The objectives of this report are to (I) describe the vegetation of the dune 

 community of Louisiana's barrier islands; (2) indicate plant species that may be used for 

 dune stabilization in Louisiana; and (3) qualitatively discuss an attempt to build and 

 stabilize a foredune ridge on Timbalier Island, Louisiana. 



FUNCTION OF STABILIZED DUNES 



How do dunes aid in reducing barrier island erosion? Firstly, coastal dunes provide 

 a reservoir of sand to the beach during storm events. Not only does the dune system 

 nourish the beach during storms, but the building of an offshore storm bar from dune 

 sands has the effect of reducing the slope of the beach and lengthening the surf-swash 

 zone so that the maximum energy dissipation of storm waves is achieved (Leatherman 

 1979a). Both effects tend to reduce erosion of the beachface. Secondly, continuous sand 

 dunes act like levees, retarding overwash and island breaching. Because one of the 

 primary causes of sand loss to an island is due to breaching and subsequent inlet dynamics 

 (Leatherman 1979b), the role of sand dunes in strengthening the island against breaching 

 is very important in controlling the overall erosion of the island. Thirdly, a well- 

 vegetated dune provides a source of vegetation to recolonize overwashed and breached 

 dunes after storms. This vegetation is important in initiating new sand accumulation and 

 dune-building processes. 



Some coastal investigators have questioned the function of barrier dunes during 

 storm conditions. Dolan (1972) maintained that large stabilized dunes are detrimental to 

 the long-term stability of. the barrier system since they were believed to interfere with 

 beach dynamics by (I) constricting the swash zone so that wave energy is dissipated over 

 a narrower area, resulting in increased turbulence and concommitant beach erosion, and 

 (2) functioning as seawalls and thus concentrating wave energy to increase the scour of 

 adjacent sand beaches. Based on this hypothesis for which no hard data were collected, 

 the U. S. National Park Service has argued that dunes are detrimental to the stability of 

 barrier islands, and in some locations should possibly be breached artificially by 

 bulldozers. As Leatherman (1979b) points out, "This management approach is rather 

 startling, considering that dune conservation programs are essentially ubiquitous 

 worldwide." 



Thus, the question exists: Do stabilized barrier dunes increase barrier island 

 erosion? Leatherman (1979a, b, c), who has intensely investigated this question, 

 concluded that Dolan's hypothesis is not substantiated by field measurements or by 

 results from previous research: 



"From laboratory tests and field observations during storm 

 conditions, it has been shown that the barrier dune does not result in 

 steepening of the upper beach foreshore. Instead, the profile continues 

 to flatten asymptotically until a critical minimum value is achieved. 

 Seaward migration and building of the outer storm bar can provide for 

 a wide enough surf-swash zone to achieve maximum energy dissipation 

 and thus define a new equilibrium profile. Dolan's (1972) emphasis on 

 the importance of the subaerial beach profile in energy dissipation and 

 wave reflection neglects the full range of interactions. The presence 

 of a dune line cannot constrict the energy dissipation process since the 

 seaward boundary (storm bar) is not a static feature. 



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