2 . Permeability. Permeable groin structures permit some sand to pass
through the groin, but experience has shown that such structures are
generally ineffective and are difficult to design, operate, and maintain. A
low impermeable groin is preferable when controlled bypassing is desired.
The height of a groin depends on the degree to which it is
3. Height.
desirable for sand to overtop the groin and replenish the downdrift beach.
The minimum height is about the same height as the beach berm height. The
outer sections of the groin generally decrease in height seaward.
Normally,
the outer section elevation is several feet above the beach profile.
Lower
groins; which follow the profile of the existing beach, help stabilize the
native beach sand but impound very little of the longshore transport.
Higher groins, projecting more than several feet above the existing beach
profile, tend to be complete littoral barriers.
Long, high groins that
eliminate bypassing are called terminal groins, and are commonly used at
the downdrift end of beach fills to keep artificially-placed sand from
moving out of the protected area.
Terminal groins can be used only where
erosion of downdrift shorelines will be slight or can be tolerated, or where
they will prevent harmful shoaling of navigation channels by bypassing sand.
Groins of adjustable height, built of panels which could be placed or re-
moved to vary the volume of bypassed sand, have been tried but tilting or
distortion of the panels and frame guides have made panel adjustments dif-
ficult; generally, adjustabiliy has been found to be impractical.
4 . Length. A groin's length must
be sufficient to create the desired beach
profile while allowing the adequate
passage of sand around the groin's outer
end. As discussed in CETN-III-11
(Protective Beaches), a design beach berm
width is chosen which provides the
required protection or recreational area;
and the slope of the beach seaward of the berm is then assumed to parallel
the existing beach. If'groins are used to protect a beach fill, the outer
end of the groin would be placed where the designed beach slope intersects
Since most of the sediment transported along the coast
the existing bottom.
moves within the breaker zone , groins that extend seaward of that zone may
force sand to flow too far offshore to be returned to the downdrift beach.
In special cases groins may be constructed shorter in order to trap a
smaller amount of sand. At the landward end, a groin must be well secured
to the shoreline to prevent flanking (scouring of the area landward of the
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