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Posthurricane survey of experimental dunes on Padre Island, Texas

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Dahl, Bill E.
Cotter, Paul F.
Wester, David B.
Drbal, Doug D.

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Coastal Engineering Research Center (U.S.)
Engineer Research and Development Center (U.S.)



Miscellaneous Report
Abstract: This report summarizes the impact of Hurricane Allen (August 1980) on dune configuration, sand accretion or erosion, and changes in the vegetation on north Padre Island. Four experimental foredunes, the result of grass plantings from 1969 to 1973, and an unplanted control section were monitored in 1975-1977 and also in 1981. The 1981 posthurricane data were compared, where possible, with the previous studies. Foredune elevation surveys were completed in March 1981; accompanying vegetation transects were made in July 1981. Hurricane Allen caused erosion of the dune face of all the experimental dunes, but caused a breach in only one dune. The beach elevations had returned to approximately prehurricane heights by the time the area was resurveyed. The unplanted control dune provided little resistance to waves generated by the storm and a large quantity of sand was deposited inland. During the past 5 years the experimental dunes have accumulated sand at an annual rate of 11.5 cubic meters per meter of beach compared with 9.3 cubic meters per meter of beach for the unplanted control area. The higher annual accumulation rate on the experimental dunes is due to the greater abundance of vegetation. Vegetation on the experimental dunes apparently continues to spread seaward at 1.5 to 1.8 meters per year. The total dune width has expanded 1.8 to 2.4 meters annually since 1976. There has been little invasion of other species into the sea oats (Uniola Paniculata) and bitter panicum (Panicum amarum plantings, even after 8 to 10 years. Landward ground cover of the unplanted control dune decreased from 28 percent in 1976 to 17 percent in 1981 due to sand deposition on existing vegetation. Landward ground cover of experimental dunes increased from 39 percent in 1976 to 56 percent in 1981, because the foredune protected vegetation from storm waves and sand deposition. Also, freshwater ponded behind the foredunes, creating, a favorable habitat for vegetation. The less salt-tolerant plants also benefited from the decreased salt spray landward of the experimental foredunes. Vegetation on the backshore was eliminated during the storm, but rapidly is becoming reestablished from residual perennial grass roots and rhizomes. Foredunes on Padre Island dissipate hurricane-generated waves, thus lessening water damage to the mainland; they are also major sand reservoirs, thereby helping hold newly deposited sand. A large, mid-island, unvegetated dune field has migrated landward 27 meters per year since 1973.


Experimental Dunes, Foredunes, Hurricane Surveys, Padre Island, Texas, Vegetation


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