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).
As explained in last month's bulletin, this is the first of a series of problems we will work through using Quick-2. We will start this month with a relatively simple exercise to ensure that you have Quick-2 loaded and running properly. For those of you who have previously used Quick-2 for more complex problems, you should still complete this exercise as we will be using the files and building on them in the future.
Before we begin, please remember that when you open Quick-2, you must create a new file for the exercise. To do this, click on 'File' and select 'New.' Give the file a name and be sure to change the file type to Quick-2 (filename.qk2). If you do not create a new file and assign the correct file type, you will have problems saving and opening your data later on.
Quick-2 Exercise and Instructions
You are preparing a Conditional Letter of Map Revision based on Fill (CLOMR-F) for a property that was designed by another engineering firm. The proposed grading plan shows the site is going to be filled so that the Lowest Adjacent Grade (LAG) is at elevation 242.25 feet (NGVD 29).
The effective Flood Insurance Rate Map (FIRM) shows that the site lies within the Special Flood Hazard Area (SFHA) of Little Toe Creek. The SFHA is designated Zone A, with no base (1-percent-annual-chance) flood elevations (BFEs) determined.
Using a USGS quad map and a USGS report on regression equations for your area, you have determined that the base flood discharge is 3,800 cfs. Using a topographic map that you obtained from the community's department of public works (with a contour interval of 10 feet), you have also determined that Little Toe Creek is approximately a trapezoid channel with the following dimensions:
Bottom Width = 63 feet
Horizontal to Vertical (H:V) side slopes = 70:1
[Downstream stream]Slope = 0.002 [feet/feet]
Invert elevation = 238.0 [feet]
Driving home from the veterinarian one day, you crossed a bridge over Little Toe Creek (not far from the proposed development) and decided Rover had been a good dog and deserved a little romp down by the stream. While searching for Rover, after he took off after a rabbit, you determined that the channel has a Manning's 'n' value of about 0.045. You had printed the table of Manning's N value descriptions contained in the Help file of Quick-2, to assist you.
FEMA's Guidelines for Determining Base Flood Elevations in Approximate A Zones states that you can use Quick-2 to determine the normal depth in the channel and use this as the BFE.
To Determine the BFE for this development, follow these steps:
- Open Quick-2
- Create a new file with the .qk2 extension
- Click on 'Method' on the menu bar
- Select Normal Depth, then trapezoidal
- Enter '1' as the cross section number - hit Enter
- Enter the Manning's 'n' value (.045) or select it from the drop-down menu - hit Enter
- Enter the [stream] slope (.002)- hit enter
- Enter the left side slope (in terms of how many feet horizontally you measure for a one foot rise in the vertical elevation of the left bank of the channel- in this case, 70) - hit Enter
- Enter the right side slope (in terms of how many feet horizontally you measure for a one-foot rise in the vertical elevation of the right bank of the channel (in this case, 70) - hit Enter
- Enter the bottom width - hit Enter
- Enter the base flood discharge (3800 cubic feet per second) - hit Enter
The button labeled "Begin Computation" should now be highlighted with a box around the label. You may either click on the button or hit "Enter", BUT DO NOT DO BOTH. By hitting "Enter" and clicking on the button, or by hitting "Enter" more than once, the cross section number at the top will be duplicated. This is not a problem with this exercise, but it will be once we start doing backwater computations.
The BIG Question:
Has the design/engineering firm proposed a high enough LAG for this property to be removed from the SFHA?
Answer: Scroll Down
According to this analysis, the BFE is 242.16', (computed using the known channel invert elevation of 238.00' plus the 4.16' depth obtained from the Quick-2 analysis) which is lower than the LAG of 242.25. Although the design does not leave much room for error, the BFE is lower than the LAG and the property could be removed from the SFHA if built as proposed. The owners of this property would not be required by FEMA to buy flood insurance, but they should be STRONGLY encouraged. Why? We'll see next month.
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Last Modified: Monday, 25-Jun-2007 11:57:20 EDT