PERMEABLE DIKES (cont.)

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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