The purpose of the Quick-2 Mailing List is to provide up-to-date information about Quick-2 software, tutorials, and other related topics pertinent to the Federal Emergency Management Agency (FEMA) and the National Flood Insurance Program (NFIP).

This month we will compute Base Flood Elevations (BFEs) using step-backwater analysis. We have a large piece of land along an unnumbered Zone A where a new subdivision is being planned. The BFEs are needed for floodplain management purposes to insure all new construction is above the 1-percent-annual chance (100-year) flood. In doing this, we will use Quick-2 to do the following:

  • Create a Backwater Profile and Determine BFE's Using the Step-Backwater Feature
  • View Rating Curves and Cross Section Plots
  • Show the Profile Plot

The results of the survey are given below:

Cross-Section 1

Station (ft) Elevation (ft NGVD 29)
0 776
178 773.5
320 767
512 762.2 (left bank)
642 758.5
842 758.1
1129 761.5 (right bank)
1316 767.6
1620 774.5

Cross-Section 2

Station (ft) Elevation (ft NGVD 29)
0 781
175 775.2
317 767.9
515 763.4 (left bank)
630 760.7
710 760.6
1133 762.8 (right bank)
1287 768.5
1490 778.8

Cross-Section 3

Station (ft) Elevation (ft NGVD 29)
0 786
156 781.3
355 769.5
575 764.1 (left bank)
705 762.7
900 762.8
1187 764.5 (right bank)
1346 771.4
1673 783.5

Cross-Section 4

Station (ft) Elevation (ft NGVD 29)
0 779.3
128 773.6
324 769.2
493 766.4 (left bank)
625 764.9
819 764.7
1095 766.1 (right bank)
1201 770.1
1347 772.9

Cross-Section 5

Station (ft) Elevation (ft NGVD 29)
0 784
149 777.6
353 772.3
502 770.5 (left bank)
617 768.4
805 768.5
1050 770.9 (right bank)
1301 773.1
1567 776.9

Base Flood Discharge 2346 cfs
Manning's n-values
  • left bank .045
  • channel: .040
  • right bank .045
Downstream Stream Slope .0038 ft/ft
Channel Reach Lengths
  • cross-section 2, left bank, channel, and right bank 650ft
  • cross-section 3, left bank, channel, and right bank 590 ft
  • cross-section 4, left bank, channel, and right bank 565 ft
  • cross-section 5, left bank, channel, and right bank 850 ft

*Note: The reach lengths between two cross sections for the left bank, channel, and right bank are required for step backwater computations. The distance for the left bank should be measured between the center of the floodplains of the left bank at each cross section. The same is true for the right bank. The channel distance should be measured along the streambed, and therefore will account for the meandering of the stream channel. For more information on reach lengths you can visit our on-line tutorial at www.fema.gov/plan/prevent/fhm/ot_main.shtm.

To Determine the BFE Using Quick-2 Step-Backwater Method:

  1. Create a new file and save it as a Quick-2 file type (.qk2)
  2. Click 'Method' and select 'Step-Backwater'
    [Hit 'Enter' after each step below]
  3. Enter '1' for the cross-section number
  4. Enter the Station from the survey data above
  5. Enter the Elevation from the survey data above
  6. Repeat steps 4 and 5 until all the data is entered for cross-section 1
  7. Click on 'Save Points'

    A screen will now come up after you 'Save Points' stating the options available for a Starting Condition for Step-Backwater Computation. It will give a choice to enter the known Water Surface Elevation (WS Elev) or Depth; or by the Normal Depth Method; or by entering an EG Slope. Because we are in an unnumbered A Zone with no detailed study we will enter the EG Slope. Just click 'OK' to clear the screen.

  8. Enter the downstream stream slope .0038 ft/ft in the 'EG Slope' box

    The bank stations are then entered by either typing in the station number or using the drop-down menu

  9. Enter the left bank station 512
  10. Enter the right bank station 1129

    Next you will then enter the manning's n-values for the left bank, channel, and right bank by either typing in the number or selecting from the drop down menus and hitting 'enter.' The cursor will automatically move to the discharge box after hitting 'enter' for the last n-value.

  11. Enter the base flood discharge 2346 cfs and hit 'enter'

When all the data is entered, the 'Compute' button will have a box around the word 'compute.' To compute the BFE, either click on 'Compute' or hit enter.

The results are displayed at the bottom of the screen in the 'Output' section. The computed BFE (located in the WS Elev box) is 760.49 ft. This is the downstream BFE for the property.

You will next have to determine the BFE at cross-section 2. Simply click on 'New X-Section' which is located under the discharge box and beside the 'Compute' button. You will then have to enter in '2' for the cross-section number and hit enter. Note that it is very important to make sure you change the cross-section number or you will have more than one section labeled '1.'

Your will now go through steps 4 through 11 entering the correct values for the left and right bank stations. The slope will be automatically calculated for cross-section 2 using the data from the previous section. The manning's n-value, and discharge will all be the same. Next be sure to enter the 'Channel Reach Lengths', these are located directly under the Manning's n-values. For cross-section 2 the reach length is 650 ft for the left bank, channel, and right bank. If this step is not completed you will not be able to compute the BFE. Now you are ready to compute the BFE, click on the 'Compute' button to show the BFE of 762.97 ft.

To enter the data for cross-section 3 simply click on 'New X-Section' and enter '3' for the number and hit enter. Follow steps 4 though 11 again only changing the left and right bank stations. The slope will also be automatically calculated for cross-section 3. Again be sure to enter the 'Channel Reach Length', 590 ft for the left bank, channel, and right bank. Click on 'Compute' to calculate the BFE. The computed BFE for cross-section 3 is 764.83 ft.

To enter the data for cross-section 4 simply click on 'New X-Section' and enter '4' for the number and hit enter. Follow steps 4 though 11 again only changing the left and right bank stations. The slope will also be automatically calculated for cross-section 4. Again be sure to enter the 'Channel Reach Length', 565 ft for the left bank, channel, and right bank. Click on 'Compute' to calculate the BFE. The computed BFE for cross-section 4 is 766.61 ft.

To enter the data for cross-section 5 simply click on 'New X-Section' and enter '5' for the number and hit enter. Follow steps 4 though 11 again only changing the left and right bank stations. The slope will also be automatically calculated for cross-section 5. Again be sure to enter the 'Channel Reach Length', 850 ft for the left bank, channel, and right bank. Click on 'Compute' to calculate the BFE. The computed BFE for cross-section 5 is 770.48 ft.

After all five cross-section are entered into Quick-2 it is very easy to view the results from any of the cross-sections. Where you entered the cross-section number there is a drop down box and you can choose any of the five cross-sections to view. You can also view the profile plot, rating curve and cross-section plot for each of the sections.

To view a cross-section plot choose what cross-section you want to view and click on 'View' and select "Xsec Plot' or click on the cross-section button in the upper right area of the screen. You can also view the rating curve on the same graph by clicking on the 'Curve Now!' button. To print the graph that you are viewing simply click on 'Print' and you can have a hard copy of the cross-sections to your property. You can also display a profile plot to give you a visual of your work. Simply click on 'View' then 'Profile Plot' or you can click on the profile button in the upper right area of the screen. You can print the profile to have a copy for you file.

By having the profile plot it will be easy for the developer to determine at what elevation to build to meet the floodplain management requirements of the community. The profile plot is a good visual tool to use when determining what elevation to build each home. This will help to ensure that the newly constructed homes meet a Federal and State regulations and will be safe from a 100 year flood event.

When the exercise is complete you will want to click on 'File' and 'Close/Save' to be sure all your information is saved. Close out of Quick-2. Now we will run a quick check to be sure all your information is in your file. Reopen the Quick-2 program by clicking on 'File' and 'Open'. You will now find your file by the name you gave it at the beginning of the exercise. Click on the file name and then on 'Open', next you will see a small screen which will include your file name and path, click on 'OK'. To view your data click on 'Method' and select 'Step-Backwater'. To view your cross sections go to the Cross Section Drop Down Box and choose the section you would like to view. Now you can retrieve the cross section information and plots as before. This will complete our exercise.

View the archive page for all Flood Hazard Mapping listservs.

Last Modified: Tuesday, 26-Jun-2007 11:57:20 EDT