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 examine the channel capacity of an irregular channel. We have a client that is looking to purchase a piece of property. The client has been told that the property is located within a floodplain. They would like us to do an analysis to determine if the home is also in the floodplain. Upon our site investigation we see that the house has no basement and the lowest adjacent grade is at an elevation of 531.9 feet. We also have taken a cross section at the upstream edge of the structure. The data from our field investigation is listed below. In this exercise we will determine the maximum discharge of the floodplain by calculating the depth (subtracting the lowest adjacent grade from the lowest elevation of the stream). We also used a USGS publication to obtain the regression equation to determine the base flood discharge. From that equation we determined the discharge to be 3710 cfs. Is the home located above the floodplain or does it require the purchase of flood insurance?

We will use Quick-2 to do the following:

  • Determine the maximum discharge using the irregular channel capacity method
  • View rating curve data to determine BFE
  • View rating curves and cross section plots

The results of our survey are given below:

Cross-Section 1

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 .0035 ft/ft
Depth Max. Depth=(LAG-Lowest Stream elev) 531.9-525.9=6 ft
  • Left bank--.055,
  • Channel--.040,
  • Right bank--.060
LAG 531.9 ft

To Determine the Channel Capacity Using Quick-2 Irregular Channel Capacity:

  1. Create a new file and save it as a Quick-2 file type (.qk2)
  2. Click 'Method' and select 'Channel Capacity' 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
  7. Click on 'Save Points'

    When all the data is entered we will need to enter the 'Depth' and 'Slope'.

  8. Enter the depth 6 ft in the 'Depth' box and the stream slope .0035 ft/ft in the 'EG Slope' box
  9. The bank stations are then entered by either typing in the station number or using the drop-down menu
  10. Enter the left bank station 258
  11. 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 compute box after hitting 'enter' for the last n-value.

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 at the bottom of the screen in the 'Output' section. The computed Discharge (located in the Discharge box) is 6245 cfs. This is the maximum discharge of the floodplain at the structure.

To determine the BFE within the cross section that is adjacent to the structure we need to view the rating curve data. To view this data go to 'View' and 'Rating Curve Data'. It will be easier to print a copy of this data rather than trying to expand the screen. Go to 'File' and 'Print' to print a copy of the rating curve data. Knowing that the base flood discharge is 3710 cfs we need to look at the data and see what the water surface elevation is at 3710 cfs. The rating curve data shows a discharge at 3655 cfs with an elevation of 530.6 feet. We know that the BFE will be slightly higher at approximately 530.7 feet. To be sure this structure is in fact out of the floodplain we can go back to our regression equation that was used to determine the base flood discharge. We know that the report shows an error in the equation of 36%. This means that the true base flood discharge is somewhere from 2365 cfs to 5032 cfs. Looking at the rating curve data we can determine that the BFE at 2365 cfs is approximately 529.7 feet and at 5032 cfs is approximately 531.35 feet. With the lowest adjacent grade to the structure being 531.9 we can say it is not located within the floodplain.

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. Also by having the rating curve data on hand we can see more accurately what the base flood elevation is. It would also be beneficial to print a copy of this also, simply click on the 'Print' button at the bottom of the graph.

Next Month: We'll look at an application for computation of a BFE for a lined and unlined trapezoidal channel.

View the archive page for all Flood Hazard Mapping listservs.

Last Modified: Monday, 25-Jun-2007 11:57:20 EDT