CHECK-RAS Exercise

Encroachment stations are not computed at a structure with weir flow, second case

CHECK-RAS will print an ST FW 09 message in the Structure Check if the left or right weir station of the floodway profile is outside the corresponding stations of the 1% annual chance flood profile (natural profile). This may occur if the user did not specify encroachment stations at the structure, or if either or both of the encroachment stations at Section 2 of the structure are within the channel bank stations of Section 3, or if encroachment method 4 is used at Section 2 while encroachment method 1 is specified at Section 3.

An example of an ST FW 09 message was provided in the previous month's bulletin covering the last two situations described in the previous paragraph. This month's bulletin provides an example for the first situation.

Please download the project file, plan files, geometric file, and flow file, which can be downloaded in WinZip format by clicking here, and unzip the files to a directory on your local computer.

Problem

Open the project file (2STFW09.prj) with the HEC-RAS program and select "plan 01" from the Steady Flow Analysis window. Click the Option menu and select "Encroachment." You will notice that Encroachment Method 1 with encroachment stations is specified under the Value 1 and Value 2 columns, except for RS 26335 Culv. Although Encroachment Method is specified as 1, encroachment stations are not specified at RS 26335. Close this 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 Std. Tables menu and select "Encroachment 1" table. You will notice that HEC-RAS did not provide any information at RS 26335 (Culvert). However, this does not mean that encroachment is computed properly at the structure. In order to find out the encroachment information at the culvert, first close the Encroachment Table 1 window and Profile Output Table window.

The information about the structure can be obtained by clicking the "View Detailed Output" button from the main HEC-RAS window. Click on the Type menu and select "Culvert" from the displayed window. The Culvert Output window will be displayed. In this window, the "Profile" box will display "100base", and "Riv Sta" box will display "26335." WeirStaLft value is equal to 4698.81, and WeirStaRgt value is equal to 5105.27. These are the values for the 1% annual chance flood profile.

To obtain the information about the floodway profile, select "100encr" by clicking the pull down arrow at the "Profile" box in the same Culvert Output window. WeirStaLft value is shown as 4697.03, and WeirStaRgt value is shown as 5108.71. The weir station values from the two profiles indicate that the floodway boundary is outside the 1% annual chance flood boundary over the road. This indicates that no encroachment was computed by HEC-RAS at RS 26335.

CHECK-RAS will provide messages relating to the above conditions so that you do not need to search the information about the encroachment at the structures from different tables. Please close all open windows except the main HEC-RAS window. Generate the 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 (2STFW09.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway and select the "Structure Check" button on the Program menu bar. When prompted with the message, "You need to specify allowable surcharge...", click the "Yes" button.

Review the resulting report and you will see the message ST FW 09 for left and right weir stations at RS 26335. If no encroachment method is specified at RS 26335, CHECK-RAS will print the message ST FW 01 informing the user that encroachment method was not specified at this river station. Because Encroachment Method 1 was specified at RS 26335, CHECK-RAS did not print the message ST FW 01 in this case.

The user should not ignore the message ST FW 09 just because the differences between the weir stations of the two profiles are small. For example, in this case the left weir station of the natural profile is 4699.08, and the left weir station of the floodway profile is 4697.15. The intent of message ST FW 09 is to inform the user that encroachment computations were not made at the structure section. In this case, the floodway width over the structure is the difference between the right weir station of 5108.47 and the left weir station of 4697.15 of the floodway profile and is equal to 411.32 feet. The user can find from "Encroachment 1" table that the floodway width at RS 26385 (Section 3 of the structure) is 150.0 feet and the floodway width at RS 26285 (Section 2 of the structure) is 155.0 feet. This proves that the floodway width over the structure is too wide because encroachment computations are not performed by HEC-RAS at RS 26335.

Solution

The HEC-RAS Hydraulic Reference Manual explains the encroachment computation process at bridges, culverts, and multiple openings on page 9-7. The following paragraphs are excerpted from that manual.

"In general, the default methodology for encroachments at bridges, culverts, and multiple openings, is to use the downstream computed encroachments through the structure, and at the cross section just upstream of the structure (the program does this automatically). There are a few exceptions to this rule."

"First, when using Method 1, the user can enter separate encroachment stations downstream of the structure, inside the structure, and upstream of the structure. Only one set of encroachments can be entered for inside of the structure."

"Second, for encroachment methods 2 through 5, the program will allow for separate encroachment calculations at a bridge, when using the energy based bridge computation method. For all other bridge computation methods (Momentum, Yarnell, WSPRO, Pressure Flow, Pressure and Weir Flow, and Low Flow and Weir Flow) the program will use the computed downstream encroachments through the bridge and at the cross section just upstream."

"At a culvert crossing or a multiple opening, when using encroachment methods 2 through 5, the program will always use the computed downstream encroachments through the structure and just upstream of the structure. The only way to override this is to use Method 1 encroachments."

Because Encroachment Method 1 is used at Section 2 (RS 26285), encroachment stations from Section 2 will not be repeated at Structure Section (RS 26335). Although Encroachment Method 1 is specified at RS 26335, HEC-RAS will not perform the encroachment computations at RS 26335 because RS 26335 does not have any encroachment stations.

The solution to the problem is provided in plan 02. Open the project file (2STFW09.prj) with the HEC-RAS program and select "plan 02" from the Steady Flow Analysis window. Click Options menu and select "Encroachment." The Encroachments window will appear. You will notice that Method 1 encroachment with encroachment stations 4880 and 5030 is specified at RS 26335 Culv. These encroachment stations are from RS 26385 (Section 3 of the structure), thereby providing a consistent floodway width from Section 3, over the structure, and at Section 2.

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. Please close all open 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 (2STFW09.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway and select the "Structure check" button on the Program menu bar. When prompted with the message, "You need to specify allowable surcharge...", click the "Yes" button.

Review the resulting report, and you will notice that the message ST FW 09 is no longer at RS 26335. This means that proper encroachment is computed at the structure section. In order to check the encroachment stations at the structure in HEC-RAS, close the CHECK-RAS program and reopen the project file (2STFW09.prj) with the HEC-RAS program. Select "plan 02" from the Steady Flow Analysis window and then close this window.

The information about the structure can be obtained by clicking the "View Detailed Output" button from the main HEC-RAS window. Click on Type menu and select "Culvert" from the displayed window. The Culvert Output window will be displayed. In this window, the "Profile" box will display 100base, and the "Riv Sta" box will display 26335. WeirStaLft value is equal to 4698.81, and WeirStaRgt value is equal to 5105.27. These are the values for the 1% annual chance flood profile, and they are the same as plan 01.

To obtain the information about the floodway profile, select "100encr" by clicking the pull down arrow at the "Profile" box in the same Culvert Output window. WeirStaLft value is shown as 4880, and WeirStaRgt value is shown as 5030. These values are the same as the encroachment station values specified at RS 26335 under Encroachments window.

Click "Profile Table" button from the HEC-RAS main window. The Profile Output Table window will appear. Click Std. Tables menu and select "Encroachment1" table. Note the surcharge values at RS 26385, 26475, and 26995. These are the river stations upstream of the structure. Close "Encroachment 1" table and Profile Output Table windows.

Select "plan 01" from Steady Flow Analysis window. Follow the same procedure as described for plan 02 to obtain the surcharge values at RS 26385, 26475, and 26995. The following table shows the differences in surcharge values between the two plans. When you are viewing the values in "Encroachment 1" table, you will also notice that the floodway widths at RS 26385, 26475, and 26995 are the same between the two plans. The following table suggests that inserting proper encroachment stations at the structure section will provide proper surcharge values upstream of the structure.

RS Plan 02 Surcharge Plan 01 Surcharge
26385 0.55 -0.04
26475 0.62 0.05
26995 0.89 0.48

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