic FIS in format but will convey the study findings in a similar format.

Flood Profile

A flood profile is a graph of the flood elevations along the centerline of a stream. The horizontal scale is the length of the stream. The vertical scale is the elevation of ground, water surfaces, etc.

The flood profile in the FIS always shows the profiles for the base or 1% annual chance (100-year) flood event. The flood profile also typically shows the 10-percent (10 year), 4-percent (25 year), 2-percent (50 year) and 0.2-percent (500 year) annual chance flood event profiles. Note: The 4-percent (25 year) flood profile is more typical on newer studies.

Other information shown on the flood profile includes the cross sections shown on the flood maps, the elevation of the stream bed, the location of stream crossings such as bridges and culverts along with other hydraulic structures like dams or weirs.

In accordance with NFIP requirements, flood profiles must be used to determine an accurate Base Flood Elevation (BFE) for a location in a riverine floodplain. BFE information displayed with a wavy line on the Flood Insurance Rate Maps are approximate BFEs rounded to the nearest foot.

Flood Profile Graph

The flood profile graph shows flood events, stream bed, cross sections, and road crossings.

In this example, the Y axis (vertical) is Elevation (Feet NGVD) and the X axis is distance, Stream distance in feet above confluence with Lake Hightower.

Graphic of a flood profile from a Flood Insurance Study. Elevation (Feet NGVD) 526, 527, 528, 529, 530, 531, 532. Stream Distance in feet above confluence with Lake Hightower 0, 50, 100, 150. Cross Section A at 12.5 feet, Cross Section B at 25 feet, Cross Section C at 70 feet, Cross Section D at 105 feet, and Cross Section E at 140 feet. Stream bed, 10-year flood, 50-year flood, 100-year flood, and 500-year flood.
Flood Profile Legend
The legend identifies how each element is displayed in the Flood Profile. Each Annual Chance Flood percent has a distinct line pattern.
Legend defining the profile lines for the base flood and other floods on the adjacent graph. Legend includes lines for 0.2 %, 1.0 %, 2.0 %, and 10 % Annual Chance Floods and Steam Bed. Hexagon with letter A Cross section location symbol is also in the Legend.
Flood Profile: Major and Minor Grids Slideshow
This slideshow explains how to calculate the size of the flood profile major and minor grids. Scroll down to see slide captions.
Flood Profile: Major and Minor Grids - Slide 1
1. FEMAs Flood Profiles have major and minor grids. The major grid refers to the larger squares that comprise the graph. In this example, each major grid has ten squares along both the vertical and horizontal axis. These are the minor grid.
Flood Profiles with major grid in bold vertical and horizontal lines. Y axis starts at 430. Next three major grids are 440, 450, and 460. X axis is the Stream Distance in Feet. Starts at 3000. Next 6 grids are labeled, 4250, 5500, 6750, 8000, 9250, and 10500. Cross sections D and E are on the profile as well as the stream centerline and four Flood event lines.
Flood Profile: Major and Minor Grids - Slide 2
2. The vertical axis shows the BFE. To calculate the major grid interval, find the difference between any two consecutive major grid values, e.g., 460 ft. – 450 ft. = 10 ft. Divide the major grid interval by the number of squares in the interval to find the minor grid value. 10 ft. divided by 10 = 1 ft. On the BFE axis, each box typically represents a half (½) or one (1) foot.
Flood Profile with BFE Axis highlighted. BFE axis is labeled at 430, 440, 450, and 460.
Flood Profile: Major and Minor Grids - Slide 3
3. You find the major and minor grid intervals on the horizontal axis by first finding the difference between the major grid and then dividing that result by the number of squares in the interval. The distance unit varies. It may be feet, or it might be miles. Make sure you are using the same unit to measure the location of your property.
Flood Profile with Distance Axis highlighted. Axis is labeled at 3000, 4250, 5500, 6750, 8000, 9250, and 10500 Feet.
Flood Profile: Major and Minor Grids - Slide 4
4. In this example the major grid interval is the difference between any two consecutive major gridlines in feet. 6750 ft. – 5500 ft.= 1250 ft
Flood Profile with Distance Axis highlighted. The distance between 5500 and 6750 is 1250 Feet.
Flood Profile: Major and Minor Grids - Slide 5
5. There are 10 minor grid lines (squares) in every major grid interval. The minor grid or square interval is determined by dividing the major grid interval by ten. (1250 ft. / 10 = 125 ft.)
Flood Profile with Distance Axis highlighted. The distance between 5500 and 6750 is 1250 Feet. There are ten squares in this grid section.
Floodway Data Tables: Cross Section Elevations

Floodway data tables are part of the FIS supporting data. Data for the Cross sections shown on the FIRM, including accurate BFE's, are listed on the Floodway data tables. To find the Base Flood Water-Surface Elevation find the cross-section ID and the corresponding elevation in the Regulatory column.

Select Floodway Data Table to open in a new window an image of the Floodway Date table that includes the following BFEs for cross section AJ, AK, and AL outlined to show the Regulatory intersection on the data table.

CROSS SECTIONDISTANCEWIDTH (FEET)SECTION AREA (SQUARE FEET)MEAN VELOCITY (FEET PER SECOND)REGULATORY

AJ

97,110

51

635

6.88

193.1

AK

97,380

144

1,411

3.10

195.8

AL

98,230

177

1,490

2.93

196.4

Floodway Data Tables: Cross Section Elevations Matched to FIRM
Use the FIS Floodway data table to obtain the accurate BFE at the cross sections shown on the FIRM. This example shows the floodway data table regulatory BFEs for cross sections AJ, AK, and AL. Note the location of the red house. In lieu of the normal process of using the Flood Profile to determine an accurate BFE, the Regulatory BFE 196.4 can be obtained from the Floodway Data Table because cross section "AL" is adjacent to the upstream corner of the structure.
FIRM with cross section AJ 193.1 ft, cross section AK 195.8 ft, and cross section Al 196.4.ft
Flood Profile and FIRM Common Elements: On Flood Profile

There are three common elements between the FIS Flood Profile and the Flood Insurance Rate Map. They are:

  1. centerline of stream (stream bed)
  2. road crossings
  3. cross section locations

Flood Profiles with arrows pointing to 1 – centerline of streambed, 2 – road crossings, and 3 - cross section locations
Here are the common elements shown on the Flood Profile. The next slide shows these same elements on the FIRM.
Flood Profile and FIRM Common Elements: On FIRM

There are three common elements between the FIS Flood Profile and the Flood Insurance Rate Map. They are:

  1. centerline of stream (stream bed)
  2. road crossings
  3. cross section locations

FIRM section with arrows pointing to 1 - centerline of streambed, 2 - road crossings, and 3 - cross section locations
Here are the same common elements that were shown of the Flood Profile as they appear on the FIRM.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM

This slideshow presents the 7 steps for identifying an accurate BFE for a specific property using the flood profile. This is the most common and important use of the FIS, especially for Surveyors and Floodplain Administrators. Note: The rounded BFE's shown on a FIRM panel were used by insurance agents to rate flood insurance policies.

Scroll down to see captions.

7 Steps: How to Determine Riverine BFE Using FIS and FIRM- Slide 1
Step 1 of 7: Find the location of the property and structure on the FIRM. In this example, first find the general location (corner of Heyden Dr and Butler Drive). Then find the exact location of the upstream portion of the structure (25 feet from center of Butler Road, 35 feet from center of Heyden Dr).
FIRM with section showing property as red square in gray shaded area and at corner of two streets. Flow and upstream corner of structure are identified.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 2
Step 2 of 7: Determine flow direction and draw BFE line for the property site. Note: This step is required no matter which method you use to determine the BFE.
FIRM with section showing property as red square in gray shaded area and at corner of two streets. BFE Line from upstream corner of structure on the property and FLOW direction are identified on the map.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 3
Step 3 of 7: Identify the FIRM/FIS common element nearest to the site BFE line. Which of these three common elements; Bald Hill Road Crossing, Cross Section D, or Private Road Crossing is the closest?
FIRM with section showing property as red square in gray shaded area and at corner of two streets. BFE Line from property and FLOW direction drawn on map. Bald hill Road Crossing, Cross section D and Private Road Crossing identified.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 4
Step 3 of 7 Continued: Cross Section D is the closest common element. Measuring to the closest common element reduces the chances of making a measuring error.
FIRM with section showing property as red square in gray shaded area and at corner of two streets. BFE Line from property and FLOW direction drawn on map. Cross section D identified as closest to property.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 5
Step 4: Measure the distance from closest common element to the property BFE line. This measurement is made along the centerline of the stream, not using a straight line directly between cross sections. Therefore you must measure along any meanders the stream contains. This can be challenging.
On the FIRM, a curved line following center line of stream has been drawn from Cross section D to the property’s BFE line.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 6
Step 4 Continued: Choose a straight increment measurement that roughly represents the curved portions of the stream line. This example uses a quarter inch which is equal to 125 ft.
The map scale is 1-inch equals 500 feet, ¼ inch equals 125 feet, and 1/5th inch equals 100 feet
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 7
Step 4 Continued: Start measuring from the Cross Section D (the nearest common element). Continue this process along the stream line until no more quarter inches can be measured without extending past the Site BFE line. (5 X 125 ft. = 625 ft.)
Two segments, each equal to 125 feet drawn along the centerline of the stream
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 8
Step 4 Continued: Now measure the remaining length of stream from you last increment to the site BFE line to determine the remaining distance. In this example the final measurement is equal to two segments on the 0 - 250 scale that is subdivided into five 50 ft. segments. (2 X 50 ft. = 100 ft.)
Close-up of BFE line with 5 X 125 feet = 650 feet line drawn from cross section D, almost to the BFE. 250 feet to 0 feet section of Scale equals 1/2 inch. 1/10 inch equals 50 feet.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 9
Step 5 of 7: Transfer the same distance that was measured in step 4 on the FIRM onto the FIS Flood Profile. In this example, the total FIRM measurement was 725 ft. downstream from Cross Section D. ( (5 X 125 ft. = 625 ft.) + (2 X 50 ft. = 100 ft.) = 725 ft.) 725 A distance of 725 feet needs to be transferred to the FIS Flood Profile.
Total of five segments that are equal to 125 feet each and a remaining segment of 100 feet drawn along the centerline of the stream from cross section D to the Property BFE line.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM- Slide 10
Step 5 Continued: First determine the profile’s “square” measurement. Begin by finding the distance that the major grid line interval represents. In this example, the major grid distance is 1,250 ft. (6,750 ft. – 5,500 ft. = 1,250 ft.).
Flood Profile - grid distance is 1,250 feet
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 11
Step 5 Continued: Next, count how many squares there are in a major grid interval. In this example there are 10. Divide the major grid distance by the number of squares. (1,250 ft. / 10 = 125 ft. per square)
Flood Profile - grid equals 10 squares of 125 feet
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 12
Step 5 Continued: Now you need to calculate how many boxes equal 725 ft. Divide 725 ft., the measured distance, by 125 ft., the distance represented by one square. (725 ft. / 125 ft. per square = 5.8 squares.) On the Flood Profile, measure downstream from Cross Section “D” the 5.8 boxes or square measurements.
Flood Profile has 5.8 squares marked downstream from cross section D
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 13
Step 6 of 7: Identify the 1% Flood Profile for the property. After measuring the distance downstream from Cross Section D on the Flood Profile, extend a vertical line up until it crosses the 1% Flood Profile line. Be sure to stop at the correct water surface profile line since up to 4 profile lines may be shown.
Flood Profile – the last line measuring 5.8 squares downstream from cross section D has been extended vertically to the 1 percent flood profile line
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 14
Step 7 of 7: Identify the BFE for the property. Extend a straight line from the point where the vertical line meets the 1% flood profile line to the horizontal axis. In this example, each square on the elevation axis equals one foot. The line intersects a little above the middle of a box between 454 and 455. BFE = 454.6. This is the correct regulatory BFE for this site using the FIS, as required by the NFIP. (Note that this is more than two foot lower than the rounded BFE shown on the FIRM directly below Cross Section D)
Flood profile – a horizontal line has been extended to the BFE axis meeting it at the 454.6 point.
7 Steps: How to Determine Riverine BFE Using FIS and FIRM - Slide 15
(Step 7 Continued) BFE = 454.6 is the correct regulatory BFE for this site using the FIS, as required by the NFIP. Note that this BFE is more than two foot lower than the rounded BFE for Cross Section D of 457 ft. (457 ft. - 454.6 ft. = 2.4 ft.)
FIRM with section showing property as red square in gray shaded area and at corner of two streets. BFE Line from property and FLOW direction drawn on map. Cross section D identified as closest to property.
How to Determine Riverine BFE Using FIS and FIRM Considerations

There can be some variance in the BFE calculations using the FIS and FIRM methods.

In our example, a correct BFE could have easily ranged from 454.4 to 454.8 feet with pencil sharpness and other factors accounting for the range. Because of this, it is recommended that you use the top of the profile line to be conservative and add a little safety margin.

Remember:

  • Flood Profiles are not necessarily “smooth” or a straight line between cross sections. These drops and rises between Cross Sections are represented in the Flood Profile.
  • The NFIP requires using the FIS document to determine the BFE for permitting development.
  • The Floodway Data Table can be used to help eliminate large errors. Look at the regulatory BFE's of the cross section immediately above and below the site and make sure the accurate BFE you determined from the flood profile falls between those two numbers.
Lesson 6 Summary

You have completed Lesson 6.

It covered Flood Profiles.

In this lesson, you learned to:

  • Identify the information contained in the Flood Profiles in a Flood Insurance Study (FIS)
  • Identify what floodplain determination must be made by using the flood profile

You also learned the seven steps to finding the BFE for a specific property using the Flood Profile and FIRM

Step 1 of 7: Find the location of the property and structure on the FIRM.

Step 2 of 7: Draw a site specific “BFE line” perpendicular to the flow of the stream.

Step 3 of 7: Identify the FIRM/FIS common element nearest to the intersection of the BFE line and Centerline.

Step 4 of 7: Measure the distance from closest common element to the property BFE line.

Step 5 of 7: Add up the measurements from the FIRM to use with the FIS Flood Profile.

Step 6 of 7: Identify the 1% Flood Profile for the property.

Step 7 of 7: Identify the BFE for the property.