Issue No. 10
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.
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:
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.
Click here to view the archive page for all Flood Hazard Mapping listservs.
Last Modified: Tuesday, 26-Jun-2007 11:57:20 EDT