Selection of Site-specific Stabilization Techniques
5.1.2 ADJUSTMENT TO BED SCOUR AND/OR BANK SUBSIDENCE
All else being equal, a stabilization method which has the ability to adjust to scour or
bank subsidence has a significant advantage over those which do not. Completely rigid
methods must be carefully designed and constructed, and perhaps even then supplemented
by flexible materials at critical points. The property of flexibility reaches its ultimate
application in the design of toe protection, discussed in 6.3.
The methods which have this property are adjustable armors, flexible mattresses, and
a few types of dikes and retards, and bendway weirs.
5.1.3 RIVER DEPTHS
The depth of water expected at the site during the construction period has a significant
impact on the range of suitable techniques.
The simplest situation is where a project is to be constructed on a stream which is dry
or nearly dry for long periods, so that the entire structure can be built above water. The range
of feasible techniques is then very broad and requires no further discussion at this point.
A more difficult situation is that of a stream which experiences flow well below
bankfull for long periods, but which has depths of several feet to several meters at the
worksite even during base flow. The range of techniques is still broad, but the selection of
the material for the underwater portion is critical. The optimum technique frequently will be
a "hybrid," involving a design for the subaqueous bank that can be reliably constructed
underwater, with a less expensive design for the upper bank. This approach is compatible
with the requirement that the toe of bank protection works be functional even when scour
occurs, as discussed in 6.3. Examples of this approach are:
Stone fill placed against and parallel to the bank, with the top elevation of the
stone being just above the water surface at the time of construction, with a
less expensive treatment used for protection of the bank above the elevation
of the stone.
Flexible mattress laid from water's edge out into the channel, with a less
expensive treatment protecting the bank slope above the mattress.
The most difficult, or at least the most expensive, situation occurs on large rivers with
depths greater than a few meters at the site even at base flow, and the toe of the underwater
bank slope far out in the stream, beyond the reach of land-based construction equipment.
Two alternative approaches in this situation are to: