CHECK-RAS Exercise

FW EM 02: Encroachment Method ___is used.

CHECK-RAS will print an FW EM 02 message if Encroachment Method 2 or 3 is used at a cross section when running the Floodway Check option in CHECK-RAS.

An example of an FW EM 02 message is provided in this month's bulletin when Encroachment Method 2 is specified at a cross section. Encroachment Method 2 uses a fixed top width and each half of the specified width is measured from the center line station of the cross section on to both overbank areas. The centerline station is computed in HEC-RAS as the average of the left channel bank station and the right channel bank station. Since this method is not based on the equal conveyance reduction principle, it should not be used in flood insurance studies. This bulletin compares the results between Encroachment Method 2 and Encroachment Method 4, which is based on the equal conveyance reduction principle. This bulletin also explains how Encroachment Method 4 computes the encroachment stations.

Please download the project file, plan files, geometric file, and flow file, which are available in the WinZip archive referenced here, and unzip the files to one directory on your local computer.

Problem

Open the project file (FWEM02.prj) with the HEC-RAS program and select Plan 01 from the 'Steady Flow Analysis' window, 'File' menu, 'Open Plan' option. Click on the Option menu from the 'Steady Flow Analysis' window and select Encroachments. The 'Encroachments' window will appear as shown in the figure below. The figure below shows that Encroachment Method 2 was specified at all the cross sections. Close the 'Encroachments' window and run HEC-RAS by clicking on the 'Compute' button. After the computation is done, close the 'Steady Flow Analysis' window.

Click the 'View Cross Sections' button from the HEC-RAS main window. The 'Cross Section' window will appear as shown below. As an example, select RS 56.97. The total floodway width assigned at this river station is equal to 3,500 feet as shown in the 'Encroachments' window. Half of the total floodway width is equal to 1,750 feet. Since the left overbank area is less than 1,750 feet, the entire left overbank area is shown in the floodway.

When CHECK-RAS is run for this plan it will provide a Summary Table and a message regarding the use of Encroachmen Method 2. Close all windows except the main HEC-RAS window. Generate a report file from the 'File' menu. After generating the report file for use with the CHECK-RAS program, please save the project.

Open the saved project (FWEM02.prj) in the CHECK-RAS program and extract the data. The 'Select Profile' window will be displayed. Select the 100-year Profile and the Floodway Profile and click on the OK button. Then click on the Floodway Check button on the program menu bar. When prompted with the message: "You need to specify allowable surcharge..." click the 'Yes' button. Click on the Report button on the program menu bar and scroll down to the ENCROACHMENT METHOD CHECK location in the displayed window. The following figure shows the FW EM 02 messages at the ENCROACHMENT METHOD CHECK location.

If you highlight the message number FW EM 02, the following message box will appear.

The Help message provides suggestions to resolve the problem. The Help message suggested using the equal conveyance reduction method, which is Encroachment Method 4 in HEC-RAS.

Solution

Close the CHECK-RAS window and open the project file (FWEM02.prj) with the HEC-RAS program. Select Plan 02 from the 'Steady Flow Analysis' window, 'File' menu, 'Open Plan' option. Click on the Option menu from the 'Steady Flow Analysis' window and select 'Encroachments'. The 'Encroachments' window will appear as shown in the figure below. The following figure shows that Encroachment Method 4 was specified at all the cross sections.

Close the 'Encroachments' window and run HEC-RAS by clicking on the 'Compute' button. After the computation is done, close the 'Steady Flow Analysis' window. Click the 'View Cross Sections' button from the HEC-RAS main window. The 'Cross Section' window will appear as shown below. As an example, select RS 56.97.

The figure above shows that by using the equal conveyance reduction method the left encroachment station is set at the left channel bank station and the left overbank area is completely outside of the floodway.

The following steps are followed by HEC-RAS when the equal conveyance reduction method is used:

  1. The target surcharge values specified for the floodway profile in the 'Encroachments' window at each cross section are added to the water-surface elevation of the same cross section from the first profile. Then the conveyance is computed at each cross section for the increased water-surface elevation of the floodway profile.
  2. Half of the difference in conveyance between the first profile and the floodway profile at a cross section is subtracted from the left overbank area of the cross section of the floodway profile to determine the left encroachment station. The other half is subtracted from the right overbank area of the cross section of the floodway profile to determine the right encroachment station.
  3. If half of the difference in conveyance cannot be obtained from one overbank, the difference will be made up, if possible, from the other overbank, except that the encroachments will not be allowed to fall within the channel portion.
  4. After the encroachment stations are determined from the above procedure, HEC-RAS will compute the true floodway water-surface elevations of the floodway profile for the encroached cross sections based on the energy equation. The difference in water-surface elevations between the first profile and the floodway profile is the computed surcharge value. The computed surcharge value may or may not be the same as the target surcharge value specified in the 'Encroachments' window.

The Plan 03 shows how step 2 is conducted within HEC-RAS. Select Plan 03 from the 'Steady Flow Analysis' window, 'File' menu, 'Open Plan' option. Close the 'Steady Flow Analysis' window. From the main HEC-RAS window click the 'Edit/Enter Steady Flow Data' button. 'Steady Flow Data' window will be displayed. From the Options menu, select 'Set Changes in WS and EG' option. The 'Set Internal Changes in WS and EG' window will be displayed as shown below.

The numbers in the last column 'Value' are obtained by adding the target surcharge values of 0.7, 0.9, 0.9, and 0.9 at RS 52.67, 53.96, 55.40, and 56.97 from the Encroachments window of plan 02 to the computed water-surface elevations of the first profile at the same cross sections from plan 02. HEC-RAS will compute the hydraulic parameters from these Known WS for the floodway profile. Click on the OK button and close the Steady Flow Data window. From the Steady Flow Analysis window, click on the Compute button. After the computation is done, close the 'Steady Flow Analysis' window. From the main HEC-RAS window, click on the 'Profile Table' button. The 'Profile Output Table' window will be displayed. Create a user-defined output table including the variables, Q Total, WS. Elev, Conv. Total, Conv. Left, Conv. Chnl, and Conv. Right, as shown below.

The difference in total conveyance between the two profiles at RS 56.97 is 212689.5 cfs. Half of that amount is equal to 106344.8 cfs. Since the Conv. Left of 104625.5 cfs for the floodway profile at RS 56.97 is less than 106344.8 cfs, the entire Conv. Left will be reduced when using the equal conveyance reduction method, Method 4. That is the reason that the left encroachment station at RS 56.97 is at the left channel bank station as discussed under Plan 02.

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Last Modified: Friday, 15-Jun-2007 11:57:20 EDT