This is a 41-percent reduction in the incident wave height. The 

 associated rate at which energy will be dissipated against the 

 shoreline will be reduced by 65 percent. In a series of field 

 experiments Knutson, Seelig, and Inskeep (in preparation, 1983) found 

 a modified version of the Dean model useful in predicting wave 

 damping in sloping, natural marshes. They found that under 

 conditions similar to those used in the above example about 64 

 percent of the energy associated with a 15-centimeter wave was 

 dissipated by only 2.5 meters of natural, sloping marsh. 



As the wave energy impacting the shore Is reduced, there is increased 

 potential for sediment deposition and decreased potential for erosion. 

 Woodhouse, Seneca, and Broome (1974) measured sediment deposition resulting 

 from marsh plantings and reported the deposition of 15 to 30 centimeters of 

 sediment along three planted profiles at Snow's Cut, North Carolina, during a 

 30-month period. 



The influence marshes have on waves depends primarily on the width of the 

 marsh ("H" in Dean's equation). The width to which a marsh can extend, under 

 optimal conditions, depends on the geographical area in which the marsh is 

 located and the tidal amplitude and slope of the shoreline. The density of 

 plants within a particular marsh depends on many variables including (1) 

 species, (2) geographical area, (3) elevation zone within the marsh, (4) 

 season, (5) substrate, (6) maturity of the marsh, (7) salinity, and (8) wave 

 climate. 



b. Soil Reinforcement . Though it is empirically evident that plant root 

 systems improve soil stability, there is little experimental evidence on this 

 subject. Gray (1974) summarized findings concerning soil reinforcement with 

 vegetation. He noted that some independent studies have shown that plant 

 roots do significantly increase soil stability (Endo and Tsuruta, 1969; 

 Manbeian, 1973). In these studies the shear strength of vegetated soils was 

 as much as two and three times greater. In addition, the shear strength of 

 soils was higher when the volume fraction or weight density of the root system 

 was greater, 



2. Coastal Salt Marshes of the United States . 



A coastfil marsh is an herbaceous (plants lacking woody stems) plant commu- 

 nity found on the part of the shoreline which is periodically flooded by salt 

 or brackish water. A number of species in the grass family (Poaceae), sedge 

 family iCyperaeea) , and rush family {Juneaoeae) commonly form coastal marshes. 



Coastal marshes occur naturally in the intertidal zone of moderate- to 

 low-energy shorelines along tidal rivers and in bays and estuaries. These 

 marshes may be narrow fringes along steep shorelines, but can extend over wide 

 areas in shallow, gently sloping bays and estuaries. Such lands were exten- 

 sive and widely distributed along the Atlantic, peninsular Florida, gulf, and 

 Pacific coasts of the United States before development by man (Fig. 1). 



There are two major groups of coastal salt marshes in the United States, 

 based on physiographic differences — marshes of the Atlantic, peninsular 

 Florida, and gulf coasts (the eastern region) and those characteristic of the 

 northern and southern Pacific coasts (the western region) . The eastern 



15 



