2. Evaluation Wave Climate Severity . 



It Is a complex task to describe wave environments in which vegetative 

 stabilization will be effective. The only method available for determining 

 the growth of wind-generated waves in relatively shallow water is empirical 

 (U.S. Army, Corps of Engineers, Coastal Engineering Research Center, 1977). 

 There are many physical and biological variables which must be acknowledged 

 when comparing wave climate to plant survival. The tidal elevation coincident 

 with a particular set of waves, as well as shore geometry and sediment grain 

 size, will greatly influence the stress placed on plantings. Also, the 

 ability of a planted area to withstand wave stress will depend on its growth 

 stage, density, vigor, and overall width. 



Little definitive information is available concerning wave climates in 

 which vegetation is likely to be an effective stabilizer. It is generally 

 held that vegetation will successfully control erosion only in areas which are 

 exposed to low and moderate wave stresses. However, this generalization does 

 not allow a thorough engineering consideration of this alternative on a 

 project-by-project basis. 



In order to establish a criteria for assessing wave climate on an indi- 

 vidual project basis, Knutson, et al. (1981) conducted a survey of 86 marsh 

 planting sites in 12 coastal states. The shoreline characteristics of fetch, 

 shore configuration, and sediment grain size were found to be the most useful 

 indicators of wave climate severity and the likelihood of planting success. 

 Though these parameters are proven indicators of planting potential, other 

 characteristics such as local winds, boat traffic, offshore depth, and tidal 

 currents should also be considered during site evaluation. The following 

 sections describe the principal shore characteristics that influence the 

 growth and decay of waves and the Impact waves have on erosion control 

 plantings . 



a. Fetch . For the coastal engineer, fetch (the horizontal distance the 

 wind blows over water to generate waves) is an important parameter in estimat- 

 ing wave height. The height of a wave formed by a constant wind blowing over 

 water of a constant depth is directly related to fetch length (U.S. Army, 

 Corps of Engineers, Coastal Engineering Research Center, 1977). This rela- 

 tionship is not linear. For example, a constant wind blowing 50 kilometers 

 per hour (13.9 meters per second) over a constant water depth of 6 meters will 

 generate a 15-centimeter wave over a fetch of about 150 meters, and a 30- 

 centimeter wave over 750 meters, a 45-centimeter wave over 2125 meters, and a 

 60-centimeter wave over 4575 meters. As fetch length increases, it has 

 incrementally less influence on wave height. 



Knutson, et al. (1981) found that fetch length was inversely related to 

 successful erosion control (Figs. 12 and 13). Eighty-nine percent of plant- 

 ings exposed to a longest fetch of less than 2.0 kilometers were either suc- 

 cessful or partially successful (no erosion landward but evidence of erosion 

 on the seaward edge of planting). Conversely, 83 percent (five out of six) of 

 the sites with a fetch of more than 18.0 kilometers were failures. 



b. Shore Configuration. Shore configuration (the shape of the shoreline) 

 can influence wave attack on the shoreline. Sites located in narrow coves may 

 be effectively sheltered from waves approaching at oblique angles and will be 



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