Indirect Techniques for Erosion Protection
Rigid double-row retards are sometimes used as "cribs," filled with various
materials to further reduce velocities behind the retard. This is a site-specific decision,
dependent on the economics of filling versus using a less permeable facing design, and
on the durability required of the filling material. Using local material such as hay or
brush reduces permeability at low cost, but at the expense of durability, and relies on
future deposition and vegetation for permanent velocity reduction. A stone filling
provides permanent toe protection as well as permeability reduction, but requires a
substantial facing to retain the stone, and will add substantially to the cost. Used tires
(perforated to reduce buoyancy) provide an inexpensive and durable filling, if regulations
permit such use. However, undermining or deterioration of the crib may result in an
unsightly redistribution of the tires along downstream river banks, adding environmental
insult to the injury of a failed structure.
(b) Some designs, such as fence-type retards, require that the bottom member be
approximately horizontal. Therefore, some leveling of the streambed along the line of
the structure may be required during construction, which limits the use of these designs
to ephemeral streams and minor scour situations, unless a material such as stone is used
to build up the base. In that case, the stone will also serve as toe protection. Otherwise,
any leveling of the streambed to expedite construction must be considered as being
temporary, lasting only until the first flow event.
(c) Carlson and Dodge (1962) present a method for determining the suitability of jack
retards for a given situation, and for estimating the amount of deposition likely to be
induced by them.
8.1.6 IMPERMEABLE RETARDS
The relative advantages and disadvantages of impermeable retards as compared to
permeable retards were discussed in 8.1.5. Most aspects of materials and structural design
are the same as for impermeable dikes (see 8.1.3). An impermeable retard of stone can be
considered to be a form of longitudinal stone toe, discussed in 7.1.4 and most aspects of
design discussed there are applicable to stone retards.
8.2 OTHER FLOW DEFLECTING METHODS
Structures other than dikes and retards may provide a means of altering hydraulic
conditions in order to resist bank erosion in bends. One of the most intractable problems of
river engineering is posed by the coupled processes of deposition of sediment on point bar
faces and scour in the thalweg of bends. Several approaches have successfully addressed
these coupled processes in some cases. These approaches alter secondary currents so that
sediment transport away from the toe of the bank is reduced. This results in a more uniform
cross-section shape, with shallower thalweg depths and a wider channel at low flow. These
approaches include Iowa vanes, bendway weirs, and sills.