Quantcast Design Considerations (Cont.)

Indirect Techniques for Erosion Protection
scour along the face of the dike without breaching the crest elevation, and
a slope at the end of the dike which is significantly flatter than the slope of
natural repose of stone.
Specify extra penetration for pile structures so that scour will not fail the
piling. However, unless the facing of the dike can adjust downward with
the scour, or the dike is constructed entirely of driven piling this approach
detracts from performance, since the total permeability of the structure is
increased as the bed beneath the structure erodes. More flow is allowed to
pass through the structure, and the scour may endanger bank stability. This
is likely to be an expensive approach as well. For example, required pile
penetration for one dike design on the Sacramento River was computed to
be 13 feet if protected from scour, 34 feet if unprotected. Even if the dikes
are constructed with adjustable facing which displaces downward with bed
scour, maintenance of the design elevation by adding more facing will be
required, unless the original design provided for lowering of the effective
height of the structure.
Add structural features such as an L-head, "hockey stick," or T-head
(sometimes called "hammerhead"), in order to move the scour away from
the dike proper. This approach actually coalesces into a retard design if
carried to the extreme of affecting overall flow rather than just local scour.
Also, the scour around the added feature itself must still be addressed by
one of the other approaches. A similar approach was used on some
Mississippi River stone dikes in the 1960's, in the form of stone "rib spurs"
built intermittently along the upstream face of dikes which were
experiencing loss of stone due to launching into the scour hole caused by
lateral flow along the upstream face. There was no conclusive evidence that
this attempt to move the scour away from the dike was more cost-effective
than simply adding additional stone to the dike cross-section to compensate
for the launching, and the practice was soon discontinued.
Use a dike design that will maintain contact with the bed as scour occurs.
Examples of this approach are jacks, "Palisades," tire-post dikes, and
anchored trees.
Use a hydraulically smooth design for the end of impermeable dikes, and
round structural members for permeable dikes (FHWA, 1985). However,
this alone is not likely to be sufficient if the dike intercepts much flow.
A safety factor is sometimes added by using two or more of the above
approaches in combination. Examples are a dike structure designed to maintain bed
contact, along with armor or mat to limit scour; or extra pile penetration at the end of
the dike, along with armor or mat for the full length of the dike as well as beyond the tip.
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