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This month we will determine the channel capacity of a rectangular channel and test the sensitivity of the manning's n-values. We will look at two different lined channels to see how the sensitivity of the manning's n-value affects our discharge. The first channel we will look at is a concrete lined channel that is finished with a gravel bottom. The second channel will be a gravel bottom with sides of riprap.

All other variables will remain the same. In this exercise we will calculate the discharge of both channels and compare their rating curves to see how sensitive the channel is to a change in manning's n-value.

  • Determine the Discharge Using the Rectangular Channel
  • Run Multiple Analysis with Varying Manning's N-Values
  • view Rating Curves
Slope .0043 ft/ft
N-value .017 (concrete finished w/gravel bottom)
N-value .033 (gravel bottom with sides of riprap)
Bottom Width 12 feet
Depth 7 feet

To Determine the Channel Capacity Using Quick-2 Rectangular Channel Method:

  1. Create a new file and save it as a Quick-2 file type (.qk2)
  2. Click 'Method' and select 'Channel Capacity' and then 'Rectangular'

    [Hit 'Enter' after each step below]

  3. Enter '1' for the cross-section number
  4. Enter the manning's n-value
  5. Enter the Slope
  6. Enter the Bottom Width
  7. Enter the Depth

    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'. The results are displayed on the right side of the screen in the 'Output' section. The computed Discharge (located in the Discharge box) is 1052.7 cfs.

After we determine the discharge for cross section 1 we will enter in cross section 2. Simply go to the 'Cross Section' box and change the number to 2 and hit enter. Because all the data will remain the same except for the manning's n-value, simply change the n-value to .033 and click on the 'Compute' button to display the discharge for cross section 2. The discharge for cross section 2 is 542.3 cfs.

Now we will run one more cross section with an n-value of .025 to see what the change in the discharge will be. Simply go to the 'Cross Section' box and change the number to 3 and hit enter. Again all input data will remain the same except the n-value. Change the n-value to .025 and hit enter. Next click on the 'Compute' button to display the discharge of cross section 3. The discharge for cross section 3 is 715.8 cfs.

Next we will take a look at the Rating Curve for each cross section. Go to 'View' and 'Rating Curve Data' to view the rating curve. With the rating curve we can compare the discharge at various water surface elevations to see how the n-value affects our discharge. For two of the cross sections that we have run in this exercise we can see that discharges in cross section 1 are approximately twice as much as in cross section 2. We also see the relationship between cross section 3 and how the sensitivity affects the output. To print this data go to 'File' and 'Print'. To view the rating curve as a graph choose the cross section you would like to view and go to 'View' and 'Rating Curve'.

Next month we will determine the BFE of parcel of land to see if it can be removed from the Special Flood Hazard Area (SFHA) using the normal depth method in Quick-2.

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