CHECK-RAS Exercise

FW EM 05: Target surcharge value of ___ is greater than the allowable surcharge value of ___.

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 05 message if a specified target surcharge value is greater than the allowable surcharge value when using Encroachment Method 4 or Encroachment Method 5 at a cross section (river station) when running the Floodway Check option in CHECK-RAS.

An example of an FW EM 05 message is provided in this month's bulletin with a specified target surcharge value of 3.50 feet, which is higher than the allowable surcharge value of 1.00 foot for Encroachment Method 4.

This bulletin demonstrates the results and possible consequences when specifying a target surcharge value greater than the allowable surcharge value.

Please download the project file, plan files, geometric file, and flow files, 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 (FWEM05.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 3.5 feet target surcharge under the Value 1 column at river stations 3.1 and 2.4. The target surcharge value at other river stations upstream of 3.1 are also specified with a target surcharge value of 3.5 feet. 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. The computed surcharge value is shown under the 'Profile Delta WS' column. Although the specified target surcharge value is 3.5 feet, the actual computed surcharge value is less than 1 foot. This usually happens when the area of analysis is in a storage area caused by the downstream controlling structure.

Close the 'Profile Output Table' window and click on the 'Water Surface Profiles' button in the HEC-RAS main window. The following figure will be displayed.

The above figure suggests that the structure upstream of river station 2.1 controls the water-surface elevation of the entire profile under study.

When CHECK-RAS is run for this plan, it will provide a Summary Table and a message regarding the use of a target surcharge value greater than the allowable 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 (FWEM05.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 05 message at the ENCROACHMENT METHOD CHECK location.

If you highlight the message number FW EM 05, the following message box will appear.

The Help message also provides suggestions to resolve the problem. The Help message suggested changing the target surcharge value from 3.5 to a value between 0.00 and the allowable surcharge value.

Solution

Close the CHECK-RAS window and open the project file (FWEM05.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 1 foot was specified at river stations 2.4 and 3.1, replacing the 3.5 feet target surcharge value. All the river stations with the target surcharge value of 3.5 feet are also changed to a target surcharge value of 1.00 foot.

The profile with target surcharge value 3.5 was maintained as a second floodway profile, 'Floodway3.50'. The second profile can be selected by clicking on the pull down arrow in the 'Profile' box as shown in the figure below.

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. Please create a user-defined table with the variables, 'W.S.Elev', 'Prof Delta WS', 'Top Wdth Act', and 'Volume'. How to create a user defined table is explained in the HEC-RAS User's Manual. A user defined table with the aforementioned variables is shown in the following figure. The variable 'Volume' is the cumulative volume from the starting river station to the selected river station.

The following figure shows the output information at the last two river stations. The first profile is the 1% annual chance flood (natural) profile, the second profile is the floodway profile with a target surcharge value of 1.00 foot, and the third profile is the floodway profile with a target surcharge value of 3.5 feet. The 'Profile Delta WS' column shows that the computed surcharge values for the two floodway profiles are about the same. However, the 'Top Wdth Act' column shows that the floodplain width of the natural profile was reduced considerably when using the target surcharge value of 3.5 and the 'Volume' column also indicates that the cumulative volume of the natural floodplain was also reduced considerably.

For this kind of situation a hydrologic analysis should be performed accounting for the storage of the floodplain when determining the discharges for the natural profile. The discharges for the floodway profile should also be analyzed by considering only the available storage within the floodway boundary. Because of the reduction in storage volume the discharges for the floodway profile will be larger than the natural profile. The limits (encroachment stations) of the floodway boundary are obtained from the equal conveyance reduction method, such as Encroachment Method 4. Final surcharge values for the stream will be determined by using the discharge values obtained from the hydrologic analysis of the natural and floodway profiles. This type of analysis is known as storage floodway analysis. The same type of analysis can be performed by using an unsteady flow analysis. If for any reason a storage floodway analysis is not to be performed, then a target surcharge value should not be more than the allowable surcharge value.

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 above figure shows the extent of reduction of the 1% annual chance floodplain when using a target surcharge of 1.00 foot and a target surcharge value of 3.5 feet.

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