CHECK-RAS Bulletin

Issue No. 9

CHECK-RAS Exercise:
Encroachment stations are not computed at a structure with weir flow.

CHECK-RAS will print the "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-percent annual chance profile. This may occur if:

An example of the "ST FW 09" message, which covers the two last situations described above, is provided in this bulletin. This bulletin also includes an explanation of how to use the Flow Distribution option to select the encroachment stations.

Please download the project file, plan files, geometric file, and flow file, which are available in WinZip format by clicking here, and unzip the files to one directory on your local computer.

Problem:

Open the project file (STFW09.prj) with the HEC-RAS program and select plan 01 from the "Steady Flow Analysis" window. Select "Encroachments..." from the "Options" menu. You will notice that Encroachment Method 4 with varying surcharge values is specified at the River Stations. 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. Select the "Encroachment 1" table from the "Std. Tables" menu. You will notice that "Enc Sta L" and "Enc Sta R" values were not computed at River Station (RS) 185. At RS 180, "Enc Sta L" is equal to 265.03 and "Enc Sta R" is equal to 315.96. At RS 185, "Ch Sta L" is equal to 264.97 and "Ch Sta R" is equal to 318.06. HEC-RAS will use the encroachment stations at Section 2 of the Structure Section and Section 3 if Encroachment method 4 is used at Section 2, the Structure Section, and Section 3. Since the encroachment stations at Section 2 (RS 180), were within the channel bank stations of Section 3 (RS 185) HEC-RAS did not compute the encroachment stations at Section 3. Close the "Profile Output Table - Encroachment 1" window.

HEC-RAS also did not encroach at the road profile at RS 182, even though Encroachment Method 4 with a surcharge value of 0.2 was specified. The information about the structure can be obtained by clicking the "View Detailed Output at XS, Culverts, Bridges and Inline Weirs" button from the main HEC-RAS window. Select "Culvert" from the "Type" menu. The "Culvert Option" window will appear. In this window, the "Profile" drop-down menu will display "100-yr" and the "Riv Sta" drop-down menu will display 182. The "Weir Sta Lft (ft)" value is equal to 115.47 and the "Weir Sta Rgt (ft)" value is equal to 472.45. These are the values for the 1-percent annual chance flood profile.

To obtain the information about the floodway profile select "floodway" from the "Profile" drop-down menu. The "Weir Sta Lft (ft)"value is shown as 114.49 and the "Weir Sta Rgt (ft)" value is shown as 474.51. The weir station values from the two profiles indicate that the floodway boundary is wider than the 1-percent annual chance flood boundary over the road.

CHECK-RAS will provide messages relating to the above conditions so that you do not need to search for information about encroachment at the structures in different tables. Please close all open windows except the main HEC-RAS window. Select "Generate Report..." from the "File" menu. After generating the report file for use with the CHECK-RAS program, please save the project.

Open the saved project (STFW09.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway in the "Select Profile" window and press OK. 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.

Press the "Report" button and review the resulting report. You will see that the ST FW 09 message is displayed for left and right weir stations at RS 182 and that the ST FW 05 message is displayed to inform you that the encroachment stations are not being computed at RS 185.

Solution:

The HEC-RAS Hydraulic Reference Manual explains the encroachment computation process at bridges, culverts, and multiple openings on pages 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."

The two messages, ST FW 05 and ST FW 09, in the CHECK-RAS report file will now be discussed. The ST FW 05 message is similar to the FW FW 02 message. The FW FW 02 message is explained in detail in the August 2001 installment of this bulletin. The ST FW 05 message is printed at RS 185 because encroachment stations are not computed by HEC-RAS, even though encroachment method and target surcharge values are specified at this RS. This happens if the encroachment stations at Section 2 (RS 180) are within the channel bank stations of Section 3 (RS 185). In order to solve this problem, Method 1 with encroachment stations that are equal to or outside the channel bank stations must be specified at RS 185.

The solution to the problem is provided in plan 02. Open the project file (STFW09.prj) with the HEC-RAS program and select plan 02 from the "Steady Flow Analysis" window. Select "Encroachments..." from the "Options" menu. The "Encroachments" window will appear. You will notice that Method 1 encroachment with encroachment stations 264 and 319 are specified at RS 185. These encroachment stations are just outside the left channel bank station of 264.97 and right channel bank station of 318.06 at RS 185. Please note that Method 4 with a target surcharge value of 0.2 is still used at RS 182 where the culvert is located; the same as in plan 01. This is not the correct modeling procedure. We will discuss the consequences in the following paragraphs.

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. Select "Generate Report..." from the "File" menu. After generating the report file for use with the CHECK-RAS program, please save the project.

Open the saved project (STFW09.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway in the "Select Profile" window and press OK. 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.

Press the "Report" button and review the resulting report. You will still see that the ST FW 09 message is displayed for the left and right weir stations at RS 182. However, the ST FW 05 message is no longer displayed at RS 185. This demonstrates that the encroachment stations are computed by HEC-RAS at Section 3 (RS 185) but that HEC-RAS is not encroaching at the structure location. This suggests that Method 1 encroachment must be used at both Section 3 and the Structure section for HEC-RAS to encroach at the structure location.

Close the CHECK-RAS program and reopen the project file (STFW09.prj) with the HEC-RAS program. Reselect plan 02 from the "Steady Flow Analysis" window. Select "Encroachments..." from the "Options" menu. The "Encroachments" window will appear. Please change Method 4 and the target surcharge value of 0.2 at RS 182 to Method 1 encroachment with encroachment stations 264 and 319, the same as at RS 185. Click OK and close the "Encroachments" window. 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. Select the "Encroachment 1" table from the "Std. Tables" menu. You will notice that a value of 2.29 is shown under the "Prof Delta WS" column at RS 185. This is the surcharge value and it is more than the allowable value of 1.00 foot. Although HEC-RAS encroached at the culvert location when Method 1 was specified at RS 182, the encroachment stations were set too close in this case. In order to reduce the surcharge value, the encroachment stations need to set wider. Because the equal conveyance reduction method (Method 4) cannot be applied at RS 182 and RS 185, we need to know the distribution of conveyance at RS 185 so that we can select the encroachment stations for Method 1 to satisfy the equal conveyance reduction criterion. Close the "Profile Output Table - Encroachment 1" window.

The solution to the problem is provided in plan 03. From the "Steady Flow Analysis" window select plan 03. Select "Flow Distribution Locations" from the "Options" menu. You will see the following statement in the box.

RIVER-1 Reach=Reach-1 RS=185 LOB=5 Channel=1 ROB=5

This statement suggests that the flow distribution option is set at specific location, RS 185, and that there will be 5 subsections on the left overbank, 1 subsection for the channel, and 5 subsections on the right overbank.

If you want to recreate this set up, please use the following procedure. Otherwise, close this window.

  1. Click the "Clear All" button in the "Set Locations for Flow Distribution" window.
  2. Under "Number of SubSections" in the "Set Specific Location Subsection Distribution" area, select 5 for "LOB," 1 for "Channel," and 5 for "ROB."
  3. Select "Starting Riv Sta" and "Ending Riv Sta" to match RS 185.
  4. Click on the "Set Selected Range" button.
  5. RIVER-1 Reach=Reach-1 RS=185 LOB=5 Channel=1 ROB=5 will be displayed in the box.
  6. Click OK and close the window.

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. Select "Generate Report..." from the "File" menu. In order to obtain the flow distribution information at RS 185, check only the Flow Distribution box in the Output area of the "Report Generator" window. To reduce the amount of data generated by HEC-RAS, do not check any boxes and do not include any Selected Summary Tables. Click on the "Set Profiles" button in the Output area and select only the 100-year profile. Click on the "Generate report" button and then the "View" button. You will see the following information in the report file. (Only "Left Sta," "Right Sta," and "%Conveyance" is included below for discussion).

Left Sta Right Sta %Con
105.99 158.98 0.28
158.98 211.98 2.80
211.98 264.97 7.73
LB 264.97 RB 318.06 86.09
318.06 397.36 2.77
397.36 476.66 0.33

The percent conveyance values are within the left and right stations and the channel portion is between LB 264.97 and RB 318.06. If we want to reduce the conveyance equally from the left and right overbanks, we may want to select station 211.98 as the left encroachment station and station 318.06 as the right encroachment station. This will reduce the conveyance from the left side by (0.28 + 2.8 = 3.08%) and from the left side by (2.77 + 0.33 = 3.1%). We will round the left and right encroachment stations to 212 and 319 and will insert these values at RS 182 and RS 185 in the "Encroachments" window.

Close the "Report Generator" window. Open the "Steady Flow Analysis" window and select "Encroachments..." from the "Options" menu. You will notice that Method 1 encroachment and encroachment stations 212 and 319 are specified at RS 182 and RS 185. Close this window. Run HEC-RAS by clicking on the Compute button. After the computation is done close the "Steady Flow Analysis" window.

Click Profile Table button from the HEC-RAS main window. The "Profile Output Table" window will appear. Select the "Encroachment 1" table from the "Std. Tables" menu. You will notice that all the surcharge values are less than 1.00. Close this window.

Please close all open windows except the main HEC-RAS window. Select "Generate Report..." from the "File" menu. Please make sure that all of the necessary boxes are checked and that "CHECKRAS" is the only table in the Selected Summary Tables box in the "Report Generator" window. Because the boxes in the "Report Generator" window were changed in the previous step, you may want to use the following checks to make sure that you have the right setup.

  1. For the present version of CHECK-RAS (Version 1.1), "No Table Borders" must be selected in the "Report File" box.
  2. All of the boxes in the "Input Data" area must be checked.
  3. All of the boxes except "Flow Distribution" and "Summary of Errors," must be checked.
  4. Select "CHECKRAS" from the "Available Summary Tables" box. Please make sure that "CHECKRAS" is the only table in the "Selected Summary Table" box.

After generating the report file for use with the CHECK-RAS program, please save the project. Open the saved project (STFW09.prj) in the CHECK-RAS program and extract the data.

If you do not want to perform the above checks, close the "Report Generator" window. Close the HEC-RAS main window. Delete the file with the ".rep" extension in the directory where your HEC-RAS files for this project reside. Open the project (STFW09.prj) in the CHECK-RAS program and extract the data. CHECK-RAS will display the following message:

"You need to create HEC-RAS report file for CHECK-RAS program"

Click OK. The HEC-RAS main window will appear. Select "Generate Report..." from the "File" menu. The "Report Generator" window will appear with all of the required boxes checked for use with the CHECK-RAS program. After generating the report file for use with the CHECK-RAS program, please save the project and close the HEC-RAS window. Click on the "Extract" button in the CHECK-RAS main window.

Select the profiles for the floodway in the "Select Profile" window and press OK. 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./

Press the "Report" button and review the resulting report. You will notice that both the ST FW 09 and ST FW 05 messages are no longer displayed under "Floodway Check."

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Last Modified: Monday, 18-Jun-2007 11:57:20 EDT