Quantcast EXISTING STRUCTURES

 
  
 
Grade Stabilization
important concern, openings, fish ladders, or other passageways must be incorporated into
the design of the structure to address the fish movement problems (Nunnally and Shields,
1985). The various methods of accomplishing fish movement through structures are not
discussed here. Interested readers are referred to Nunnally and Shields (1985), Clay (1961),
and Smith (1985) for a more detailed discussion.
The environmental aspects of the project must be an integral component of the design
process when siting grade control structures. A detailed study of all environmental features
in the project area should be conducted early in the design process. This will allow these
factors to be incorporated into the initial plan rather than having to make costly and often less
environmentally effective last minute modifications to the final design. Unfortunately, there
is very little published guidance concerning the incorporation of environmental features into
the design of grade control structures. One source of useful information can be found in the
following technical reports published by the Environmental Laboratory of the Corps of
Engineers, WES (Shields and Palermo, 1982; Henderson and Shields, 1984; and Nunnally and
Shields, 1985).
12.2.5 EXISTING STRUCTURES
Bed degradation can cause significant damage to bridges, culverts, pipelines, utility
lines, and other structures along the channel perimeter. Grade control structures can prevent
this degradation and thereby provide protection to these structures. For this reason, it is
important to locate all potentially impacted structures when siting grade control structures.
The final siting should be modified, as needed, within project restraints, to ensure protection
of existing structures.
It must also be recognized that grade control structures can have adverse as well as
beneficial effects on existing structures. This is a concern upstream of hydraulic control
structures due to the potential for increased stages and sediment deposition. In these
instances, the possibility of submerging upstream structures such as water intakes or drainage
structures may become a deciding factor in the siting of grade control structures.
Whenever possible, the engineer should take advantage of any existing structures
which may already be providing some measure of grade control. This usually involves
culverts or other structures that provide a non-erodible surface across the streambed.
Unfortunately, these structures are usually not initially designed to accommodate any
significant bed lowering and, therefore, can not be relied on to provide long-term grade
control. However, it may be possible to modify these structures to protect against the
anticipated degradation. These modifications may be accomplished by simply adding some
additional riprap with launching capability at the downstream end of the structure. In other
situations, more elaborate modifications such as providing a sheet pile cutoff wall or energy
dissipation devices may be required. Damage to and failure of bridges is the natural
consequence of channel degradation. Consequently, it is not uncommon in a channel
stabilization project to have several bridges that are in need of repair or replacement. In
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