Selection and Design of Channel Rehabilitation Methods
appropriate solutions to local instability problems; however, when system-wide channel degradation exists,
a more comprehensive treatment plan must be implemented utilizing a combination of bank stabilization and
grade control. Therefore, design of an effective grade control structure, or a system of structures, requires
at a minimum, a common definition of stability, and a design procedure that relies on the balance between
supply and transport of a desired sediment yield from the upstream watershed and channel system. As will
be shown later in this chapter, reliance on emprical design relationships for grade control structures is self-
defeating.
Equally important to the design fundamentals is an understanding of the functions of a grade control.
In the broadest sense, the term grade control can be applied to any alteration in the watershed which
provides stability to the streambed. The most common method of establishing grade control is the
construction of in-channel grade control structures. There are basically two functions of grade control
structures. One type of structure is designed to provide a hard point in the streambed that is capable of
resisting the erosive forces of the degradational zone. This is somewhat analogous to locally increasing the
size of the bed material. Lane's relation (Section 3.4.1) would illustrate the situation by QS+ % QsD50+,
where the increased slope (S+) of the degradational reach would be offset by an increase in the bed
material size (D50+). For this discussion, this will be referred to as a Bed Control Structure. The other
structure is designed to function by reducing the energy slope along the degradational zone (QS- % QsD50).
This will be referred to as a Hydraulic Control Structure. The distinction between the processes by which
these structures operate is important whenever grade control structures are considered.
Because of the complex hydraulic behavior of grade control structures, it is difficult to designate
a single function that will apply without exception to each structure. For many situations, the function of
a structure as either a bed control structure or hydraulic control structure is readily apparent. However,
there may be circumstances where a single function of a structure as strictly a bed control or hydraulic
control structure may be less evident and, in many cases, the structure may actually have characteristics of
both. It also must be recognized that the hydraulic performance and, therefore, the function of the structure,
can vary with time and discharge. This can occur within a single hydrograph or over a period of years as
a result of upstream or downstream channel changes.
6.2.1
TYPES OF GRADE CONTROL STRUCTURES
There are certain features which are common to most grade control structures. These include a
control section for accomplishing the grade change, a section for energy dissipation, and protection of the
upstream and downstream approaches. However, there is considerable variation in the design of these
features. For example, a grade control structure may be constructed of riprap, concrete, sheet piling,
treated lumber, soil cement, gabions, compacted earth fill, or other locally available material. Also, the
shape (sloping or vertical drop) and dimensions of the structure can vary significantly, as can the various
appurtenances (baffle plates, end sills, etc.). The applicability of a particular type of structure to any given
situation depends upon a number of factors such as: hydrologic conditions, sediment size and loading,
channel morphology, floodplain and valley characteristics, availability of construction materials, project
objectives, and time and funding constraints. The successful use of a particular type of structure in one
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