CHECK-RAS Exercise

Encroachment stations are specified within the channel bank stations

According to the definition of "regulatory floodway" contained in Section 59.1 of the NFIP regulations, a regulatory floodway means "the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height." The base flood for NFIP purposes is the 1% annual chance flood. If the 1% annual chance flood elevations are above the channel bank elevations, the regulatory floodway width in most of these cases contains the channel width plus any adjacent land. However, when the 1% annual chance flood is contained within the channel, the regulatory floodway width is defined as the width of the channel. FEMA does not allow for the regulatory floodway to be mapped narrower than the channel width.

CHECK-RAS will display FW FW 01 message if the encroachment stations are specified within the channel bank stations.

An example of FW FW 01 message is provided in this month's bulletin.

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

Problem

Open the project file (FWFW01.prj) with the HEC-RAS program and select "plan 01" from the "Steady Flow Analysis" window. Click the Option menu and select "Encroachment." You will find that Encroachment Method 1 was used at all the cross sections. 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.

Click the "Profile Table" button from the HEC-RAS main window. The Profile Output Table window will appear. Create a user-defined table as shown in Figure 1. Figure 1 shows only part of the full table. The variables in the table are W.S.Elev, Prof Delta WS (Surcharge), EncrWD, TopWdthAct, EncStaL, ChStaL, ChStaR, EncStaR, QTotal, Area, and Vel Total. Instructions for creating a user-defined table are explained on pages 8-18 of the HEC-RAS User's Manual, Version 2.2, September 1998.

Figure 1. User-Defined Table FWFW01

You will notice in the above table that at River Stations 2700 and 2650 the EncStaL value is more than the ChStaL and the EncStaR value is less than the ChStaR. That means the encroachment stations were specified within the channel bank stations.

CHECK-RAS will provide messages relating to the above conditions so that you do not need to search the information about the encroachment stations at the cross sections. 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.

Open the saved project (FWFW01.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway and select the "Floodway 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 FW FW 01 for left and right encroachment stations at RS 2700 and RS 2650 indicating that they are within the channel bank stations. When you highlight the message number FW FW 01, the Help message will provide you with the following information:

  1. The definition of the floodway.
  2. That the Encroachment Method 1 was used to specify encroachment stations.
  3. A suggestion to check the location of the channel bank stations.
  4. A suggestion either to revise the channel bank stations or encroachment stations so that encroachment stations are not within the channel banks.

Solution

Let us review the cross sections at RS 2700 and RS 2650. Open the project file (FWFW01.prj) with the HEC-RAS program and select "plan 01" from the Steady Flow Analysis window, if plan 01 is not in the box for "Plan" in the HEC-RAS main window. Click the "View Cross Sections" button from the HEC-RAS main window. The Cross Section window will be displayed. Click on the pull-down arrow in the "River Sta" box of this window and select RS 2700. The Cross Section window with the cross sectional view of RS 2700 is shown in Figure 2.

Figure 2. Cross Sectional View of RS 2700

Although the Encroachment is shown as a solid line in the Legend box, you cannot see the encroachment stations from this window. If you want to see the encroachment stations, click on the Options menu from this window (Cross Section window) and select the "Zoom In" option. Zoom into the area of the left edge of the water surface, and you will see the encroachment line in the vertical direction.

In the Help message for FW FW 01, it is suggested that the channel bank stations should be located at the ground stations beyond which relatively flat overbank area exists. "Relatively flat" is to be interpreted by comparing the slope of the ground points before the channel bank station and the slope of the ground points after the channel bank station. In this case (RS 2700), the channel bank stations are properly selected. Therefore, the encroachment stations at RS 2700 should be moved to be the same as the channel bank stations. The same conclusion will be reached when you view RS 2650.

FEMA recommends the use of the equal conveyance reduction principle in FEMA 37, Flood Insurance Study Guidelines and Specifications for Study Contractors, to establish the regulatory floodway. Encroachment Method 4 in HEC-RAS uses this principle to compute the encroachment stations. This method computes encroachment stations so that the encroachment stations will not be placed within the channel bank stations and, therefore, satisfies Section 59.1 of the NFIP regulations.

The solution to the problem is provided in plan 02. Open the project file (FWFW01.prj) with the HEC-RAS program and select "plan 02" from the Steady Flow Analysis window. Click the Options menu and select "Encroachment." The Encroachments window will appear. You will notice that Method 4 with a surcharge value of 1 is specified at RS 2700 and RS 2650.

Close the Encroachments window and run HEC-RAS by clicking 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. Generate the 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 (FWFW01.prj) in the CHECK-RAS program and extract the data. Select the profiles for the floodway and select the "Floodway 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 message FW FW 01 is no longer at RS 2700 and RS 2650. This means that encroachment stations are no longer within the channel bank stations at these river stations. To check the encroachment stations at RS 2700 and RS 2650 in HEC-RAS, close the CHECK-RAS program and reopen the project file (FWFW01.prj) with the HEC-RAS program. Select "plan 02" from the Steady Flow Analysis window and then close this window.

Click the "Profile Table" button from the HEC-RAS main window. The Profile Output Table window will appear. The results from plan 02 are shown in Figure 3. They are from the user-defined table FWFW01. (Creation of the user-defined table is explained under the Problem section.)

Figure 3. The Results from Plan 02

You will notice that EncStaL is the same as ChStaL and EncStaR is the same as ChStaR at RS 2700 and RS 2650 because of the use of Encroachment Method 4 at these river stations. The EncrWD values in the above figure are computed by subtracting the EncStaL from EncStaR. Because the EncStaL and EncStaR are the same as ChStaL and ChStaR respectively, EncrWD is the same as the channel width at RS 2700 and RS 2650. The TopWdthAct values in the above table are smaller than the EncrWD (channel width) values at RS 2700 and RS 2650. This means the 1% annual chance flood is contained within the channel at these river stations. The TopWdthAct is the water-surface width across the cross section for the given flood. At RS 2700 and RS 2650, the EncrWD (channel width) values will be used as the regulatory floodway width instead of the TopWdthAct values because the 1% annual chance flood is contained within the channel.

You will also notice that the velocities under the Vel Total column for RS 2700 and 2650 are much higher than the other cross sections. This is because the flow area of RS 2700 and RS 2650 under the Area column is much smaller than the other cross sections. This can create hazardous conditions within the channel area. FEMA defines the minimum floodway width to be equal to the channel width to prevent unsound development within the channel banks, which are unstable areas.

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