This table includes the change in water surface

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Unformatted text preview: a map. This table includes: the change in water surface elevations from the first profile; the top width of the active flow area; the percentage of conveyance reduction in the left overbank; the left encroachment station; the distance from the center of the main channel to the left encroachment station; the station of the center of the main channel; the distance from the center of the main channel to the right encroachment station; the right encroachment station; and the percentage of conveyance reduction in the right overbank. An example of the Encroachment 2 standard table is shown in Figure 10.6. 10-9 Chapter 10 Performing a Floodway Encroachment Analysis Figure 10.6 Example of the Encroachment 2 Standard Table The last encroachment table, Encroachment 3, provides the minimum floodway data for reporting. This table includes: the active flow top width; the flow area (including any ineffective flow area); the average velocity of the entire cross section; the computed water surface elevation; the base water surface elevation (profile 1); and the change in water surface from the first profile. An example of this table is shown in Figure 10.7 10-10 Chapter 10 Performing a Floodway Encroachment Analysis Figure 10.7 Example of the Encroachment 3 Standard Table Floodway Encroachments With Unsteady Flow Encroachment analyses can also be performed with the unsteady flow computations module within HEC-RAS. However, only method one (user placed encroachments) has been added to the unsteady flow computations. A suggested methodology for performing an encroachment analysis with an unsteady flow model is the following: 1. First, develop the unsteady flow model of the river system and calibrate it to the extent possible. 2. Develop an unsteady flow plan of the 100 yr event in order to establish the base floodplain. 10-11 Chapter 10 Performing a Floodway Encroachment Analysis 3. 4. Perform a steady flow encroachment analysis using the available steady flow encroachment methods to calculate an approximate floodway. 5. Copy the unsteady flow plan to a new plan (using the Save As option), and give it a name that represents the encroached plan. 6. Adjust the boundary conditions file to reflect an increased water surface elevation at the downstream boundary for the range of possible flows. If using a rating curve, you will need to develop a new rating to reflect the encroached condition at the downstream boundary. If you are using normal depth or critical depth, no change is necessary, since the program will calculate a new water surface with the encroachments. 7. Go to the Options menu of the unsteady flow analysis window and select Unsteady Encroachments. This will bring up the Unsteady flow Encroachment editor shown in Figure 10.8. 8. Import the final encroachments from the steady flow encroachment run in to the unsteady flow encroachment editor. This is accomplished by pressing the button labeled “Get Encroachments From Steady Flow Plan”, and then selecting the appropriate plan and profile number from the steady flow encroachment analysis. 9. Run the unsteady flow model with the encroachments and compare the output of the encroached unsteady flow plan with the output from the base unsteady flow plan. 10. 10-12 Develop a steady flow plan that incorporates the peak flows from the unsteady flow run as the 100 yr event for the model. Set up the model for two profiles with the same flows. Adjust the encroachments as necessary to stay within the limits for increased water surface elevations. Re-run the unsteady flow model. Repeat this process until a final floodway is achieved. Chapter 10 Performing a Floodway Encroachment Analysis Figure 10.8 Unsteady Flow Encroachment Editor. 10-13...
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This note was uploaded on 03/16/2013 for the course ENGINEERIN 401 taught by Professor Tommas during the Spring '13 term at University of Alberta.

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