Lesson 4 Objectives

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

  • Identify floodplain boundary information included in a Flood Insurance Study (FIS)
  • Identify floodway information included in a FIS
FIS 4.0 Floodplain Management Applications

This section includes two sub-sections:

4.1 – Floodplain Boundaries

4.2 − Floodways

FIS 4.1 – Floodplain Boundaries

Floodplain boundaries show the areas that would be inundated by a flood of a given frequency. The Flood map shows the floodplain boundaries for the flood having a 1-percent annual chance (100-year flood) and, in some areas, the flood having a 0.2-percent annual chance (500-year flood).

This section indicates the scales, contour intervals or topographic accuracy, and dates of the topographic model or maps used to delineate the floodplain boundaries.

The floodplains are delineated using flood elevations at cross sections or transects and by interpolating between cross sections or transects using topographic maps.

For newer flood maps, digital elevation models are typically derived from LiDAR and are used to develop digital cross sections to improve flood elevation accuracies.

Regulatory Floodway

A "Regulatory Floodway" means 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 these floodways to ensure that there are no increases in flood elevations.

For streams and other watercourses where FEMA has provided Base Flood Elevations (BFEs), but no floodway has been designated, the community must review floodplain development on a case-by-case basis to ensure that increases in water surface elevations beyond a certain amount do not occur, or identify the need to adopt a floodway if adequate information is available. 

FIS 4.2 – Floodways

This section defines the floodway and explains how it is used for floodplain management.

Also, this section lists which streams have floodways and describes how floodways were determined.

Floodway cross section. See Appendix A for text description of the diagram
Floodway Data Table
The Floodway Data Table presents the results of the floodway analyses at the cross sections shown on the flood maps.

FLOODING SOURCE

FLOODWAY

BASE FLOOD WATER SURFACE ELEVATION (FEET NGVD)

CROSS SECTIONDISTANCE WIDTH (FEET)SECTION AREA (SQUARE FEET)MEAN VELOCITY (FEET PER SECOND)REGULATORYWITHOUT FLOODWAYWITH FLOODWAYINCREASE

Rocky River

A

B

C

D

E

F

G

H

I

J

K

 

4.395

5.537

9.610

10.995

12.695

13.845

14.513

16.625

18.209

20.849

25.360 

 

115

13

100

85

245

270

230

180

415

230

340

 

1,233

142

323

861

1,887

2.403

2,553

2,000

2,566

2,381

2,924

 

6.1

9.2

8.4

7.2

5.1

4.5

3.7

4.2

3.9

4.0

3.6

 

9.9

10.4

10.9

11.2

1.3

11.5

11.6

11.7

12.5

13.0

14.0

 

9.9

10.4

10.9

11.2

1.3

11.5

11.6

11.7

12.5

13.0

14.0

 

10.0

10.5

11.1

11.3

11.4

11.5

11.6

11.7

12.7

13.2

14.2

 

0.1

0.1

0.2

0.1

0.1

0.0

0.0

0.0

0.2

0.2

0.2

Select this link to access a text version of the Floodway Data Table.

Floodway Data Table Slideshow
This slideshow looks at each of the Floodway Data Table columns and provides a graphical representation of the data. Scroll down to see slide captions.
Floodway Data Table Slide 1
Column 1: Flooding Source: Cross Section
Flood Data Table Slide 1
Shows how the Flood Data Table Cross Section column relates to cross sections that are identified graphically on flood maps, including stream centerline, 100 Year Flood elevation, Cross-Section , a cross section letter identification, i.e. A. Shows Plan View Section "A" and Cross-Section View "A"
Floodway Data Table Slide 2
Column 2: Flooding Source: Distance
Flood Data Table Slide 2
Shows what the Flood Data Table Distance column references on a flood map, including stream centerline, Zone designations i.e. Zone AE, Zone A, and Cross section letter identifications (A, B, C, D, E)
Floodway Data Table Slide 3
Column 3: Floodway: Width (Feet)
Flood Data Table Slide 3
Shows what the Flood Data Table Floodway Width column references on a flood map. Inset of Floodway width a cross section B between 106 and 107.
Floodway Data Table Slide 4
Column 4: Floodway: Section Area (Square Feet)
Flood Data Table Slide 4
Shows what the Flood Data Table Floodway Section Area column references on a floodway cross section diagram that shows a shaded Floodway Section Area between encroachments. The surface level is the Flood elevation when confined within the floodway. The diagram also shows the area of floodplain that could be used for development by raising the ground and the Flood Elevation before encroachment on the floodplain.
Floodway Data Table Slide 5
Column 5: Floodway: Mean Velocity (Square Feet)
Flood Data Table Slide 5
Shows what the Flood Data Table Floodway Section Area column references in a diagram. Mean velocity is the 10-year flow divided by the cross-sectional area of the floodway. The velocity is measured in feet per second. In the equation q divided by a equals v; q equals discharges, a equals area, and v equals Mean Velocity. For example; 2,000 cfs divided by 500 feet flow area equals 4 feet per second.
Floodway Data Table Slide 6
Column 6: Base Flood Water Surface Elevation (Feet NGVD): Regulatory
Flood Data Table Slide 6
Shows how the Flood Data table Regulatory Base Flood Elevation Regulatory column includes backwater effects from other streams.
Floodway Data Table Slide 7
Column 7: Base Flood Water Surface Elevation (Feet NGVD): Without Floodway
Flood Data Table Slide 7
Shows how the Flood Data table Regulatory Base Flood Elevation Without Floodway column does not account for backwater effects from other streams.
Floodway Data Table Slide 8
Column 8: Base Flood Water Surface Elevation (Feet NGVD): With Floodway
Flood Data Table Slide 8
Shows how the Flood Data table Regulatory Base Flood Elevation With Floodway column references the shaded With Floodway Flood Elevation between the encroachments. The diagram shows the area of floodplain that could be used for development by raising the ground and the Flood Elevation before encroachment on the floodplain. It also point to the Flood elevation before encraochment on floodplain level.
Floodway Data Table Slide 9
Column 9: Base Flood Water Surface Elevation (Feet NGVD): Increase
Flood Data Table Slide 9
Shows how the Flood Data table Regulatory Base Flood Elevation Increase column references the distance between the Floodway Elevation when Confined within the floodway and the Flood Elevation before encroachment on the floodplain. Also shown in the diagram are area of floodplain that could be used for development by raising the ground and the encroachments. The increase in surcharge is identified and a note: Surcharge is not to exceed 1.0 foot (FEMA requirement) or lesser amount if specified by State or Community
Lesson 4 Summary

You have completed Lesson 4.

It covered Section 4.0: Floodplain Management Applications and its two sub-sections. .

4.1 – Floodplain Boundaries

4.2 − Floodways

In this lesson, you learned to:

  • Identify floodplain boundary information included in a Flood Insurance Study (FIS)
  • Identify floodway information included in a FIS