BARRIER ISLAND SEDIMENT MANAGEMENT USING ANTHROPOGENIC SAND DUNES

Anchored sand dunes were created in a run-off channel on Santa Rosa Island, Okaloosa County, FL (N30° 23'14” W086° 49'22”). The installation site is 155 feet by 135 feet and lies at the north end of a run-off channel. The design uses 10 anchored dunes of two geometric shapes that create a tortuous path for floodwater and surface winds to follow. This artificial meandering path attenuates energy of flowing water allowing sediment to drop out of transport in the energy dissipation spaces of the anchored dune field. The new topography allows eolian deposition on the leeward side of the structures. The anchored dune design prevents erosion of sediment contained inside the buried structure. The concept of anchored dunes uses burial of a permeable structure at the core of each dune. Once buried, the anchored dune resembles a normal dune because there is no visible evidence of the buried structure. Only when the burial sediment is removed by erosion would the buried structure be revealed.

In this experiment, heavy gage wire baskets lined with permeable geotextile fabric are filled and buried to create the core of each dune structure. Individual 3'X3'X3' baskets are connected in a 5 unit string to create 3'X15' rectangular units. The geotextile material allows groundwater to flow through the sediment in the buried baskets. The 3'X15' basket units connect and stack together so that several units combine to make a core structure that is 6' high. The entire structure is buried with enough sand to create a 3' thick dune face. Because the tops of the baskets are not closed, vegetation planted on the dune faces will grow their root systems into the basket sediment.

Several advantages are provided by this design. The long fetch of impounded flood water in the run-off channel is eliminated. Sediment eroded by overwash flooding into the run-off channel will flow into spaces designed for energy dissipation. Sediment contained inside the basket structure is conserved. Vegetation rooted in the basket sediment will survive at a higher rate. The permeable buried structure allows natural hydrological conditions to exist below the surface of the dune field. In windy conditions, the topography creates additional areas of differential air pressure that either reduce eolian transport or enhance eolian deposition within the anthropogenic dune field.