Indirect Techniques for Erosion Protection
8.1.3.2 Design Considerations
Design considerations for impermeable dikes beyond the general factors discussed
previously for dikes are as follows:
(a)
Stone gradation for stone dikes is less critical than for riprap armor, which is
fortunate, because there is no widely accepted method for designing stone gradation
for dikes. Stone displacement due to scour will tend to be self-healing if the
maximum stone size is adequate, and enough stone is present.
A larger maximum stone size required for dikes than would be used for riprap
armor on the same stream, because turbulence and local acceleration of flow adjacent
to the dike creates large hydraulic forces. Also, if stone is being placed in large
depths and/or high velocities, larger sized stones will suffer less displacement as they
fall through the water column, thus control of placement is easier and the amount of
stone which falls outside the design cross-section will be reduced. The range of
maximum stone sizes commonly used in practice is from 200 pounds to 5,000 pounds,
depending on the depth of water, velocity of flow, and the amount of flow being
intercepted by the structure, all of which influence the displacement forces on the
stone and the amount of scour which will occur during and after construction.
The gradation of stone below the maximum size is dependent to a large degree
on the economics of quarrying and handling the stone. Ideally, stone will be well
graded, with a low percentage of spalls and waste particles. However, too restrictive
a gradation will increase the cost of quarrying beyond the benefits gained. In general,
the higher the cost of transporting the stone to the project site, and the more severe
the hydraulic conditions, the more justified a strictly controlled gradation, since
transportation costs for "waste" material in the stone is the same as for the high-
quality stone.
(b)
The crown width of stone dikes depends primarily on the amount of anticipated scour
adjacent to the structure (see "Structural scour protection" in 8.1.1 above) and the
height of the structure. As a practical matter, a crown width of about 2 feet is the
smallest that can feasibly be constructed, while still providing a minimal amount of
stone to launch into any scour that may occur. Crown width should be increased
beyond that if the maximum stone size is larger than 2 feet, or if significant scour
adjacent to the structure is expected and the height of the dike is so small that the
amount of stone available to launch off the downstream side slope will be insufficient
to retain an effective dike height.
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