or other improvements that would significantly alter the post-project water and sediment loads, then it may
be necessary to calculate a new channel forming discharge. In these instances, the new channel forming
discharge would have to be calculated using the effective discharge analysis, or a specified recurrence
interval flow based on post-project conditions. The use of the bankfull discharge would not be appropriate
in these situations, because the bankfull morphology would reflect pre-project morphology, and would not
be correlated with the post-project flows. The new post-project channel forming discharge is used in the
determination of the stable channel dimensions discussed in the next step.
PRELIMINARY DESIGN METHODOLOGY - DETERMINATION OF STABLE
Computation of stable channel dimensions can be accomplished with a number of channel design
methods (Chapter 5, Sections 5.3.2 - 5.3.7). The selection of the appropriate method is a function of a
number of factors such as level of study (reconnaissance, feasibility, detailed design, etc), funding and time
constraints, complexity of project and stream characteristics, consequences of failure of the design, and
available data. For instance, during early recon studies, it may be appropriate to utilize some of the less
computationally intense empirical methods. However, as the level of study increases, it might be necessary
to conduct more rigorous analyses using SAM, HEC-6, or other numerical methods. It should also be
noted that in some situations it may not be necessary to calculate all three variables. For example, if the
rehabilitation plan simply calls for the layout of a series of grade control structures to stabilize the channel,
then it may only be necessary to calculate a stable slope to be used in spacing the structures.
The following is a brief summary of the applicability of the various methods discussed in
Sections 5.3.2 through 5.3.7.
Maximum permissible velocity and tractive force. The maximum permissible velocity and
tractive force design methods were discussed in Chapter 5, Sections 5.3.2 and 5.3.3. These methods are
most applicable to reconnaissance level studies where the purpose is to quickly assess various alternatives.
Neither of these methods specify a complete design channel design because they can be satisfied by various
combinations of width, depth and slope. Additionally, these two methods are generally not applicable to
situations where a significant bed material load exists.
Regime and hydraulic geometry. The regime theory
of channel design
relationships developed from field data collected from numerous river and canal systems (Chapter 5,
Section 5.3.4). The USACE regime method (USACE, 1994) recommends using locally or regionally
developed equations for channel design. If this is not available, graphs are provided for estimating width,
depth, and slope of the channel given the channel forming discharge and bed material description. As with
the permissible velocity and tractive force methods, the regime approach is more suited to reconnaissance
level studies, and caution is advised when attempting to use these methods for detailed design. The reader
is referred to the limitations discussed in Section 5.3.4.