|
|
||
 Indirect Techniques for Erosion Protection
8.1.4.1 Typical Application
The typical application of retards is where the channel is to be realigned, but the bend
curvature, bank erodibility, debris load, or hydraulic conditions are too severe for dikes to be
effective or economical. In some cases where channel realignment is not a factor, retards may
be the preferred method if less expensive than bank armoring.
8.1.4.2 Design Considerations
Design considerations for retards beyond those discussed in Section 8.1 involve
location, height, and tiebacks.
(a) If a change in channel alignment is not required, the preferred location for the retard
from the standpoint of economy and efficiency is at a point slightly riverward from the
toe of the bank slope. The location of the retard in plan view is determined by identifying
that point on surveyed bank cross-sections, then plotting on a plan view that point's
location at each cross-section. A smooth alignment can then be drawn through those
points which "control" the overall alignment. Those points will be the ones which are
farthest out in the channel. If the existing bank alignment is fairly smooth, then the
retard alignment will pass through or near all the "preferred" points. However, if the
existing alignment is irregular, then the retard alignment must necessarily lie riverward
of many of the preferred points. If a pronounced single irregularity causes the retard to
be located unacceptably far out in the channel upstream and downstream of the
irregularity, then the alternative is to smooth the bankline irregularity by excavation.
(b) The height, or elevation, of the retard is determined by considering the factors discussed
previously for dikes. The elevation of the retard can be varied around a bend as the
attack against the bank and/or as the erodibility of material varies. This complicates
design and construction somewhat, and is seldom done, but does have the potential to
increase the efficiency of the design. The United Nations (1953) described some
European work as having the retard highest at the apex of the bend, sloping downward
to a minimum elevation at the upstream and downstream ends. A concern about that
approach, however, would be that the downstream limb of a bend is often where the
attack against the bank is greatest at higher flows, and the risk of a low elevation there
is greater than for a low elevation at the upstream end. This is especially true after the
work has been in place long enough for the normal downstream movement of scour
pools and bars to have increased the hydraulic forces along the downstream portion of
a retard in a bend.
(c) Tiebacks (sometimes called "baffles") are mandatory where the retard is located well in
front of the bank and in short radius bends, and are recommended in all cases. For
simplicity of design and construction, they are often of the same structural design as the
retard, but can be of a less costly design if site conditions permit a less conservative
approach. The length of tiebacks is determined by the distance from the bank to the
210
2 0
|
||
|
||
