Fundamentals of Engineering Design
separated into those caused by rainfall and those due to fluvial processes of the stream. Streambank
erosion and failure processes are discussed in Section 3.4.3.2.
5.3
COMPUTATIONAL METHODS FOR STABLE CHANNEL DESIGN
A preliminary channel design based on stability evaluation should be conducted early in project
planning to screen out alternative designs that would present serious stability problems and to identify future
needs. As planning progresses, successive evaluations with increasing detail may be required. This
approach is essential to insure that the final channel design thoroughly addresses stability problems thus
avoiding costly future channel maintenance efforts.
Channel design computations are based on a design discharge. The design discharge can be based
on a computed hydrological event such as a 10-year storm event, or it can be based on the channel forming
discharge which is responsible for shaping channel morphology. This section presents both preliminary and
detailed design methods based on the channel forming discharge. Section 5.3.1 presents a detailed
description of how to compute the channel forming discharge. Sections 5.3.2 - 5.3.5 present preliminary
design methods, while Sections 5.3.6 - 5.3.9 present more computationally intensive methods for a detailed
design.
5.3.1
CHANNEL FORMING DISCHARGE
An alluvial river adjusts the bankfull shape and dimensions of its channel of the wide range of flows
that transport boundary sediments. However, for stable channels that are in equilibrium, a single, dominant
flow can be identified which would produce a channel with the same morphological characteristics as the
natural sequence of events. The concept of a single prevailing water and sediment discharge to which the
river width, depth, slope, hydraulic roughness, and planform are adjusted is an attractive simplification. The
single discharge can be used to assess general trends between channel morphologic characteristics and a
single discharge. The single prevailing discharge represents a spectrum of discharges and is referred to as
the channel forming discharge. The best situation for design would be to have gaged-defined water stage
and discharge, and sediment discharge relationship defined at each site. In channel stabilization projects,
the channel will be unstable, and therefore, it is unlikely a gage will exist.
Appendix A is a thorough examination of the proper computation procedure for effective discharge.
5.3.2
SLOPE-DRAINAGE AREA CURVE
The slope-drainage area curve is an empirical regional stability relationship that defines the stable
channel slope, or equilibrium slope, as a function of drainage. The relationship is developed by field
assessment to determine stable channel reaches. The slope of the stable reaches are determined by field
survey and the drainage area at each stable site is determined from topographic maps. This slope is used
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