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 re-visit the property we looked at last time. Our exercise this week will be to compute a Base Flood Elevation using actual field surveyed points and we'll do some sensitivity analyses. In doing doing this, we will use Quick-2 to do the following:
- determine the BFE using the irregular channel, normal depth feature
- run multiple analyses using different n-values and discharges
- view rating curves and cross section plots
As you recall from last month, the grading design for this lot did not leave much room for error. Being a conscientious engineer, you decided to go out and survey the channel section, look at the error in the computed base flood discharge, and evaluate variations in n-values.
The results of the survey are given below:
| Station(ft) | Elevation(ft NGVD 29) |
|---|---|
| 0 | 247 |
| 200 | 245 |
| 300 | 242.5 |
| 600 | 242 (left bank) |
| 730 | 241 |
| 900 | 238 |
| 963 | 238 |
| 1010 | 239 |
| 1050 | 241 |
| 1200 | 242 (right bank |
| 1375 | 242.5 |
| 1525 | 243.5 |
| 1700 | 247 |
From last month's Listserv, we have the following data:
Base Flood Discharge: 3,800 cfs
Manning's n-value: 0.045
Downstream stream slope: 0.002 ft/ft
To determine the BFE using Quick-2 irregular channel, normal-depth method:
- Create a new file and save it as a Quick-2 file type (.qk2)
- Click 'Method' and select 'Normal Depth,' then 'Irregular'<[> [Hit 'Enter' after each step below]
- Enter '1' for the cross section number
- Enter the Station from survey data above
- Enter the Elevation from survey data above
- Repeat steps 4 and 5 until all the data is entered
- Click on 'Save Points'
You will then see the cursor blinking in the 'E.G. Slope' box
Assuming steady, uniform flow, the E.G. Slope can be approximated by the Stream slope
- Enter the stream slope 0.002 ft/ft
The bank stations are then entered by either typing in the station number or using the drop-down menu
- Enter the left bank station 600
- Enter the right bank station 1200
The first analysis will assume that both the channel and over bank areas have the same n-value
- Enter the n-values for the left bank, channel, and right bank by either typing in the number (.045)
or selecting from the drop down menus and hitting 'enter' for the last n-value.
The cursor will automatically move to the discharge box
- Enter the base flood discharge 3800 cfs and hit 'enter'
When all the data is entered, the 'Compute' button will be 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 242.58 ft. The LAG for the property is 242.25 ft. Using the more detailed geometry we see that the property can not be removed from the SFHA.
To determine how sensitive the BFE is to n-values, simply click on the drop down menus for the n-value parameters, select a value from the list of possible coefficients, and click on 'Compute."
Try using values slightly above and below .045, like .04 or .05. Also it is a good idea to change the cross section ID each time you change parameters and compute a new water surface elevation. This way, you will be able to instantly recall the results of a previous calculation (using the cross section drop down box). It should be noted that even if you do not change the cross section number, the summary and detailed outputs under the View menu, will still record all of your computations.
Now, the USGS publication that gave us the regression equation to determine the base flood discharge reports that the error in the equation is 33%. This means the true discharge is somewhere between 2,546 cfs and 5,054 cfs. To test the sensitivity of the BFE to the discharge, simply click in the discharge box, enter a new discharge and click on 'Compute." Again, it is a good idea to change the cross section ID each time you change parameters and compute a new water surface elevation.
Quick-2 can automatically create a rating curve for the channel section. 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 a cross section plot you can click on 'View,' and select 'Xsec Plot,' or click on the cross-section button in the upper right area of the screen.
While viewing the cross section plot, you can click on the 'Curve Now!' button and the rating curve will be plotted on the same graph as the cross section. Click on 'Print,' and you can show you're client why they should put at least another foot of fill on their site! (Hint: using the Rating Curve, or computing directly, see what water surface elevation is determined for a discharge of 5,000 cfs.)
View the archive page for all Flood Hazard Mapping listservs.
Last Modified: Monday, 25-Jun-2007 11:57:20 EDT