Quantcast Design Considerations (Cont.)

 
  
 
Indirect Techniques for Erosion Protection
(f) Dike bankheads must be designed so that erosion does not flank the structure; that is,
disconnect it from the bank. Some local erosion is acceptable, but it must be limited.
There are two basic approaches:
(1) Excavate a trench into the bank and extend the dike back into the trench (called the
dike "root").
(2) Pave the downstream bank with an armor, and if conservative design is called for,
also pave a lesser distance upstream. This usually involves grading the bank and
placing riprap.
Specific guidance here is at least as difficult as for other dike design parameters.
The best guide, unfortunately, is previous experience in similar circumstances, which is
no comfort if similar experience is lacking. The difficulty lies in predicting velocity fields
and the depth and precise location of the scour hole which will develop at an unprotected
bankhead. For very expensive hydraulic structures, this difficulty is often resolved by
large-scale physical models, which is usually impractical for bank protection projects.
The following are "rules of thumb" based on experience, but they cannot be
considered formal guidance:
For dikes in straight reaches, approach (1) above involves extending the
dike root into the bank a minimum distance equal to the bank height. If the
depth at high flow of local scour holes in the adjacent area, such as around
erosion-resistant bank material or other obstructions to flow, can be
observed or estimated, a more conservative approach is to extend the root
into the bank a distance of the bank height plus that scour depth. If eroded
"eddy pockets" downstream of existing protrusions into the channel are
observed, the root should be at least as long as the maximum landward
extent of those pockets. For areas of severe erosion, such as in bends, the
root should be longer. Examples of extremes from practice: A root length
of 300 feet is commonly used on Mississippi River dikes, but as little as 10
feet has been successful on very small tributary streams.
USACE Mead Laboratory model tests described in USACE (1981) suggest
that lateral erosion between dikes, thus required dike root length for
approach (1), is related to stream depth (or bank height), velocity of flow,
and dike length.
When using approach (1), backfilling over the dike root, routing surface
drainage away from the backfilled area, and vegetating the disturbed area
will help prevent post-construction erosion and will improve the aesthetics
of the project. Design of the backfill can be simple or sophisticated,
depending upon specific site conditions. The simplest approach is simply to
replace the excavated material in the most expeditious way (with due
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