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 for a property using step-backwater analysis. We have a piece of property located in an unnumbered Zone A and would like to know if in fact it is in the floodplain. We have field surveyed 3 cross-sections to use in the step-backwater analysis. 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
- determine if the property is in or out of the floodplain
The results of the survey are given below:
|Station (ft)||Elevation (ft NGVD 29)|
|147||498.0 (left bank)|
|243||498.2 (right bank)|
|Station (ft)||Elevation (ft NGVD 29)|
|233||502.2 (left bank)|
|334||501.8 (right bank)|
|Station (ft)||Elevation (ft NGVD 29)|
|600||506.3 (left bank)|
|725||506.5 (right bank)|
|Base Flood Discharge||3000 cfs|
|Downstream Stream Slope||.0025 ft/ft|
|Channel Reach Lengths||cross-section 2, left bank, channel, and right bank 450ft|
|cross-section 3, left bank, channel, and right bank 490 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. Lowest Lot Elevation of the Property: 503.5 to 510.4 ft
To determine the BFE using Quick-2 step-backwater method:
- Create a new file and save it as a Quick-2 file type (.qk2)
- Click 'Method' and select 'Step-Backwater
[Hit 'Enter' after each step below]
- Enter '1' for the cross-section number
- Enter the Station from the survey data above
- Enter the Elevation from the survey data above
- Repeat steps 4 and 5 until all the data is entered for cross-section 1
- 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.
- Enter the downstream stream slope .0025 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
- Enter the left bank station 147
- Enter the right bank station 243
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.
- Enter the base flood discharge 3000 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 501.16 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 450 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 502.60 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', 490 ft for the left bank, channel, and right bank. Click on 'Compute' to calculate the BFE. The computed BFE for cross-section 3 is 507.13 ft.
After all three 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 three cross-sections to view. You can also view the rating curve and cross-section plot for each of the sections.
To automatically create a rating curve for cross-section 1 be sure that you have the data for cross-section 1 displayed. Then click on 'View' and select 'rating curve' or click on the yellow rating curve button in the upper right-hand side of the screen. To view the rating curve for cross-section 2 or 3 simply change the cross-section number. If you want a hard copy of the rating curves you can print them by clicking on the 'Print' button. The rating curve can be a very helpful tool when working with floodplains. Using the rating curve we can view at what discharge our property would actually be in danger of being inundated by the 1% annual chance (100 year) flood. For example if you look at the rating curve for cross section 1 it would take more than twice the discharge (6000+ cfs) to equal the lowest elevation on the property. This is important information in that even if there is a + or -50% error in our calculations our structures would be safe from flooding. With a + or -50% error the discharge would be 4500 cfs which is still under the 6000 cfs it would take to inundate the structure according to the rating curve.
The final part of this exercise is to compare the lowest lot elevations of the property with the base flood elevations calculated by Quick-2. The lowest lot elevations varied from 503.5 feet at the downstream side to 510.4 feet at the upstream edge of the property. Knowing from our Quick-2 analysis that the BFE ranges from 501.16 feet at the downstream side to 507.13 at the upstream edge of the property this area is in fact above the floodplain. If you are looking to build on this piece of land you now know that the structure would 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: Monday, 25-Jun-2007 11:57:20 EDT