Grade Stabilization
single hydrograph or over a period of years as a result of upstream or downstream channel
changes.
12.2
DESIGN CONSIDERATIONS FOR SITING GRADE CONTROL
STRUCTURES
Design considerations for siting grade control structures include determination of the
type, location and spacing of structures along the stream, along with the elevation and
dimensions of structures. Siting grade control structures is often considered a simple
optimization of hydraulics and economics. However, these factors alone are usually not
sufficient to define the optimum siting conditions for grade control structures. In practice, the
hydraulic considerations must be integrated with a host of other factors, which vary from site
to site, to determine the final structure plan. Some of the more important factors to be
considered when siting grade control structures are discussed in the following sections.
12.2.1 HYDRAULIC CONSIDERATIONS
One of the most important steps in the siting of a grade control structure or a series
of structures is the determination of the anticipated drop at the structure. This requires some
knowledge of the ultimate channel morphology, both upstream and downstream of the
structure which involves assessment of sediment transport and channel morphologic
processes.
The hydraulic siting of grade control structures is a critical element of the design
process, particularly when a series of structures is planned. The design of each structure is
based on the anticipated tailwater or downstream bed elevation which, in turn, is a function
of the next structure downstream. Heede and Mulich (1973) suggested that the optimum
spacing of structures is such that the upstream structure does not interfere with the deposition
zone of the next downstream structure. Mussetter (1982) showed that the optimum spacing
should be the length of the deposition above the structure which is a function of the
deposition slope (Figure 12.1). Figure 12.1 also illustrates the recommendations of Johnson
and Minaker (1944) that the most desirable spacing can be determined by extending a line
from the top of the first structure at a slope equal to the maximum equilibrium slope of
sediment upstream until it intersects the original streambed.
Theoretically, the hydraulic siting of grade control structures is straightforward and
can be determined by:
H = (So - Sf)x
(12.1)
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