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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
View Plan View
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
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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- Spring '13