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Vortex Sand Dune Fencing
A new sand fence technology was
patented by the University of South Florida. Designed and
developed by Rip Kirby at the USF Coastal Research Lab, this
patent pending design will be field tested during the
2005-06 winter storm season on Santa Rosa Island in NW
Florida.
Dune Fence Research Project -
Field Testing
The research will be conducted on undeveloped property
controlled by Eglin AFB on Santa Rosa Island. Several large
overwash fans were created by Hurricane Ivan in Septement
2004. During research into the post-storm recovery of the
barrier island, it was found that a half meter of vertical
elevation per month was being lost from the surface of the
overwash fans during the winter storm season. This loss was
caused by the erosion of the surface sand by wind, or
aeolian transport. A proposal to place sand fencing on the
overwash fans was presented to Eglin AFB in January 2005.
Final approval for the project was received in July, 2005.
Installation of the fence sections will commence in late
November and be completed by mid-December.
Fence Design
During research into sand dune fencing, the
literature consistently pointed out two things.
First, everyone who installed sand fence used an
industry standard snow fence design that was
adopted for coastal environments. This standard is
typically constructed of No.1 Aspen wooden pickets
(3/8" x 1.5" x 48"), woven with 5 double strands of
13 gauge galvanized wire (see The
Kalinich Fence Company
for details) with a 50% void spacing. Second, in
terms of layout pattern and sand deposition
efficiency, it didn't seem to matter which pattern
was used (---, T, V, W, ///, +++, or ===)
because all the different patterns had very similar
or identical rates of deposition.
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When I examined the mechanism by which the fence worked, it
was obvious that 50% of the available sand in aeolion
transport was passing through the void space and not being
trapped. The reason was that turbulence on the leeward side
of the fence was not very high and that the capability to
generate a vortex that could capture the aeolian sand was
minimal. In addition, the snow fence worked best when the
wind direction was perpendicular to the fence line. This
happens less frequently than predicted by the prevailing
wind pattern since that pattern is an average of the wind
directions.
Therefore, I designed a fence picket that would create a
non-scouring vortex and work in an environment with varying
wind direction. The picket has a triangular cross section
instead of a rectangular cross section. As importantly, the
deployment pattern mimics a natural dune field that evolves
on the surface of an overwash fan...alternating rows of
offset asterick shaped structures 8 feet in diameter with an
8 foot gap between the structures. The next row is offset 8
feet to allow the sand moving through the 8 foot gap in the
upwind row to be captured by this next offset downwind row.
Computer models showed the triangular pickets would create
4-10 times as much turbulence and vortices as the
rectangular pickets. Literature confirmed that multiple
lines of sand fence would cature more sand than a single
line. Thus, a three-dimensional approach to creating a dune
field was invented.
Click HERE
to download a PDF file of this patent pending fence design.
As the fence brochure is updated, the new version will be
placed online for download. Check back for
updates.
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