Indirect Techniques for Erosion Protection
8.1.2 PERMEABLE DIKES
8.1.2.1 Advantages
The advantages of permeable dikes as compared to impermeable dikes are that they
are equally, if not more effective when used on streams with relatively high concentrations
of suspended sediment, and are often less costly.
8.1.2.2 Disadvantages
The disadvantages are that they are less durable than stone dikes and some other
impermeable dike materials, and are usually considered less aesthetic, although the visual
impact may ultimately be lessened by the growth of vegetation.
8.1.2.3 Design Considerations
Design considerations beyond those general considerations discussed previously for
dikes involve materials, structural design, and miscellaneous items.
(a) Posts and piles for permeable dikes, and the main members of jacks, may be wood, steel,
or concrete. The economic feasibility of using treated wood for decay prevention is a
project-specific decision, as discussed by Petersen (1986). However, water quality
considerations may preclude the use of some preservatives. Some early jack designs
were patented, and although their use has become practically generic, the present legal
status of these patents is unclear. Other shapes, such as tetrahedrons, are sometimes
used. The function of tetrahedrons is identical to jacks, but they are stronger and more
expensive than jacks made of the same components.
At least one proprietary design of permeable dikes exists, called "Palisades."
They are constructed of panels of synthetic netting attached to pipes driven into the
stream bank and bed. The panels can slide down the pipes to adjust to changing
contours of the bank and bed.
Anchored trees or brush provide an "all-in-one material."
The primary
shortcomings are durability, and in some regions, availability.
The most common facings are boards and wire fencing of various types. For pile
dikes in deep streams, the piles are closely spaced without a separate facing material
(Peterson, 1986). This design retains the original permeability ratio even if the bed
beneath a dike scours, as long as the dike does not fail from loss of pile penetration, and
it also makes construction of a permeable dike practical even in fairly deep water.
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