CHECK-RAS Bulletin

Issue No. 12

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.

An example of an FW EM 01 message is provided in this month's bulletin for not specifying the encroachment method at the bridge section.

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 (FWEM01.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 1 was used at all the cross sections, except at RS 28669.0 BR. 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 profile and the second line is the floodway profile. Surcharge values are shown under the 'Prof Delta WS' column and they are less than the allowable surcharge value of 1.00 foot. Therefore, it appears that the surcharge value is correct. However, notice that the 'Top Wdth Act' values at BR U and BR D sections for 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 BR U and BR D sections. These discrepancies are the result of not having an Encroachment Method at RS 28669.0.

Note that, 'Enc Sta L' and 'Enc Sta R' are computed at RS 28698.5 and RS 28639.5. These river stations are at Section 3 and Section 2 of the Bridge, respectively. Section 3 is located at the upstream toe of the road embankment and Section 2 is located at the downstream toe of the road embankment. The 'Top Wdth Act' column shows that the floodway widths at RS 28698.5 and RS 28639.5 are equal to 830 feet. The floodway widths at RS 28669.0 BR U and RS 28669.0 BR D are equal to 1314.23 feet and 1303.75, respectively.

This indicates that although the floodway fringe areas are filled in at Section 3 and Section 2 of the bridge, some portions of the road that are blocked by the filled areas upstream and downstream of the bridge are considered effective for the floodway profile run. This 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 entire road profile is considered to be conveying flow in the floodway analysis even for the areas that are blocked by fill in the fringe areas at the upstream and downstream of the road.

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 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 (FWEM01.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 the following window will appear.

If you highlight the message number FW EM 01, the following message box will appear:

The Help message provides the reasons why the error message was printed and how to resolve the problem. Let us investigate what will happen if we use Encroachment Method 1 at the bridge section using the same encroachment stations from Section 3.

Solution
Close the CHECK-RAS window and open the project file (FWEM01.prj) with the HEC-RAS program. 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, at RS 28669.0 BR Encroachment Method 1 was inserted under Method column and the left and right encroachment stations from RS 28698.5 were inserted as Value 1 and Value 2 columns, respectively. Notice that the encroachment stations at RS 28639.5, RS 28669.0 BR, RS 28698.5 are the same enabling the same waterway opening to be reserved for the flood to go over the road. However, in order to set the encroachment stations to be the same, the centerline stations within the channels of all three cross sections should also be the same. 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.

The first line for each River Station (RS) is the 1-percent annual chance profile and the second line is the floodway profile. Surcharge values are shown under 'Prof Delta WS' column and they are now more than the allowable surcharge value of 1.00 foot, starting from RS 28669.0 BR U. Notice that the 'Top Wdth Act' values at the BR U and BR D sections for floodway profile are narrower than the values for the 1-percent annual chance flood profile. The values are also narrower than the floodway widths at RS 28639.5 and RS 28698.5. Close the 'Profile Output Table' window and click on the 'Cross Sections' button from the main HEC-RAS window. Select the 28669.0 BR U section by scrolling down the arrow on the River Sta: block. The following window will appear.

The floodway width under the 'Top Wdth Act' column is the water surface width. Some portions of the road deck are above the floodway water surface elevation within the encroachment stations. Therefore, the 'Top Wdth Act' values at RS 28669.0 BR U are narrower than the 'Top Wdth Act' values at RS 28639.5 and RS 28698.5 even though the same encroachment stations were specified at all three cross sections.

The surcharge values are more than the allowable surcharge value of 1.00 foot because of the inclusion of Encroachment Method 1 and the corresponding encroachment stations at RS 28669.0 BR. Therefore, the encroachment stations or encroachment method should be modified. Since the 'Enc Sta L' value of 4895 is the same as the Left Ineffective Flow stations at Section 3 (RS 28698.5) and Section 2 (RS 28639.5) only the 'Enc Sta R' needs to be adjusted at Section 3, Bridge Section, and Section 2 in the Encroachments window.

Close the 'Cross Section' window and select plan 03 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, Encroachment Method 1 with 'Enc Sta L' of 4895 and 'Enc Sta R' of 5900 were specified at RS 28639.5, RS 28669.0 BR, and RS 28698.5, respectively. 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 '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 profile and the second line is the floodway profile. Surcharge values are shown under 'Prof Delta WS' column and they are less than the allowable surcharge value of 1.00 foot. The 'Top Wdth Act' values at the BR U and BR D sections for floodway profile are narrower than the values for the 1-percent annual chance flood profile. The 'Top Wdth Act' values at RS 28639.5 and RS 28698.5 are equal to 1005. They are 175 feet wider than the values in plan 01 and plan 02.

Close the 'Profile Output Table' window and click on the 'XYZ Perspective Plot' button from main HEC-RAS window to see the plan view of the 1-percent annual chance floodplain and the floodway. The following window will be displayed.

The above figure shows a smoother floodway and there are no areas on the road that can be blocked by the fill in the fringe areas upstream and downstream of the bridge.

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