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.

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.

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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