SHORE STABILIZATION WITH SALT MARSH VEGETATION 



by 

 Paul L, Knutson and W.W. Woodhouse, Jv* 



I. INTRODUCTION 



Shore erosion is a common problem in the bays, sounds, and estuaries of 

 the coastal United States. A wide variety of structures have been developed 

 and used to control this erosion. However, due to environmental objections 

 and economic limitations it is often impractical to use even the most innova- 

 tive of these structures. This is particularly true for relatively low wave 

 energy areas where erosion may be costly but is not yet catastrophic. Low- 

 cost, nonstructural techniques are now available for controlling erosion in 

 salt and brackish water, low wave energy areas of the contiguous United States 

 using native marsh plants. Vegetation, where feasible, is usually lower in 

 cost than structures and may be more effective. 



This report provides comprehensive guidelines on the use of marsh plants 

 to control shore erosion resulting from wind waves and tidal currents. The 

 report has been carefully organized to facilitate its use as a reference 

 document. Each major section addresses a specific facet of project planning, 

 design, or construction; most sections end with a summary of information 

 generally presented in the form of graphs, tables, and matrices. 



II. BACKGROUND 



This section provides a background of information on the subject of using 

 marsh plants to control shore erosion. It discusses the role of marshes in 

 providing stability to the shore, describes natural coastal marshes by region, 

 and provides an introduction to the concept of encouraging marsh establishment 

 to reduce shore erosion. 



1. Role of Marshes in Shore Stabilization . 



Marsh plants perform two functions in abating erosion. First, their 

 aerial parts form a flexible mass which dissipates wave energy. As wave 

 energy is diminished, both the offshore transport and the longshore transport 

 of sediment are reduced. Optimally, dense stands of marsh vegetation can 

 create a depositional environment, causing accretion rather than erosion of 

 the shoreface. Second, many marsh plants form dense root-rhizome mats which 

 add stability to the shore sediment. This protective mat is of particular 

 importance during severe winter storms when the aerial stems provide only 

 limited resistance to the impact of waves. 



a. Wave Attenuation and Sediment Trapping. Wave attenuation in marshes 

 has not been studied extensively. Wayne (1975) measured small waves passing 

 through a smooth cordgrass marsh at Adams Beach, Florida. Dean (1979) gives 

 the following empirical methodology for describing wave dampening in marshes, 

 based on empirical estimates of the fluid drag forces occurring on vertical 

 cylinders and laboratory observation of various arrays of cylinders: 



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