CHECK-RAS Bulletin

Issue No. 8

CHECK-RAS Exercise:
Negative Surcharge and the Selection of the Bridge Modeling Approach

CHECK-RAS will print an ST FW 06 message at Section 3, upstream bridge internal section, downstream bridge internal section, and Section 2 if the negative surcharge value is less than -0.09.

HEC-RAS may compute a negative surcharge of less than -0.09 when the bridge computation method for the 1-percent annual chance flood profile and the floodway profile are selected differently.

An example of an ST FW 06 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:
Open the project file (STFW06.prj) with the HEC-RAS program and select plan 01 from the "Steady Flow Analysis" window. Close this window and click the Geometric button from the main HEC-RAS window. Click the Brdg/Culv button from the Geometric Data window. Click the Bridge Modeling Approach button from the Bridge Culvert Data window. The Bridge Modeling Approach Editor window will appear. You will notice that under the Low Flow Methods, both Energy and Momentum boxes are checked and the circle in front of Highest Energy Answer is selected. Close this window. Run HEC-RAS from the Steady Flow Analysis window 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 the "Bridge Comparison" option. The Bridge Comparison Table will appear. You will notice that under the "Br Sel Method" column, the Energy/Weir methodis used for the 1-percent annual chance flood profile and the Energy only method is used for the floodway profile. Select the Encroachment 1 Table from Std. Tables. You will notice that a negative surcharge of -0.33 is at River Station (RS) 3.3.

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 (STFW06.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway and select the Structure check button. When prompted with the message: "You need to specify allowable surcharge..." click the "Yes" button.

Review the resulting report. You will notice that under the Type of Flow check, CHECK-RAS stated as low and weir for the first profile (1-percent-annual chance flood profile) and low flow for second profile. CHECK-RAS based the type of flow from the energy grade line at Section 3, Min Top Rd elevation, and the maximum low chord elevation. For the first profile, the energy grade elevation is equal to 233.42 and the water-surface elevation is equal to 233.36 at Section 3. The Min Top Rd elevation is equal to 233.36 and the maximum low chord elevation is equal to 236.37. Under the "Selected Method" column, the method used by HEC-RAS is provided. HEC-RAS used Energy/Weir method for the first profile and Energy for the second profile. From this data it appears that the weir flow is computed for the first profile by HEC-RAS because the energy grade elevation at Section 3 is higher than the Min Top Rd elevation. However, you will notice that the water-surface elevation of 233.36 at Section 3 is the same as the Min Top Rd elevation and therefore, water is not going over the road. Therefore, the weir flow for the first profile is questionable.

Under the Ineffective Flow check, an ST IF 02 message is displayed for Section 2. Since this message is related to weir flow and the weir flow computation is questionable we will ignore this message for this run.

Under the Floodway check, an ST FW 06 message is displayed. It is stated as follows:
RS: 3.3 This is Section 3 ST FW 06 Negative surcharge value of -0.33 occurs at this section.

If you highlight ST FW 06, a CHECK-RAS Help Message box will appear. You will find different reasons why a negative surcharge can occur. One of the reasons is if the type of flow between the two profiles are different. Since we know that two different types of flow occur in this case, we will try to solve the problem by reviewing the bridge modeling approach in HEC-RAS.

Solution:
The solution to the problem is provided in plan 02. Open the project file (STFW06.prj) with the HEC-RAS program and select plan 02 from the "Steady Flow Analysis" window. Close the "Steady Flow Analysis" window and click the Geometric Data button from the main HEC-RAS window. Click the Brdg/Culv button from the Geometric Data window. Click the Bridge Modeling Approach button from the Bridge Culvert Data window. The Bridge Modeling Approach Editor window will appear. You will notice that under the Low Flow Methods, only the Momentum box is checked and the circle in front is selected. The reason to select the momentum is explained in the following paragraphs.

Four low flow methods, energy, momentum, Yarnell, and WSPRO, are available in the HEC-RAS program. The flow through the structure can be supercritical or subcritical, although the stream itself is in subcritical regime. The momentum is the only method that can analyze both regimes through the structure when subcritical analysis is performed for the stream. If HEC-RAS cannot find a solution when using the momentum method it will use the energy method. After HEC-RAS provides a solution with the momentum method, the user should review the results and make a decision as to whether other methods should also be used for this structure. If the results are satisfactory then the user does not need to switch to other methods. It is not advisable to select the Highest Energy Answer for the low flow methods because it does not base on the proper type of flow through the structure. The user should decide the proper type of flow.

The bridge modeling methods should be the same between the two profiles for the floodway analysis to achieve the surcharge values within the allowable value, unless the type of flow changes due to the encroachment. For example, the type of flow is low flow for the 1-percent annual chance flood profile but it changes to pressure flow for the floodway profile due to the encroachment and that may cause the surcharge value to be above the allowable value. Even for this case, the user should investigate whether the type of flow changes are due to the encroachment or due to the bridge modeling method selected in HEC-RAS.

Close the Bridge Modeling Approach Editor window. Run HEC-RAS from the Steady Flow Analysis window 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 "Bridge Comparison". The Bridge Comparison Table will appear. You will notice that under the "Br Sel Method" column, the Momentum method is used for both the 1-percent annual chance flood and the floodway profile. Select Encroachment 1 Table from Std. Tables. You will notice that there is no negative surcharge at RS 3.3.

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.

Review the resulting report. You will notice that under the Type of Flow check, CHECK-RAS indicated a low flow for both the first profile (1-percent annual chance flood profile) and the second profile (floodway profile). CHECK-RAS based the type of flow from the energy grade line at Section 3, Min Top Rd elevation, and the maximum low chord elevation. At Section 3, the energy grade elevation is equal to 232.93 and the water-surface elevation is equal to 232.46 for the first profile and the energy grade elevation is equal to 233.18 and the water-surface elevation is equal to 232.74 for the second profile. The Min Top Rd elevation is equal to 233.36 and the maximum low chord elevation is equal to 236.37. Since the energy grade elevation at Section 3 for both profiles are lower than the maximum low chord elevation and the Min Top Rd elevation CHECK-RAS indicated a low flow. Under the "Selected Method" column the method used by HEC-RAS is provided. HEC-RAS used Momentum method for both profiles. Since the water-surface elevations from both profiles at Section 3 are lower than the maximum low chord and the Min Top Rd elevation the method used in the HEC-RAS model is acceptable. You will notice that an ST IF 02 message is no longer under the Ineffective Flow check and an ST FW 06 message is no longer under the Floodway check. The surcharge value at RS 3.3 is now equal to 0.29.

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Last Modified: Monday, 19-Jun-2007 11:57:20 EDT