CHECK-RAS Bulletin
Issue No. 13
CHECK-RAS Exercise:
Second case, FW EM 01: Encroachment Method is not specified at a cross section
CHECK-RAS will print an FW EM 01 message if a method of encroachment is not specified at a cross section when running the Floodway Check option in CHECK-RAS.
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 (2FWEM01.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. As you can see from the figure shown below, Encroachment
Method was not specified from RS 1800 Culv to RS 2794.978. 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 'Profile Table' button from the HEC-RAS main window. The 'Profile Output Table' window will appear. Click on the 'Std. Tables' menu and select 'Encroachment 1'. The following window will be displayed.
The first line for each River Station (RS) is the 1-percent annual chance flood profile and the second line is the floodway profile. Notice that the 'Top Wdth Act' values for the floodway profile are wider than the values for the 1-percent annual chance flood profile. Also, notice that 'Enc Sta L' and 'Enc Sta R' values are not listed at the same River Stations. These discrepancies are the result of not having an Encroachment Method at those River Stations.
The intention of the requester in this case is to not encroach upon the lake upstream of RS 1800. However, according to Flood Insurance Study mapping standards, the floodway width should not be wider than the 1-percent annual chance floodplain width. The requester analysis can be illustrated by clicking the 'XYZ Perspective Plot' button from the main HEC-RAS window. Close the 'Profile Output Table' window and click on the 'XYZ Perspective Plot' button. The following window will be displayed. You can adjust the view by clicking on the 'Rotation Angle' and 'Azimuth Angle' arrows.
Notice that the 1-percent annual chance floodplain is covered by the floodway at three cross sections upstream of the embankment.
CHECK-RAS will provide a Summary Table and a message regarding the problem in the analysis. 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 (2FWEM01.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway and click on 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 01 messages at the ENCROACHMENT METHOD CHECK location.
If you highlight the message number FW EM 01, the following message box will appear.
The Help message provides suggestions to resolve the problem. A user-defined table needs to be created to obtain the starting and ending stations of the 1-percent annual chance floodplain.
Solution:
Close the CHECK-RAS window and open the project file (2FWEM01.prj) with the HEC-RAS program.
Select plan 01 from the 'Steady Flow Analysis' window. Since plan 01 has already been run already close,
the 'Steady Flow Analysis' window.
Click the 'Profile Table' button from the HEC-RAS main window. The 'Profile Output Table' window will appear. A user-defined table with the variables W.S.Elev, Prof Delta WS, Top Wdth Act, Left Sta Eff, Enc Sta L, Ch Sta L, Ch Sta R, Enc Sta R, Rght Sta Eff, and Q Total should be created. How to create a user-defined table is explained on page 8-18 of HEC-RAS User's Manual, Version 2.2, September 1998. Click on the newly created user-defined table from the 'User Tables' menu and the following window will be displayed.
The starting station for the 1-percent annual chance floodplain is shown on the first line for each River Station (RS) under the 'Left Sta Eff' column. The ending station is shown under the 'Rght Sta Eff' column. Note down the starting and ending stations of the 1-percent annual chance floodplain from RS 1859.895 to RS 2794.978. These stations are to be used as the encroachment stations such that the boundary of the floodway and the 1-percent annual chance floodplain will be the same. Close the 'Profile Output Table' window and select plan 02 from the 'Steady Flow Analysis' window. Click on the Option menu and select Encroachments. The Encroachments window will appear as shown in the figure below. As shown in the figure, starting and ending stations are inserted as Value 1 and Value 2 using Encroachment Method 1 from RS 1859.895 to RS 2794.978. The same encroachment method and encroachment stations from RS 1859.895 are used at RS 1800 Culv. 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 on the 'Profile Table' button from the HEC-RAS main window. The 'Profile Output Table' window will appear. Click on the previously created user-defined table from the 'User Tables' menu and the following window will be displayed.
The first line for each River Station (RS) is the 1-percent annual chance flood profile and the second line is the floodway profile. Notice that the 'Top Wdth Act' values for the floodway profile are the same as the values for the 1-percent annual chance flood profile. Also, notice that 'Enc Sta L' and 'Enc Sta R' values are listed at the same River Stations.
This will preserve the 1-percent annual chance floodplain boundary within the lake and it will satisfy the Flood Insurance Study mapping requirements. The FW EM 01 messages will not be displayed if CHECK-RAS is rerun for plan 02.
Click here to view the archive page for all Flood Hazard Mapping listservs.
Last Modified: Tuesday, 19-Jun-2007 11:57:20 EDT