Fundamentals of Engineering Design
where qtv is the unit channel width total load transport in volume per time. The term Gr is the gradation
coefficient calculated as follows:
D84
D50
1
Gr '
@
%
(5.35)
2
D50
D16
Equation (5.34) is dimensional. The particle size, D 0, is in millimeters and all other variables are in
5
standard English units.
5.3.7.2 Model Application
The stable channel design procedure developed by Firenzi (1998) simultaneously solves equations
of flow resistance and sediment transport. The Manning equation is used as a relationship between
roughness and hydraulic geometry. The program user may specify one of two sediment transport
relationships: the Meyer-Peter and Mller bedload equation or the Simons, Li and Fullerton total load
equation. These equations have been reviewed in the previous section. The program partitions the design
cross section into three subsections according to Einstein (1950). Partitioning the cross section in this
manner allows for Manning's n to be specified separately for the bed and banks, where the Limerinos
(1970) equation is used for the channel bed. Allowing Manning's n to be different for the bed and banks
of the channel makes the program applicable to small rivers where it is not valid to neglect the effects of
bank roughness.
Three degrees of freedom are assumed in the method: width, depth, and slope. In the absence of
a third equation to satisfy the three degrees of freedom, a table of solutions is generated by the program.
It is left to the user to incorporate a third governing principle. The minimum slope can be selected as an
extremal hypothesis design according to Chang (1980). Note that minimum slope corresponds to minimum
stream power when a single design discharge is used. A stable channel curve can be used to graphically
evaluate the design at minimum slope. Figure 5.43, presented in Section 5.3.6.5 is an example of a stable
channel curve.
5.3.8
HEC-6
HEC-6 is a one-dimensional numerical model designed to simulate and predict changes in river
profiles due to scour and/or deposition over average time periods. The model is based on movable
boundary, open channel flow hydraulics with time periods normally in years, although single flood events
with days or months are also possible. Various features in HEC-6 include: network stream analysis
capability, channel dredging, assortment of levee and encroachment alternatives, and several methods for
computing sediment transport rates (USACE, 1993). The following sections present an overview of the
computational process and the four input categories: geometry and hydraulics, sediment, hydrology, and
special commands.
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