Quantcast Design Considerations (Cont.)

 
  
 
Indirect Techniques for Erosion Protection
structures can be accepted. That guidance is then qualified by a recommendation that
impermeable dikes not be used in bends. However, that pessimistic viewpoint may have
been influenced by unsuccessful use in sharp radius bends, or by failures due to
inadequate bankhead design.
Even if one of the approaches discussed above is used to quantify spacing, the
location of individual dikes may need to be modified according to site conditions. For
example, the project site may have localized "plunge pools" or "shelves" because of
variations in bed or bank material, or other local anomalies. If so, dike locations can
perhaps be adjusted so that no one dike requires a large volume of material or unusually
long piling, or conversely, so that no one dike is built with insufficient volume of material
or pile penetration to be stable against future local scour.
If dike spacing is determined by using an approach based on projections of
tangents to streamlines or to the thalweg, the engineer should be aware that if the
channel upstream of the project is migrating, the alignment of incoming flow and the
thalweg may change with time. A conservative approach would be advisable in such
cases if the predicted future condition will result in a more direct impingement of flow
on the bank which is to be protected.
(d) The optimum angle that dikes should have with respect to the direction of flow is a
subject upon which there is much disagreement. The controversy may be due to the
influence of less obvious, and perhaps overlooked, factors overriding the effect of angle
at a specific site. In the absence of compelling evidence to the contrary, dikes which are
constructed on the shortest path from the bankline to the desired new channel alignment
will be the shortest, thus the cheapest. Usually, this path will be approximately
perpendicular to flow, or the bankline, or a compromise between the two. FHWA
(1985) suggests that angle is not critical to permeable dikes, but that better performance
may be obtained with impermeable dikes if the upstream dike in a system is constructed
at an angle of about 150 degrees, with subsequent dikes having successively smaller
angles, reaching a minimum of 90 degrees for the downstream dike. Whether results are
better to the extent of outweighing the additional cost for longer structures is a matter
for debate.
Permeable dikes are sometimes angled downstream to shed debris and ice,
although if debris and ice loads are consistently heavy, permeable dikes may not be the
appropriate protection method to begin with. In any event, the "shedding" effect should
be considered to be only an additional safety factor, and should not lead to disregarding
debris and ice loads in structural design.
Contrary to intuition, dikes angled downstream may form downstream scour
holes nearer to the bank than if they were perpendicular to the bank or angled upstream
to the flow, because overtopping flows will tend to form an erosive "roller," or plunging
flow, immediately adjacent and parallel to the structure, to the detriment of bank
stability.
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