Quantcast Design Considerations

 
  
 
Indirect Techniques for Erosion Protection
Hardware and fasteners, such as nails, bolts and cable, will be largely dictated by
the choice of other materials. Corrosion resistant hardware must be used unless the
work is temporary.
(b) Structural design is an iterative process. The goal is to achieve the required height and
permeability in the most economical way, considering the cost of materials and the
construction techniques that will be used. The variables for fence-type permeable dikes
are:
Lateral loads (drag force of current, impact of debris);
Spacing, size, and penetration of piles;
Size of sub-components (boards, fencing, cables, anchors); and
Supplementary bracing.
The vulnerability to failure from lateral loads increases with dike height, since the
moment arm of the force is greater, and the amount of debris carried by the stream, as
well as the speed of impact, is likely to increase as river stage increases. As the height
of the dike increases, this combination dictates an increase in the size of the structural
members, as well as an increase in pile penetration for those designs using driven piles.
These factors cause the cost to increase dramatically as the dike height increases.
Typical practice for penetration of piles or posts is that at least 1/2 to 2/3 of the
total length should be in the ground. Factors that influence required penetration are the
nature of the bed and sub-bed material, the potential for scour, and anticipated lateral
loads from hydraulic loading and floating debris or ice. The nature of the material
through which the piles or posts are to be driven must be known in order to determine
if driving will be feasible. Encountering unanticipated difficulties during the driving
operation may cause contractual difficulties as well as perhaps necessitating redesign of
the work.
If previous experience has developed a design that has been successful in
applications similar to the project at hand, it is more prudent to apply that experience
rather than over-extending the safe bounds of theory with numerical structural analysis
using imprecise assumptions. Figure 8.1 shows some typical designs of permeable dikes.
(c) Some miscellaneous design considerations are as follows:
The facing material should be attached to the upstream side of dikes.
Large trees which may be undermined and fall onto the dikes should be
removed. Otherwise, existing vegetation should be preserved to the
greatest extent possible. If clearing of the bank is necessary to provide
construction access, stumps should be left in the ground, since regrowth of
some species will occur.
Cuts made in treated wood members should be recoated with a preservative.
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