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 reexamine the site discussed last month and assume that it is located near the confluence of a main stem river and a tributary. The effective Flood Insurance Rate Map (FIRM) shows that the site lies within the Special Flood Hazard Area (SFHA), the area subject to inundation by the base (1-percent-annual-chance) flood. The SFHA is designated Zone A, with no base flood elevations (BFEs) determined. However, the main channel is an area of detailed study with BFEs determined. The property owner is looking to build a home on the site and would like to know if the natural ground is above the base flood or if fill will need to be placed to elevate the structure.
To start this exercise, let's assume we have a coincident peak situation, where both the main stream and tributary flows peak simultaneously. Therefore, we should use the known water surface elevation of the main stream as the starting point for our calculations for the tributary. Using the information in the Flood Insurance Study (FIS) profile, we determine the BFE at the confluence to be 502.5 feet. As you may recall from last month's exercise, we started with the existing slope because there was no detailed study in the area. We will use the same three cross-sections that were surveyed in last month's exercise and assume that the starting water surface elevation for Cross Section 1 is controlled by the BFE on the main stream. We will test the sensitivity of the BFE to the Manning's n-values.
The final part of the exercise will be to compare the existing ground elevation to the BFE and determine if fill is needed to elevate the structure above the floodplain. We will assume that the upstream side of the structure is at Cross Section 2 and will control the fill elevation. In doing so, we will use Quick-2 to do the following:
- Determine the BFE using the step backwater method
- View rating curves and cross section plots
- View the detailed input/output data
- Run multiple analyses using different Manning's n-values
The results of the survey are provided 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||3000cfs|
|Channel Reach Lengths||cross-section 2, left bank, channel, and right bank 450ft cross-section 3, left bank, channel, and right bank 490 ft|
To determine the BFE using Quick-2's 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 have a coincident peak we will enter the known Water Surface Elevation (WS Elev). Just click "OK" to clear the screen.
Enter the water surface elevation 502.5 ft. in the "WS Elev" box and hit "enter".
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, enter the Manning's n-values for the left bank, channel, and right bank by typing 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". A window asking for the slope of the flooding source will appear on your screen.
- Enter a slope of .0007 and hit "enter".
The results will be displayed at the bottom of the screen in the "Output" section. The computed BFE (located in the WS Elev box) is 502.50 ft.
You will next have to determine the BFE at cross-section 2. Click on "New X-Section" 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". It is very important to make sure you change the cross-section number, or you will have more than one section labeled "1".
You 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 502.56 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 507.16 ft.
After all three cross-sections are entered into Quick-2, you may view the results from any of the cross-sections. A drop-down box will appear where you entered the cross-section number; you may choose any of the three cross-sections to view. You can also view the rating curve, cross-section plot, and profile plot for each of the sections. We will also take a look at the detailed input/output data.
Viewing the cross-section plot, rating curves, and profile plot was covered in detail in last month's exercise so we will briefly go through the steps. To view a cross-section plot, choose the cross-section you would like to view, click on "View", and select "Xsec Plot" or click on the cross-section button in the upper right area of the screen. You may also view the rating curve on the same graph by clicking on the "Curve Now!" button. To print the graph, click on "Print". To automatically create a rating curve for cross-section 1, please ensure that the data for cross-section 1 is displayed. Then click on "View" and select "rating curve", or click on the yellow rating curve button on the upper right-hand side of the screen. To view the rating curve for cross-section 2 or 3, 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 profile plot can be viewed by clicking on "View" and "Profile Plot". To print a copy, click on the "Print" button.
You can also view a detailed copy of the input/output data for your file. Go to the tool bar at the top of the screen and click on "View" and "Detailed Input/Output". This is a printout of all the cross-sections and the data used. This is a simple way to view all the data from our exercise. Each individual cross-section is labeled with the station number and elevation. Be sure to print out a copy of the input/output file because we will need to reference it later in the exercise. To print a copy, go to "File" and "Print".
To determine how sensitive the BFE is to n-values you must set up a new file and run the three crosssections again. Because we are calculating backwater elevations, the program will assume it is upstream of the last one entered if a new section is provided. To start these calculations, go to "File" and "Save/Close". Next, open a new file and give it a different name than the one just completed. You will now complete steps 1 through 10 using the same data, only changing the Manning's n-values. For this run, try lower values, such as .055 or .050 for the over banks and .035 for the channel value. The results shown here may differ, depending on your choices for the n-values. To observe the sensitivity of the n-values, we can look at the Input/Output data table. Print a copy of it and compare how sensitive the BFE is to a change in n-value.
Our final task in this exercise is to determine if fill is needed to elevate the proposed building. We know from our calculations that the BFE at cross section 2 is 502.56 feet. The natural ground at the building site is at an elevation of 502.40 feet. We can see that it would be wise to add a minimum of a half a foot of fill to elevate the structure above the floodplain. However, many communities have ordinances that require structure to be elevated at least one foot above the BFE, which in this case would require at least 1.2 feet of fill; therefore, we would encourage the homeowner to consult with local community officials to ensure compliance with the community's floodplain management regulations. By practicing sound engineering and floodplain management practices, communities and homeowners may reduce potential damage and/or losses to life and property.
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Last Modified: Monday, 25-Jun-2007 11:57:20 EDT