CHECK-RAS Exercise

FW EM 04: Target surcharge value is equal to zero.

A target surcharge should be specified greater than 0.00 but no more than the allowable surcharge value.

CHECK-RAS will print an FW EM 04 message if a target surcharge value of zero is specified for Encroachment Method 4 at a cross section (river station) when running the Floodway Check option in CHECK-RAS.

An example of an FW EM 04 message is provided in this month's bulletin with a zero surcharge value specified at a river station for Encroachment Method 4.

HEC-RAS will add the target surcharge value specified for the floodway profile at a river station for Encroachment Method 4 to the water-surface elevation of the first profile (natural profile) at the same river station to compute the conveyance (the user must make sure that the first profile represents the 1% annual chance flood profile). The increased conveyance compared to the natural profile is then subtracted equally from left and right overbank areas to obtain the left and right encroachment stations.

When the target surcharge value is equal to zero, HEC-RAS will not compute the encroachment stations. It will consider the first ground station as the left encroachment station and the last ground station as the right encroachment station. When HEC-RAS computes the water-surface elevations for the floodway profile, the water-surface elevation at the river station with zero target surcharge will usually be higher than the water-surface elevation of the natural profile because the downstream floodway water- surface elevation at the downstream river station is usually higher than the water-surface elevation of the natural profile. This will create the floodway width at the river station with zero target surcharge value wider than the top width of the natural profile. This is not acceptable for the flood insurance study because, by definition, the floodway width should be narrower than the natural water-surface top width.

In some cases the computed surcharge value at the river station with zero target surcharge can be more than the allowable surcharge value because of the smaller velocity head associated with the higher water-surface elevation and the larger area for the floodway profile at this river station.

Specifying a zero target surcharge value can produce computed surcharge values greater than the allowable surcharge value of one foot, violating the NFIP regulatory requirements. Since HEC-RAS will not compute encroachment stations when zero surcharge value is specified, the floodway width computed may be wider than the 1% annual chance floodplain, also violating FEMA mapping standards. Therefore, the target surcharge value should be specified as greater than 0.00 and no more than the allowable surcharge value.

This bulletin demonstrates the results obtained by specifying a target surcharge value of zero and how to correct the problem.

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 a directory on your local computer.

Problem

Click the 'Open Project' button in the HEC-RAS main window. Select the directory where the downloaded HEC-RAS files were stored. Select the project file (FWEM04.prj). Click the 'Steady Flow Analysis' button in the HEC-RAS main window. Select Plan 01 from the 'Steady Flow Analysis' window, 'File' menu, 'Open Plan' option. Click the 'Option' menu in 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 4 was specified with zero target surcharge under the Value 1 column at river station 709.55. The target surcharge value at other river stations are above zero surcharge value and no more than the allowable surcharge value of 1.00 foot. Close the Encroachments window and run HEC-RAS by clicking the 'Compute' button. After the computation is done, close the 'Steady Flow Analysis' window.

Click the 'Profile Table' button in the HEC-RAS main window. The 'Profile Output Table' window will appear. Click the 'Std. Tables' menu and select 'Encroachment 1'. The 'Encroachment 1' table will appear as shown below. At RS 709.55 the 'Profile Delta WS' (surcharge) is more than the allowable surcharge value of 1.00 foot and the 'Top Wdth Act' of the floodway profile is wider than the 'Top Wdth Act' of the first profile (natural profile).

Close the 'Profile Output Table' window and click on the 'X-Y-Z Perspective Plot' button in the HEC-RAS main window. The following figure will be displayed./

The third cross section from the bottom of the screen is the river station 709.55 and the floodway width covers the floodplain width at this river station.

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

Open the saved project (FWEM04.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 the 'OK' button. Then click 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 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 04 message at the ENCROACHMENT METHOD CHECK location.

If you highlight the message number FW EM 04, the following message box will appear.

The Help message provides suggestions to resolve the problem. The Help message suggested changing the target surcharge value from zero to a value between 0.00 and the allowable surcharge value.

Solution

Close the CHECK-RAS window and open the project file (FWEM04.prj) with the HEC-RAS program. Select Plan 02 from the 'Steady Flow Analysis' window, 'File' menu, 'Open Plan' option. Click 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 a target surcharge value of 0.3 was specified at the river station 709.55, replacing the 0 target surcharge value.

Close the 'Encroachments' window and run HEC-RAS by clicking the 'Compute' button. After the computation is done, close the 'Steady Flow Analysis' window. Click the 'Profile Table' button in the HEC-RAS main window. The 'Profile Output Table' window will appear. Click the 'Std. Tables' menu and select 'Encroachment 1'. The 'Encroachment 1' table will appear as shown below. At RS 709.55, the 'Profile Delta WS' (surcharge) is less than the allowable surcharge value of 1.00 foot and the 'Top Wdth Act' of the floodway profile is narrower than the 'Top Wdth Act' of the first profile (natural profile).

Close the Profile Output Table window and click on the 'X-Y-Z Perspective Plot' button from the HEC-RAS main window. The following figure will be displayed.

The third cross section from the bottom of the screen is the river station 709.55 and the floodway width is narrower than the floodplain width at this river station.

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