Quantcast CHAPTER 12: GRADE STABILIZATION

 
  
 
CHAPTER 12
GRADE STABILIZATION
12.1 GRADE CONTROL CONCEPTS
Implementation of bank stabilization measures without proper consideration of the
stability of the bed can result in costly maintenance problems and failure of structures. For
this reason, it is essential to consider the stability of the bed as part of any bank stabilization
scheme. Bank stabilization measures are generally appropriate solutions to local instability
problems, such as erosion in bendways. However, when system-wide channel degradation
exists, a more comprehensive treatment plan, which usually involves some form of grade
control, must be implemented.
In the widest sense, the term "grade control" can be applied to any alteration in the
watershed which provides stability to the streambed. By far the most common method of
establishing grade control is the construction of in-channel grade control structures. There are
basically two types 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 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
type of structure is designed to function by reducing the energy slope along the degradational
zone to the point that the stream is no longer capable of scouring the bed (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 develop an "ideal" classification scheme that will apply without exception to all situations.
For many situations, the classification of a structure as either a bed control structure or
hydraulic control structure is readily apparent. However, there may be circumstances where
a distinct classification 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
classification of the structure, can vary with time and discharge. This can occur within a
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