CHECK-RAS Exercise

Relationships among the Abutment Stations, Ineffective Flow Stations, and Encroachment Stations at a Bridge (Continued from previous month's exercise).

The previous month's exercise discussed the CHECK-RAS message ST FW 03. CHECK-RAS will print this message if the encroachment stations at Section 2, Structure Section, and Section 3 are within the abutment stations, indicating that the encroachment stations are blocking the opening area of the structure.

Please download cr_nov01.zip (in WinZip format) for use with this month's exercise.

This month's exercise will explain the CHECK-RAS message ST IF 06. This message deals with the placement of ineffective flow stations at Section 3 and Section 2 of the structure. This exercise will make the user aware of the fact that some messages are related to one another, such as Message ST IF 06 and Message ST FW 03. It will also explain what other related computation steps the user should perform to resolve the statements in the messages.

An example of an ST IF 06 message is provided in this bulletin. Please reply to this mailing list if you have questions or feedback concerning other situations that are not documented here.

The same project file, plan files, geometric files, and the flow file from the previous month exercise will be used. If you do not have those files please download the files that are available in the WinZip archive referenced here, and unzip the files to one directory on your local computer.

Problem

Open the project file (STFW03.prj) with the HEC-RAS program and select plan 01 from the "Steady Flow Analysis" 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 under Options menu to enlarge the view of the bridge opening area.

The HEC-RAS Hydraulic Reference Manual, Version 2.2, September 1998, provides guidance on the placement of ineffective flow stations at a bridge on pages 5-6. The following paragraph is excerpted from the HEC-RAS Hydraulic Reference Manual.

The ineffective flow areas should be set at stations that will adequately describe the active flow area at Sections 3 and 2 of the structure. (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 general, these stations should be placed outside the edges (abutments) of the bridge opening to allow for the contraction and expansion of flow that occurs in the immediate vicinity of the bridge. On the upstream side of the bridge (Section 3) the flow is contracting rapidly. A practical method for placing the stations of the ineffective flow areas is to assume a 1:1 contraction rate in the immediate vicinity of the bridge. In other words, if Section 3 is 10 feet from the upstream bridge face, the ineffective flow areas should be placed 10 feet away from each side of the bridge opening (abutments). On the downstream side of the bridge (Section 2), a similar assumption can be applied.

You will notice that the ineffective flow stations are outside the bridge opening area. However, we do not know whether they are placed at the recommended locations according to HEC-RAS. We will use CHECK-RAS to evaluate the location of the ineffective flow stations. Please close all open windows except the main HEC-RAS window. Generate a 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 (STFW03.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 IF 06 at RS 3.3 and RS 3.2 under Ineffective Flow Check. In order to explain how CHECK-RAS computes the ineffective flow stations, the ST IF 06 message at RS 3.3 is copied from the CHECK-RAS report file and displayed below.

RS: 3.3 This is Section 3
ST IF 06 Low flow occurs at Bridge
The velocity head at Section 3 is more than 0.5
The computed left ineffective flow station of 4953.42 is less than
the selected left ineffective flow station of 4959.3
The left ineffective flow station should be adjusted.

The computed left ineffective flow station at Section 3 (LIfStaCU) is equal to the upstream leftmost abutment station of each group of structure (LAbutStaU) minus the maximum height of the bridge or culvert or gate opening of that group.

The required data to compare the ineffective flow stations can be obtained from the Summary Table of Structure Check and the detailed information of each structure in the CHECK-RAS report file for the Structure Check.

The upstream left abutment station of the bridge is computed by CHECK-RAS as 4969.30. The upstream bridge opening height is obtained by subtracting the upstream internal bridge section Minimum Channel Elevation of 220.49 from the upstream Maximum Low Chord Elevation of 236.37, and is equal to 15.88. Therefore, the computed left ineffective flow station is equal to (4969.30 - 15.88 = 4953.42).

According to the HEC-RAS Hydraulic Reference Manual, the left ineffective flow station at Section 3 should be located away from the left abutment station the same distance as that between Section 3 and the upstream face of the structure. This distance can be obtained from Deck/Roadway Data window for the structure. However, this distance is not reliable to use because different users specify different distances.

In order to have a consistent check for all the HEC-RAS files, CHECK-RAS assumes that the distance between Section 3 and the upstream face of the structure should be at least equal to the opening height of the structure. This assumption is based on the fact that the embankment height at the abutment locations can be as high as the bridge opening height and the embankment side slopes are usually flatter than 1H:1V. The true distance between Section 3 and the upstream face of the structure is equal to the horizontal scale of the side slope of the embankment to the height of the embankment at the abutment location.

The HEC-RAS report file printed the starting station and the left ineffective flow station as a pair of ineffective flow stations connected by the assigned left ineffective flow elevation, if the user selected the Normal Ineffective Flow Areas option in the Cross Section Data window. CHECK-RAS selected the ineffective flow station closest to the left abutment station at RS 3.3 as the selected left ineffective flow station and is equal to 4959.3.

CHECK-RAS will print an ST IF 06 message if the computed left ineffective flow station is less than the selected left ineffective flow station and satisfies two other conditions. One of the conditions is that the type of flow must be a low flow or pressure flow because ineffective flow stations are necessary to define the effective flow area for these types of flow. The other condition is that the velocity head at Section 3 should be more than 0.5 foot. This condition is included because the contraction loss between Section 4 and Section 3 will not be more than 0.15 foot if the velocity head at Section 3 is 0.5 foot and the velocity head at Section 4 is equal to zero and the contraction loss coefficient is equal to 0.3. If the velocity head is less than 0.5 foot and the computed left ineffective flow station is less than the selected left ineffective flow station, the widening of the ineffective flow stations will further reduce the contraction loss value and, therefore, it may not be worth changing the ineffective flow stations in the original model. Because several conditions must be satisfied to print the ST IF 06 message the user should check the location of Section 3 and the ineffective flow stations if ST IF 06 appears in the CHECK-RAS report file.

The same procedure is applied to checking the selected right ineffective flow station for Section 3 and the selected left and right ineffective flow stations for Section 2.

In order to resolve the message ST IF 06, the plan view of the bridge is reevaluated and found that Section 3 and Section 2 are not at the proper location. The plan view indicates that Section 3 should be located 25 feet from the upstream face of the bridge and Section 2 should be located 30 feet from the downstream face of the bridge. Because of the changes in the location of Section 3 and Section 2, the distances at Section 4, Section 3, and Section 2 need to be modified. The distance at Section 3 is the summation of the distance between Section 3 and upstream face of the bridge, deck width, and the distance between downstream face of the bridge and Section 2. The deck width is given in Deck/Roadway Data window for this bridge and is equal to 16 feet. Therefore, the distance at Section 3 should be equal to (25 + 16 + 30 = 71 feet).

Let us review the distances in plan 01. Open the project file (STFW03.prj) with the HEC-RAS program and select plan 01 from the "Steady Flow Analysis" window. Click the Geometric Data button from the HEC-RAS main window. From Geometric Data window, click Tables menu and select Reach Lengths. The "Edit Downstream Reach Lengths" window will appear.

The following table shows the reach lengths at Section 4, Section 3, Section 2, and Section 1 in the "Edit Downstream Reach Lengths" window.

River Station Bridge Section LOB Channel ROB
3.4 Section 4 50 50 50
3.3 Section 3 56 56 56
3.25 Bridge      
3.2 Section 2 200 200 200
3.1 Section 1 363.25 363.25 363.25

You will notice that the reach length of 56 at Section 3 is different from the computed reach length of 71. In order to adjust the distances at Section 4 and Section 2, the following steps should be taken to find the distance between Section 3 and the upstream face of the bridge, the bridge deck width, and the distance between downstream face of the bridge and Section 2 in plan 01.

Click "OK" to close the "Edit Downstream Reach Length" window. Click the Brdg/Culv button in the Geometric Data window. Click the Deck/Roadway button from the Bridge Culvert Data window. The Deck/Roadway Data Editor window will be displayed. The distance between Section 3 and the upstream face of the bridge is specified as 20 feet under the Distance column and the bridge deck width of 16 feet is specified under the Width column. Click "OK" to close the window.

The distance between the downstream face of the bridge and Section 2 can be computed by subtracting the summation of the distance between Section 3 and the upstream face of the bridge and the bridge deck width from the reach length at Section 3. For plan 01, the distance between the downstream face of the bridge and Section 2 is computed as (56-(20 + 16) = 20) feet.

Let us compare the distances at the structure between plan 01 and the actual lay out of Section 3 and Section 2 on the bridge plan. From the explanation given in the previous paragraphs, you will notice that the distance between Section 3 and the upstream face of the bridge from the bridge plan is 5 feet further away from the upstream face of the bridge and the distance between the downstream face of the bridge and Section 2 from the bridge plan is 10 feet further away from the downstream face of the bridge. Therefore, the reach length at Section 4 needs to be reduced by 5 feet, the reach length at Section 2 needs to be reduced by 10 feet, and the reach length at Section 3 needs to be increased by 15 feet due to the new location of Section 3 and Section 2.

The following table shows the new reach lengths at Section 4, Section 3, Section 2, and Section 1 after relocating Section 3 and Section 2.

River Station Bridge Section LOB Channel ROB
3.4 Section 4 45 45 45
3.3 Section 3 71 71 71
3.25 Bridge      
3.2 Section 2 190 190 190
3.1 Section 1 363.25 363.25 363.25

The distance between Section 3 and the upstream face of the bridge and the distance between the downstream face of the bridge and Section 2 should be used to locate the ineffective flow stations. The following table shows the abutment stations and the ineffective flow stations in plan 01.

River Station Bridge Section Left Abutment Left Ineffective Right Abutment Right Ineffective
3.3 Section 3 4969.3 4959.3 5025.2 5035.3
3.2 Section 2 4969.3 4959.3 5025.2 5035.3

CHECK-RAS already provides the message ST IF 06 indicating that the locations of the ineffective flow stations are inappropriate in plan 01. The following table shows the proper location of left and right ineffective flow stations based on the HEC-RAS criteria after placing Section 3 and Section 2 at the proper location. As discussed in the previous paragraphs, the distance between Section 3 and the upstream face of the bridge should be equal to 25 feet and the distance between the downstream face of the bridge and Section 2 should be equal to 30 feet. The left and right ineffective flow stations at Section 3 should be placed 25 feet away from the upstream internal bridge section left and right abutment stations. Also, the left and right ineffective flow stations at Section 2 should be placed 30 feet away from the downstream internal bridge section left and right abutment stations.

River Station Bridge Section Left Abutment Left Ineffective Right Abutment Right Ineffective
3.3 Section 3 4969.3 4944 5025.2 5050
3.2 Section 2 4969.3 4939 5025.2 5055

If the centerline stations at Section 2, the downstream internal bridge section, the upstream internal bridge section, and Section 3 are the same and the shape of the bridge opening area at the downstream internal bridge section and at the upstream internal bridge section are the same then the abutment stations at the downstream internal bridge section and the upstream internal bridge section will be equal. The user should be aware of this fact when modeling a bridge.

According to the CHECK-RAS Structure report file, the type of flow is low flow for the 1-percent-annual-chance flood. Therefore, the total discharge should be maintained within the ineffective flow stations. For the floodway profile, the 1-percent-annual-chance flood should assume to be maintained within the ineffective flow stations for the first floodway run. Therefore, the encroachment stations at Sections 3 and 2 should be made equal to the ineffective flow stations using Encroachment Method 1. If the results from the first floodway run show that weir flow occurs for the floodway profile and the computed surcharge values are more than the allowable value then the floodway width should be widened by reassigning the encroachment stations.

Solution

The solution to the problem is provided in plan 02. Open the project file (STFW03.prj) with the HEC-RAS program and select plan 02 from the "Steady Flow Analysis" window. Click Options and select Encroachment. The Encroachments window will appear. You will notice that the encroachment stations are set the same as the revised left and right ineffective flow stations at river stations 3.3, 3.25, and 3.2. The left encroachment station is equal to 4944 and the right encroachment station is equal to 5050 at RS 3.3 and RS 3.25 BR, and the left encroachment station is equal to 4939 and the right encroachment station is equal to 5055 at RS 3.2, using Encroachment Method 1.

Close the Encroachments window and close the "Steady Flow Analysis" window. Click the Geometric Data button from the main HEC-RAS window. Click the Cross Section button from the Geometric Data window. Select RS 3.3 and double click on the Normal Ineffective Flow Areas statement at the bottom of the window. You will see that the left ineffective flow station has been changed to 4944 and the right ineffective flow station has been changed to 5050 in the Ineffective Flow Area dialog box. For RS 3.2 the left ineffective flow station has been changed to 4939 and the right ineffective flow station has been changed to 5055. Close the Cross Section Data window.

From the Geometric Data window, click the Tables menu and select Reach Lengths. You will notice that the reach lengths for the left overbank, channel, and the right overbank at RS 3.4, RS 3.3, and RS 3.2 reflect the revised distances. Close the Reach Length window.

From the Geometric Data window, click Brdg/Culv button. Click the Deck/Roadway button from the Bridge Culvert Data window. The Deck/Roadway Data Editor window will be displayed. The revised distance of 25 feet between Section 3 and the upstream face of the bridge is specified under the Distance column and the bridge deck width of 16 feet is specified under the Width column. Click "OK" to close the window.

After closing the Geometric Data window, click the "Steady Flow Analysis" button from the main HEC-RAS window. Click the compute button from the "Steady Flow Analysis" window. Please close all open windows except the main HEC-RAS window. Generate a 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 (STFW03.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 ST IF 06 message under the Ineffective Flow Check and the ST FW 03 message under the Floodway Check are no longer there. However, the ST FW 08 message is still at RS 3.25 and it also appears at RS 3.2.

The ST FW 08 message from the CHECK-RAS report file is displayed below for discussion.

RS: 3.25 This is Bridge
ST FW 08 The left station effective of 4976.55 for the natural profile.
is less than the left channel bank station of 4986.3
The left encroachment station of 4944 is less than the left
station effective of 4976.55 for the natural profile.
The left encroachment station should be adjusted.

The Left Station Effective (Left Sta Eff) and the Right Station Effective (Right Sta Eff) for the natural profile are located at the left and right edges of the natural floodplain, respectively. Since the Left Sta Eff of 4976.55 is less than the left channel bank station of 4986.3 the 1-percent-annual-chance floodplain is outside the channel portion on the left side. However, the left encroachment station of 4944 is less than the Left Sta Eff of 4976.55. This means that the left encroachment station is outside the 1-pecent-annual-chance floodplain on the left side. By definition, a floodway cannot be wider than the natural floodplain. Therefore, the left encroachment station cannot be outside of the natural floodplain. The left encroachment station should be equal to or greater than the Left Sta Eff of the natural profile.

To eliminate the ST FW 08 message at RS 3.25, the left encroachment station should be made equal to the Left Sta Eff of 4976.55 and the right encroachment station should be made equal to the Right Sta Eff of 5022.51.

Before revising the encroachment stations, let us view the RS 3.25 BRU. Open the project file (STFW03.prj) with the HEC-RAS program and select plan 02 from 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 under the Options menu to enlarge the view of the bridge opening area.

You will notice that the channel bank stations are located at the toe of the abutment slopes. The overbank roughness coefficient of 0.06 is assigned to both the left and right abutment slopes within the bridge opening area. For this bridge the abutment slopes are constructed with concrete and therefore a roughness coefficient of 0.015 should be assigned to the abutment slopes. In order for HEC-RAS to compute a proper roughness coefficient for the entire bridge opening area with one composite n-value, depending upon the depth of the flow, the channel bank stations should be assigned at the bridge abutment locations. We will discuss how to assign the channel bank stations for the bridge opening area when we explain plan 03. Therefore, we will not change the encroachment stations at RS 3.25 in plan 03.

Please also view RS 3.4, 3.3, 3.2, and 3.1. You will notice that the channel bank stations are not located at the ground points beyond which relatively flat overbank areas exist. The channel bank stations appear to be located at the edges of the low flow channel. The channel bank stations should be moved up to the ground stations so that the ground stations within the channel bank stations will be located within a confined section. The roughness coefficients will be assigned using the horizontal variation in n-values option. We will discuss in detail when we review plan 03.

We also have an ST FW 08 message for RS 3.2. The message reads as follows:

RS: 3.2 This is Section 2
ST FW 08 The right station effective of 5044.6 for the natural profile
is greater than the right channel bank station of 5021
The right encroachment station of 5055 is greater than the right
station effective of 5044.6 for the natural profile.
The right encroachment station should be adjusted.

We have set the right ineffective flow station and the encroachment station to be the same at 5055. Since the right station effective of 5044.6 is less than the right ineffective flow station of 5055 the 1-percent-annual-chance flood plain does not reach the right ineffective flow station. Therefore, the right encroachment station of 5055 should be made equal to the right station effective of 5044.6 at RS 3.2.

The above suggestions are included in plan 03. 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 Encroachments window will appear. The right encroachment station is changed from 5055 to 5044.6 at RS 3.2. The right encroachment station of 5023.19 is changed to 5047.8 representing the new right channel bank station at RS 3.1.

We will review the changes made to the cross section data. Close the Encroachment window and the "Steady Flow Analysis" window. Click the Geometric Data button from the main HEC-RAS window. Click the Cross Section button from Geometric Data window. The channel bank stations are changed at RS 3.4, 3.3, 3.2, and 3.1. The following Table shows the channel bank stations in plan 02 and in plan 03.

  Plan 02 Plan 03
River Station Left Bank Right Bank Left Bank Right Bank
3.4 4986.3 5021 4984.6 5046.9
3.3 4986.3 5021 4955.9 5052.4
3.2 4986.3 5021 4984.5 5037.3
3.1 4986.3 5021 4968.8 5047.8

Select RS 3.4 from the Cross Section Data window. Click the Options menu and you will notice that Horizontal Variation in n Values item is checked. Click the Options menu again. The n-values are assigned under n-Val column in Cross Section X-Y Coordinates box. The channel bank stations are assigned in Left Bank and Right Bank columns in Main Channel Bank Stations box. The n-value of 0.05 is assigned at the first ground station 3438.5. This n-value will be used by HEC-RAS until next n-value is encountered. The next n-value of 0.05 is assigned at the left bank station of 4984.6 from Plan 03. The n-value of 0.035 is assigned at the left bank station of 4986.3 from Plan 02. The n-value of 0.06 is assigned at the right bank station of 5021 from Plan 02. The n-value of 0.06 is assigned at the right bank station of 5046.9 from Plan 03. By using the Horizontal Variation in n-Values option channel portion between Left Bank station of 4984.6 and the Right Bank station of 5046.9 is modeled with three different n values of 0.05, 0.035, and 0.06.

The same approach is applied to other river stations. We will now review the changes made to the bridge internal cross sections. Close the Cross Section Data window. Click the Brdg/Culv button from the Geometric Data window. Click the Options menu from Bridge Culvert Data window and select Internal Bridge Cross Sections.

The Bridge Cross Sections window will display. The left and right abutment stations are assigned as the left and right channel bank stations for upstream and downstream internal bridge cross sections. The left abutment station is equal to 4969.3 and the right abutment station is equal to 5025.2. The ground station corresponding to 4969.3 and 5025.2 is inserted into the Cross Section X-Y Coordinate box of the upstream and downstream internal cross section. The elevations of the new ground stations are obtained by interpolating between the elevations of the adjacent ground stations. The three n-values, .015, .035, and .015 are assigned between the new channel bank stations 4969.3 and 5025.2 representing the abutment slopes and the low flow channel.

After closing the Geometric Data window, click the "Steady Flow Analysis" button from the main HEC-RAS window. Click the compute button from the "Steady Flow Analysis" window. Please close all open windows except the main HEC-RAS window. Generate a 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 (STFW03.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 there are no ST FW 08 messages. The user may also create a user-defined table in HEC-RAS by including a variable Mann Comp, which is the Manning's composite n-value within the channel, to evaluate what n-values are used by HEC-RAS based on the assignment of different n-values within the channel portion. The computation of composite n-value is explained on page 2-6 of HEC-RAS Hydraulic Reference Manual. The user will find that the channel composite n-value of 0.044, 0.032, 0.031, and 0.046 were computed at RS 3.2, 3.25 BRD, 3.25 BRU and 3.4, respectively, by HEC-RAS.

View the archive page for all Flood Hazard Mapping listservs.

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