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SE GSA - Ivan Impact

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.

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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Date Last Modified: 11/11/05
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