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CHECK-RAS Exercise

Water-surface elevation of lower discharge is higher than the water-surface elevation of higher discharge

CHECK-RAS will print the "MP WS 01" message in the Profile check if the water-surface elevation of the lower discharge is higher than the water-surface elevation of the higher discharge. This is known as criss-crossing of water-surface elevations. Criss-crossing of water-surface elevations is not permitted when plotting profiles for Flood Insurance Studies. This month's exercise will explain the effect of the selection of the critical depth and conveyance computation methods on the criss-crossing of water-surface elevations.

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

Problem

Open the project file (MPWS01.prj) with the HEC-RAS program and select plan 01 from the "Steady Flow Analysis" window. Select "Conveyance Calculations..." from the "Options" menu. You will notice that the "At breaks in n values only" option is selected for this plan. Select "Critical Depth Computation Method..." from the "Options" menu. You will notice that "Parabolic method (Faster Method)" is selected for this plan. 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. You will need to create a user-defined table to analyze the output information of this plan because the standard tables do not have all of the variables that you will need to evaluate. The following procedure describes how to create the user-defined table for this plan.

Select "Standard Table 1" from the "Std. Tables" menu in the "Profile Output Table" window. Select "Define Table..." from the "Options" menu. The "Create a Table Heading" window will appear. Add and delete the variables in the "Select Variables" tab so that the final table includes only the following variables:

QTotal, EGElev, WSElev, Crit WS, Conv.Total, Conv.Left, Conv.Chnl, and Conv.Right

Select the "Additional Options" tab and activate the following checkboxes:

Get the cross sections inside bridge nodes, Get all culverts in a culvert group, Note Location of Internal Boundaries (Bridges, Culverts, etc.), and Cross Sections.

Click OK. Select "Save Table..." from the "Options" menu. Enter "ConvComp" as the name of the table in the window that appears and click OK. You now have a new user table called ConvComp. Select "ConvComp" from the "User Tables" menu.

Scroll through the table and you will notice that critical depth occurred for all the discharges at River Station (RS) 19870. You will also notice that the critical water-surface elevation for the discharge of 3413.00 cfs is 586.05, but the critical water-surface elevation for the discharge of 3789.00 cfs is only 585.82. We will discuss this issue later.

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 (MPWS01.prj) in the CHECK-RAS program and extract the data. Select #10-yr, #50-yr, #100-yr, and #500-yr as the 10-year, 50-year, 100-year, and 500-year profiles, respectively, in the "Select Profile" window and press OK.

Press the "Report" button and review the resulting report. You will notice that the MP WS 01 message is displayed at RS 20206, 20061, 20006, 19981, 19955, 19912, and 19870 under the "Water Surface Elevation Check" section of the report. As an example, the message at RS 20206 reads as:

	RS:		20206
	MP WS 01	WSEL of 591.39 for profile number 4 is greater than the
			WSEL of 591.04 for profile number 5

If you highlight "MP WS 01," the CHECK-RAS Help Message box will appear. You will find different reasons why a higher discharge can have lower water-surface elevations. However, the cause of criss-crossing water-surface elevations for this case is not one of the reasons provided. It is caused by the computation of critical depth in HEC-RAS. The critical water-surface elevation in HEC-RAS is computed based on the minimum energy at a cross section. The minimum energy is the summation of critical water-surface elevation and the corresponding velocity head. The velocity head includes the velocity distribution coefficient and is related to conveyance computations.

Pages 2-13 of the HEC-RAS Hydraulic Reference Manual provide an explanation of the critical depth determination. There are two methods, parabolic and secant, in HEC-RAS to compute the critical depth. The parabolic method has been set as the default method in HEC-RAS. The default method can be changed from the "Steady Flow Analysis" window by selecting "Critical Depth Computation Method..." from the "Options" menu. A dialog box will appear and the user can select between the secant method and the parabolic method. The secant method is described as "Multiple Critical Depth Search" in the dialog box.

We will try to solve the problem by reviewing the critical depth computations in HEC-RAS.

Solution

We will revisit the results from plan 01 before we solve the problem. Open the project file (MPWS01.prj) with the HEC-RAS program and select plan 01 from the "Steady Flow Analysis" window. Select "Conveyance Calculations..." from the "Options" menu. You will notice that the "At breaks in n values only" option is selected for this plan. Select "Critical Depth Computation Method..." from the "Options" menu. You will notice that "Parabolic method (Faster Method)" is selected for this plan. 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 "ConvComp" table from the "User Tables" menu. You will notice that critical depth occurred for all the discharges at RS 19870. You will also notice that the critical water-surface elevation for the discharge of 3413.00 cfs is 586.05, but the critical water-surface elevation for the discharge of 3789.00 cfs is only 585.82. It appears that the critical depth computations are not computed correctly at RS 19870. The critical water-surface elevation should increase with an increase in discharge. As suggested by the HEC-RAS Hydraulic Reference Manual, we will change the Critical Depth Computation Method.

Close the "Profile Output Table" window and select plan 01 from the "Steady Flow Analysis" window. Select "Critical Depth Computation Method..." from the "Options" menu. Select "Multiple Critical Depth Search..." and click OK. Run HEC-RAS by clicking on the Compute button. After the computation is done, close the "Steady Flow Analysis" window. Please note that the Conveyance Calculations method is left as the "At breaks in n values only" option for this run.

As explained in the previous paragraphs, the critical depth computations are related to the conveyance computations. We will use HEC-2 style conveyance to determine whether or not the critical depth computations are improved. The solution to the problem is provided in plan 02. Select plan 02 from the "Steady Flow Analysis" window. Select "Conveyance Calculations..." from the "Options" menu. You will notice that the "Between every coordinate point (HEC2 Style)" option is selected for this plan. Select "Critical Depth Computation Method..." from the "Options" menu. You will notice that "Parabolic method (Faster Method)" is selected for this plan. 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 "ConvComp" table from the "User Tables" menu. You will notice that critical depth occurred for all the discharges at RS 19870. You will also notice that the critical water-surface elevation increases with an increase in discharge. For the discharge of 3413.00 cfs, the critical water-surface elevation is 587.18 and the critical water-surface elevation for the discharge of 3789.00 cfs is 587.53. It appears that the critical depth computations are computed in a consistent manner at RS 19870 when HEC-2 style conveyance is selected. You can review the difference in conveyance between the two methods by comparing the Conv. Total values from the two plans in user table "ConvComp."

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.

Press the "Report" button and review the resulting report. You will notice that there are no messages under the "Water Surface Elevation Check" section of the report.

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