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 determine normal depth in an irregular channel that has a supercritical flow. Supercritical flow occurs when the normal depth is less than the critical depth. FEMA does not accept supercritical depth elevations for use in determining Base Flood Elevations (BFE), unless the channel has been specifically designed to operate under supercritical flow conditions. In this exercise we will calculate the normal depth of an irregular channel.

This exercise will illustrate how to determine the BFE when there is a supercritical flow.

  • Determine the BFE Using the Irregular Channel, Normal Depth Feature
  • View the Critical Depth as the BFE
  • View Rating Curves and Cross Section Plots
Station (ft) Elevation (ft NGVD 29)
0 535.0
175 529.8
258 527.2 (left bank)
261 525.9
356 525.9
359 526.8 (right bank)
427 530.5
616 535.1

Slope .016 ft/ft
N-values Left bank--.055, Channel--.040, Right bank--.060
Discharge 6000 cfs

To Determine the Normal Depth Using Quick-2 Irregular Channel Method:

  1. Create a new file and save it as a Quick-2 file type (.qk2)
  2. Click 'Method' and select 'Normal Depth' and then 'Irregular'
    [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
    Click on 'Save Points'
    When all the data is entered we will need to enter the 'Slope'.
  7. Enter the stream slope .016 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
  8. Enter the left bank station 258
  9. Enter the right bank station 359
    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.
  10. Enter the 'Discharge' 6000 cfs and hit 'enter'
    When all the data is entered, the 'Compute' button will have a box around the word 'compute.' To compute the Discharge, either click on 'Compute' or hit enter.

You will now get a 'Supercritical Warning: FEMA does not accept Supercritical Elevations to establish Base Flood Elevations. Use the Critical depth elevation instead.' Simply click on 'OK'.

The results are displayed at the bottom of the screen in the 'Output' section. The computed Critical Depth (located in the 'Crit Depth' box) is 530.44 feet. This is the BFE at cross section 1.

We can see in the 'Output' section that the normal water surface elevation (located in the 'WS Elev' 530.04 feet) is less than the critical depth of 530.44 feet. As we view the Rating Curve and Cross Section Plots we need to remember that the normal water surface elevation is shown on the graph. The actual BFE is the critical depth that is slightly higher.

Next we will take a look at the Rating Curve and Cross Section Plot. Go to 'View' and 'X Sect Plot' to view the cross section. At the bottom of the graph click on 'Curve Now !' and the rating curve will be plotted on the same graph. With the rating curve and cross section plot on the same graph we get a good representation of what is going on at the site. It would also be beneficial to print a copy of this also, simply click on the 'Print' button at the bottom of the graph. We can also see the rating curve data in a word file. Go to 'View' and 'Rating Curve Data'. To print this data go to 'File' and 'Print'.

Next month we look at the channel capacity of a rectangular channel.

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