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 Surface Armor for Erosion Protection
7.1.3.3 Disadvantages
A windrow also has the same disadvantages as trenchfill. Also, it is rather wasteful
of stone when it is placed on top of the stream bank, because the self-launching process is not
as efficient when the stone must launch down the entire bank height rather than only below
the bottom of a trench excavated to a lower elevation.
7.1.3.4 Typical Applications
Where stone or other suitable windrow material is relatively inexpensive, construction
of a windrow behind the existing bank may be cost-effective, if the simplicity of design and
construction offset the relatively inefficient use of material. As with trenchfill, the key to
efficient performance is a relatively uniform rate of launching at any given point. Therefore,
sites with predominantly non-cohesive bank materials are the most suitable.
Windrow may be appropriate for emergency situations, where urgency overrides cost,
there is limited time for detailed design, and high river stages and velocities prevent normal
construction operations. The site conditions, availability of materials, equipment, and labor,
in practice dictate the design, which must be performed concurrently with mobilization of
resources and the beginning of construction. The approach is to quickly feed into the stream
a resistant material at the critical points, continuing the operation until the crisis passes and
a well-designed, permanent solution can be engineered.
7.1.3.5 Design Considerations
The design of windrow is approached in the same way as trenchfill, except that no
trench design is required. Geotechnical analysis is recommended to determine if the risk of
mass bank failure during or after launching is acceptable, although it is impossible to obtain
the same degree of geotechnical safety with windrow as with more conventional methods, so
that some risk is unavoidable.
Based on laboratory model studies conducted at the U.S. Army Waterways
Experiment Station, a rectangular shape for the windrow was found to be the best windrow
shape (USACE, 1981). This shape supplies an initial surge of stone which counters the
thinning effect of the scour in the toe zone of the forming revetment. The remaining portion
of the windrow then provides as ready supply of stone to produce a uniform paving.
However, this shape does require the excavation of a trench for placement of the stone. The
second best windrow shape was the trapezoidal shape. It has one advantage over the
rectangular shape in that no trench is needed to contain the windrow stone. This shape
supplies a steady supply of stone similar to the rectangular shape. The triangular shape was
the least desirable shape. This shape supplies more stone initially, but the quantity of stone
diminishes as the windrow is undercut.
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