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.
  3. 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.
  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 9-8.
  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. 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.

Figure 9 8 Unsteady Flow Encroachment Editor