CHECK-RAS Bulletin

Issue No. 6

CHECK-RAS Exercise:
Relationship among the Abutment Stations, Ineffective Flow Stations, and Encroachment Stations at a Bridge.

HEC-RAS Hydraulic Reference Manual, Version 2.2, September 1998, provides guidance on the placement of ineffective flow stations at a bridge on page 5-6. In general, these stations should be placed outside the abutments of the bridge opening to allow for the contraction and expansion of flow that occurs in the immediate vicinity of the bridge. For low flow and pressure flow conditions, the total discharge should be confined between the left and right ineffective flow stations at Section 3, within the bridge abutments through the internal bridge cross sections, and Section 2 of the bridge. Section 3 is located at the upstream toe of the embankment or the wing walls and Section 2 is located at the downstream toe of the embankment or the wing walls.

In HEC-RAS, the encroachment stations computed from Encroachment Method 4 have no relationship to the abutment stations or the ineffective flow stations. Therefore, HEC-RAS may compute encroachment stations that are located within the abutment stations of the bridge. Although surcharge values may still be less than the allowable value, it is not advisable to allow filling in the fringe areas of Section 3 and Section 2 because this would block the bridge opening area. CHECK-RAS will print the message ST FW 03 if the encroachment stations are within the abutment stations.

An example ST FW 03 message is provided in this bulletin.

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

Problem:
In HEC-RAS, open the project file (STFW03.prj) and select plan 01 from the "Steady Flow Analysis" window. Click Options and select Encroachment. The Encroachment window will appear. You will notice that the Encroachment Method 4 with the surcharge value of 1.0 foot is specified at RS 3.3 (Section 3) and RS 3.25 (Bridge section) and Encroachment Method 1 is used at RS 3.2 (Section 2). After closing the Encroachment window, click the Compute button to run plan 01. After closing the "Steady Flow Analysis" window, click the "View Cross Sections" button from the main HEC-RAS window. View River Station (RS) 3.25 BRU. Use the "Zoom In" option to enlarge the view of the bridge opening area. You will notice that the encroachment stations are within the bridge opening area. 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.

In CHECK-RAS, open the saved project (STFW03.prj) 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 03 at RS 3.2 and RS 3.3 under Floodway Check.

The RS 3.2 is Section 2 of the bridge, and the RS 3.3 is Section 3 of the bridge. HEC-RAS duplicates Section 2 at the downstream internal section of the bridge and Section 3 at the upstream internal section of the bridge. CHECK-RAS computes the bridge abutment stations at the downstream internal section and the upstream internal section. Since Section 2 and Section 3 of the bridge are closely related to the downstream and upstream internal bridge sections, respectively, it is appropriate to compare the computed bridge abutment stations at the internal bridge sections to the encroachment stations at Section 2 and Section 3 to evaluate whether allowing the floodplain at Section 2 and Section 3 to encroach up to the encroachment stations will block the flow into the bridge opening area.

The message ST FW 03 will only be considered when the 1-percent-annual-chance floodplain at Section 3 and Section 2 is wider than the bridge opening area. If the 1-percent-annual chance floodplain is smaller than the width between the bridge abutments, it is possible that the encroachment stations can be within the bridge abutments. In that case, CHECK-RAS will not print the message ST FW 03.

The starting station of the 1-percent-annual-chance floodplain is called at left station effective in HEC-RAS, and the ending station of the 1-percent-annaual-chance floodplain is called at right station effective in HEC-RAS. If the left station effective value is less than the left abutment station, CHECK-RAS considers that the floodplain is wider than the bridge opening on the left side. If the right station effective value is greater than the right abutment station, CHECK-RAS considers that the floodplain is wider than the bridge opening on the right side. CHECK-RAS analyzes this condition for each side of the floodplain.

If the above condition is true, then CHECK-RAS will print the message ST FW 03 if the left encroachment station is greater than the left abutment station indicating that the left encroachment station is within the bridge opening area. CHECK-RAS will also print the message ST FW 03 if the right encroachment station is less than the right abutment station indicating that the right encroachment station is within the bridge opening area.

According to the ST FW 03 messages in the CHECK-RAS report file, both left and right encroachment stations at RS 3.2 (Section 2) and RS 3.3 (Section 3) are within the bridge abutment stations and will block the bridge opening area.

Solution:
The solution to the problem is provided in plan 03. (We will discuss plan 02 in the next exercise). Open the project file (STFW03.prj) with the HEC-RAS program and select plan 03 from the "Steady Flow Analysis" window. Click Options and select Encroachment. The Encroachment window will appear. You will notice that the encroachment stations are specified beyond the bridge abutment stations at RS 3.2 (Section 2), RS 3.25 (Bridge Section), and RS 3.3 (Section 3) using Encroachment Method 1.

Close the Encroachment window and click the Compute button to run plan 03. After closing the "Steady Flow Analysis" window, click the "View Cross Sections" button from the main HEC-RAS window. View River Station (RS) 3.25 BRU. Use the "Zoom In" option to enlarge the view of the bridge opening area. You will notice that the encroachment stations are located outside the bridge opening area. 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.

Review the resulting report, and you will see that there are no messages under Floodway Check.

The selection of the encroachment stations will be explained in detail in the next exercise. Please rerun plan 01 in HEC-RAS and check against the CHECK-RAS program. After running Structure Check, open the CHECK-RAS report file. You will notice that there are messages ST IF 06 and ST FW 08 other than ST FW 03. The ST IF 06 message deals with the location of the ineffective flow stations, and the ST FW 08 message deals with the location of encroachment stations in relationship to the floodplain boundary. You will also notice that the channel bank stations are located within the confined portion of the channel. They should be located at the ground stations beyond which relatively flat overbanks exist. These issues are addressed in plan 02, and we will discuss them in the next exercise. You may want to run plan 02 in HEC-RAS and check it against CHECK-RAS on your own to evaluate what changes were made from plan 01 and what messages remain to be resolved.

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