SHORE EROSION CONTROL WITH SALT MARSH VEGETATION 



by 

 Paul L. Knutson and Margaret R. Inskeep 



I. INTRODUCTION 



Erosion in salt and brackish water areas of the contiguous United States 

 can be controlled either structurally or with recently developed nonstructural 

 techniques using native marsh plants. Vegetation, where feasible, is usually 

 more cost-effective than structures built to control erosion. This report 

 provides a method for determining site suitability, establishes guidelines for 

 planting marshes to control erosion, and compares the costs of vegetation to 

 structural methods. 



II. BACKGROUND 



Coastal marshes are herbaceous plant communities along shorelines 

 periodically flooded by salt or brackish water. Vulnerability to wave attack 

 during early stages of establishment prevents the natural invasion of marshes 

 along many shorelines. Even mature natural marshes may suffer permanent 

 damage from severe storms. A common form of damage is the formation of a 

 scarp or bank on the seaward edge of the marsh. Once a scarp is formed it 

 becomes a focal point of continued erosion. 



Both natural and planted marshes (Fig. 1) function in three ways to reduce 

 shore erosion: (1) wave attenuation, (2) sediment capture, and (3) sediment 

 stabilization. Both wave attenuation and sediment capture are directly 

 related to stem density and marsh width (landward to seaward) (Dean, 1978). 

 Stem density is dependent on many variables, including (1) species, (2) 

 geographical area, (3) elevational zone within the marsh, (4) season, (5) 

 maturity of the marsh, and (6) wave climate. Marsh width is influenced by 

 shore slope and tidal range. During the winter when the aerial stems provide 

 only limited resistance to wave energy, the sediment stabilizing function of 

 the plant roots becomes critical. Root mass may increase the shear strength 

 of soils by a factor of 2 or 3 (Gray, 1974). 



Planted marshes can often succeed on shorelines where natural processes 

 have failed to provide plant cover. These marshes, like natural marshes 

 proceed through a cycle which includes a period of establishment, a period of 

 stability, and a period of erosion, where wave-induced erosion is a factor 

 (Knutson, et al., 1981). The functional life of planted marshes will be 

 shorter in areas where waves are more severe. 



III. DETERMINING SITE SUITABILITY 

 1. Evaluating Wave Climate . 



a. Indicators of Wave Severity . In brackish and salt water tidal areas, 

 wave climate severity has a major influence on marsh establishment. Three 

 shoreline characteristics — fetch, shore configuration, and sediment grain 

 size — are useful indicators of wave climate severity and planting success 

 (Knutson, et al. , 1981). Fetah, the distance the wind blows over water to 

 generate waves, is inversely related to successful erosion control. Shore 



