protection, and maintenance. On the Atlantic and Gulf coasts of the United 

 States, hurricanes may provide the design criteria. The frequency of 

 occurrence of the design hurricane at any site may range from once in 20 

 years to once in 100 years. On the North Pacific Coast of the United States, 

 the weather pattern is more uniform and severe storms are likely each year. 

 The use of H s as a design height under these conditions could result in 

 significant annual damage due to a frequency and duration of waves greater 

 than H s in the spectrum. Higher wave heights such as H 10 or H 5 may be 

 advisable to reduce maintenance costs {Shore Protection Manual 1984). 



Structural Stability 



Structural stability analyses are performed to determine required armor 

 units or to predict expected level of damage that will occur for a given 

 structure exposed to selected design wave and water level conditions. 

 Structural stability can be divided into two types: static and dynamic. 

 Conventional breakwaters have been designed to remain statically stable or 

 allow zero damage to rigid and semirigid structures and less than 5 percent 

 damage to rubble-mound structures for wave conditions not exceeding design 

 conditions. Recent efforts (Ahrens 1987,1989; Van der Meer 1990, 1991; 

 Sheppard and Hearn 1989) have focused on the design of dynamically stable 

 structures such as reef breakwaters where initial crest heights are allowed to 

 be reshaped due to wave attack. The stability of such structures is measured 

 in terms of reduction in crest height due to wave attack. 



The stability of a rubble-mound structure can be influenced by a number of 

 parameters including wave and water level conditions, armor characteristics 

 (size, shape, placement methods, etc.), crest elevation and width, structure 

 slope, and overall structure permeability. A number of dimensionless 

 parameters including the stability coefficient, stability number, and spectral 

 stability number have been developed by various researchers (Shore Protection 

 Manual 1984; Ahrens 1984,1987) to incorporate the influence of 

 environmental variables and structural design parameters into a single 

 parameter. Such parameters are useful in analyzing the influence of each 

 variable on the overall stability of the structure. 



Stabilities of three different types of rubble-mound breakwaters are 

 presented to aid in the design of a nearshore breakwater for shoreline 

 protection. The three types are defined as conventional, statically stable low- 

 crested, and dynamically stable reef breakwaters. Methods are presented for 

 each structure type to assess the structure's stability and determine stone 

 dimensions and crest elevations required to provide a stable structure. 



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Chapter 4 Structural Design Guidance 



