Quantcast RETARDS

Indirect Techniques for Erosion Protection
The method to be used to construct the dikes may also influence the choice of
crown width. If land-based equipment is to be used, but the area where a dike is to
be constructed is underwater or otherwise impassable, specifying a crown wide
enough for the operation of hauling and handling equipment should be considered,
since the additional crown width will strengthen the dike as well as expediting
construction. Whether this is cost effective for a given structure will depend on the
capabilities of the work force, the cost of stone, and the height of the structure, since
the additional volume of stone required for a wider crown will increase exponentially
with the height of the structure.
The slope of natural repose can be specified for side slopes of stone dikes. Providing
extra stone to launch into any scour hole that may occur adjacent to the structure can
be accomplished more efficiently by increasing the crown width, as discussed in
"Structural scour protection" in 8.1.1 above, than by attempting to construct a flatter
side slope to accomplish the same purpose. Specifying the slope of natural repose
simplifies construction, because then only the elevation and crown width of a dike
require control in the latter stages of construction, which is especially advantageous
if the side slopes of a dike are underwater. For pre-construction estimates of stone
quantities, the slope of natural repose is commonly assumed to be 1 vertical on 1.5
horizontal, although some variation can be expected depending on stone gradation,
construction procedures, and site conditions.
The slope of the riverward end of a stone dike is often designed flatter than the slope
of natural repose, as discussed in "Structural scour protection" in 8.1.1.
Dikes with a core of earth or other material, with an armor on the surface, are not
commonly used because they provide a smaller factor of safety against unanticipated
scour and other severe hydraulic conditions than do sturdier structures. Baird and
Klumpp (1992) report scour problems with such dikes on the Rio Grande River. A
filter of some type between the core material and the armor is likely to be required,
which increases the cost. Also, construction of this type of dike underwater is not
usually practicable. In spite of these shortcomings, the potential for cost savings may
be considerable if the cost of stone or other conventional dike materials is very high.
The relative advantages and disadvantages of retards were compared to dikes in
Section 8.1.1.
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