The Process
Background investigations include assimilating data and information in areas such as watershed
geology, sediment, geography, hydrology, and historical channel stability. Background investigations
provide a historical perspective in which past behavior is examined to provide some indication of the future
trends in channel adjustments.
Geomorphic assessments provide the process-based framework to define past and present
watershed dynamics, develop integrated solutions, and assess the consequences of remedial actions. This
is an essential part of the design process, for either local stabilization treatment or development of a
comprehensive plan for an entire watershed.
The existing-condition status of the watershed and channel is determined through field investigations.
The field tasks focus on the study area. These tasks involve assimilating the necessary field samples, data,
and observations that are required to support analysis techniques. These investigations provide a qualitative
description of existing channel processes and bank characteristics, as well as quantitative data related to
sediment characteristics (types, gradations, and transport), channel hydraulics, and system stability.
Ultimately, the background and field investigation data are combined with computational methods
for stable channel design. The result of the initial cycle through the systems approach provides information
on channel hydraulics and sediment transport characteristics, watershed dynamics, and system stability
necessary to begin the preliminary design phase of channel rehabilitation.
2.4
PRELIMINARY DESIGN
Figure 2.4 describes the sequence of events for preliminary design. At this juncture, the initial stable
channel design from the systems approach is evaluated against proposed goals. If goals are satisfied by
the existing condition design, then no further work is required, and the work can proceed to the design of
local stabilization and habitat enhancement features. If the goals are not satisfied due to system instability
or a need to modify design parameters to meet project goals, an iterative design process is initiated in which
design parameters such as channel forming discharge and stable channel dimensions are re-evaluated, and
measures for restoring stability such as grade control, bank stabilization, and planform properties are
considered.
The preliminary design phase uses the data, information, findings, and analysis techniques presented
in Chapter 5 to formulate a stable channel design that will meet project goals. As shown in the preliminary
design process flow chart (Figure 2.4), the process can be limited in scope if it is determined, after
background and field investigations, that the system is stable and project goals can be met without
additional design efforts. At this point, detailed design procedures can commence. More often than not,
this is not the case. If, upon initial investigations, channel instabilities are present, or project goals cannot
be accomplished with the existing conditions, a preliminary design process must be undertaken to develop
a channel design which will achieve the desired goals.
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