VI. STABILITY OF COASTAL STRUCTURES 



by 

 R.Y. Hudson 



1. Introduction . 



The economic design of a stable coastal structure is a difficult prob- 

 lem involving the complex interaction of waves and structure. Waves may 

 be of the short -period type generated by storm winds with periods from 

 about 1 to 25 seconds and heights from about 1 to 40 feet or more; or 

 waves may be of seismic origin (tsunamis), which are long waves with per- 

 iods ranging from about 5 to 35 minutes and heights nearshore up to 30 

 feet or more. The magnitude and distribution of wave pressures on coast- 

 al structures vary with the type and geometry of the structure, the depth 

 of water and bottom configuration immediately seaward of the structure, 

 the stage of tide relative to the crest of the structure at the time the 

 wave action occurs, and the wave dimensions. Since accurate determina- 

 tion of wave forces on other than structures of simple shape in rela- 

 tively deep water cannot be calculated, hydraulic models are commonly 

 used to obtain sufficiently accurate data from which to make engineering 

 decisions. 



This section discusses the design, operation, accuracy, utilization 

 of test data, and costs of hydraulic models, the results of which are 

 used for optimum designs of coastal structures concerning their stability 

 under wave attack. The types of structures considered are (a) rubble- 

 mound breakwaters, jetties, and wave absorbers; (b) vertical -wall break- 

 waters and jetties; (c) composite breakwaters; (d) seawalls; and (e) 

 floating, pneumatic, and hydraulic breakwaters. 



Rubble-mound breakwaters, jetties, and wave absorbers are used exten- 

 sively where the depths are not prohibitive and suitable quarrystone is 

 available locally or within transporting distance at competitive prices. 

 These structures are also used if the foundation materials are such that 

 the stability of a vertical-wall structure would be endangered. Rubble- 

 mound structures can be constructed in stages if the natural foundation 

 material is incapable of supporting the weight of the completed structure 

 without considerable settlement, with enough lapse of time between the 

 stages to allow settlement to take place. Rubble-mound wave absorbers 

 may be used inside harbors to reduce the wave action to acceptable levels. 

 Rubble wave absorbers are similar in construction to rubble breakwaters 

 except that they may have flatter slopes, a thicker cover layer with a 

 maximum of voids, and they are usually backed by a vertical, impervious 

 bulkhead. 



Vertical -wall breakwaters and jetties may be more economical when 

 the foundation material is firm, homogeneous, and not easily scoured. 

 Vertical-wall breakwaters may also be used as additional pier facilities 

 if enough top width is provided and overtopping can be prevented. The 

 foundation is seldom such that a vertical-wall, gravity-type structure 



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