Quantcast EFFECTS ON TRIBUTARIES

 
  
 
Grade Stabilization
12.2.9 EFFECTS ON TRIBUTARIES
The effect of main stem structures on tributaries should be considered when siting
grade control structures. As degradation on a main stem channel migrates upstream it may
branch up into the tributaries. Therefore, the siting of grade control structures should consider
effects on the tributaries. If possible, main stem structures should be placed downstream of
tributary confluences. This will allow one structure to provide grade control to both the main
stem and the tributary. This is generally a more cost effective procedure than having separate
structures on each channel.
12.2.10 SUMMARY
The above discussion illustrates that the siting of grade control structures is not simply
a hydraulic exercise. Rather, there are many other factors that must be included in the design
process. For any specific situation, some or all of the factors discussed in this section may
be critical elements in the final siting of grade control structures. It is recognized that this
does not represent an all inclusive list since there may be other factors not discussed here that
may be locally important. For example, in some cases, maintenance requirements, debris
passage, ice conditions, or safety considerations may be controlling factors. Consequently,
there is no definitive "cookbook" procedure for siting grade control structures that can be
applied universally. Rather, each situation must be assessed on an individual basis.
12.3 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 situation does not necessarily ensure it will be effective in
another. Some of the more common types of grade control structures used in a variety of
situations are discussed in the following sections. For more information on various structure
designs, the reader is referred to Neilson et al. (1991), which provides a comprehensive
international literature review on grade control structures with an annotated bibliography.
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