For a detailed discussion on each of these methods

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Unformatted text preview: in HEC-RAS. For a detailed discussion on each of these methods, the user is referred to Chapter 10 of the Hydraulic Reference Manual. The goal of performing a floodplain encroachment analysis is to determine the limits of encroachment that will cause a specified change in water surface elevation. To determine the change in water surface elevation, the program must first determine a natural profile with no encroachments. This base profile is typically computed using the one percent chance discharge. The computed profile will define the floodplain, as shown in Figure 10.1. Then, by using one of the 5 encroachment methods, the floodplain will be divided into two zones: the floodway fringe and the floodway. The floodway fringe is the area blocked by the encroachment. The floodway is the remaining portion of the floodplain in which the one-percent chance event must flow without raising the water surface more than the target amount. 10-2 Chapter 10 Performing a Floodway Encroachment Analysis ∆ Water Surface Natural Water Surface Encroached Water Surface Cross Section View Plan View Main Channel Floodway Fringe Floodway Floodway Fringe 100 - Year Floodplain Figure 10.1 Floodway Definition Sketch Entering Floodplain Encroachment Data Within HEC-RAS, the data for performing a steady flow floodplain encroachment analysis are entered from the Steady Flow Analysis window. Encroachment information is not considered as permanent geometry or flow data, and is therefore not entered as such. The encroachment information is saved as part of the existing Plan data. To bring up the floodplain encroachment data window, select the Encroachments option from the Options menu of the Steady Flow Analysis window. When this option is selected an Encroachment window will appear as shown in Figure 10.2 (except yours will be blank when you first open it). As shown in Figure 10.2, there are several pieces of data that the user must supply for an encroachment analysis. The encroachment analysis can only be performed for profiles 2 through 15 (or what ever number has been set by the user in the flow data editor). Encroachments are not performed on profile one because most of the encroachment methods rely on having a base profile for comparison. 10-3 Chapter 10 Performing a Floodway Encroachment Analysis Figure 10.2 Floodplain Encroachment Data Editor The data for an encroachment analysis should be entered in the following manner: Global Information. Global information is data that will be applied at every cross section for every profile computed. The first piece of global information is the Equal Conveyance Reduction selection box at the top of the Encroachment data editor window. Equal conveyance reduction applies to encroachment methods 3, 4, and 5. When this is turned on, the program will attempt to encroach, such that an equal loss of conveyance is provided on both sides of the stream. If this option is turned off, the program will encroach by trying to maintain a loss in conveyance in proportion to the distribution of natural overbank conveyance....
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