shore alinement configuration and construction of offshore breakwaters. These 

 include ecology, safety, esthetics, and breakwater gap currents. Structural 

 aspects such as foundation design, scour protection, cross-section shape, and 

 armor stability and placement are discussed in Chapter 7, Section III. 



a. Ecological Considerations . The design analysis should include an 

 appraisal of the total impact of the project, environmental as well as eco- 

 nomical. Rounsefell (1972) discusses the ecological effects of offshore con- 

 struction, and Thompson (1973) examines the ecological effects of offshore 

 dredging and beach nourishment. Although these studies suggest that offshore 

 breakwaters generally do not cause long-term undesirable ecological changes, 

 each proposed project site is unique and must be examined for a possible 

 negative impact to the ecological system. 



If a double tombolo (or any other shoreline adjustment that traps water) 

 forms, it is possible that the reduced exchange of water will cause the 

 entrapped water to become stagnant. This is more likely to occur in regions 

 of small tidal ranges. Generally offshore breakwaters have adequate circu- 

 lation to prevent accumulation of waterborne pollutants in their lees. 



b. Esthetics . If a breakwater is to be constructed to protect a recrea- 

 tional beach, esthetics should be taken into consideration. For example, 

 bathers usually prefer that their view of the horizon is not obstructed, so 

 this may be a factor in selecting the structure height. However, the 

 effectiveness will be limited as overtopping becomes more common. 



c. F low Through Breakwater Gaps ♦ Of possible concern when sizing off- 

 shore breakwater gaps are return flow currents. These currents occur when the 

 structure is nearly impermeable and low crested, causing the water that passes 

 into its lee by wave overtopping to return only through the gaps or around the 

 ends of the structure. The return flow can become particularly strong if the 

 breakwater is long, has only a few gaps, and has two tombolos that prevent 

 flow around the exterior ends of the structure. These currents can cause 

 severe scour at the ends of each segment, which may result in the partial 

 failure of the breakwater. The strong currents are also a hazard to swimmers. 

 A method for estimating the magnitude of these currents is presented by Seelig 

 and Walton (1980). Return flow currents can be reduced by raising the break- 

 water crest elevation, enlarging the gaps between segments, and increasing 

 structure permeability. 



d. Construction Considerations . Because of the difficulty in quantita- 

 tively predicting shoreline changes associated with segmented offshore break- 

 waters, it may be wise to first build small segments with large gaps and 

 partially close the gaps in response to the shoreline adjustment. In this 

 way the desired protection is eventually attained. If feasible, the expected 

 shoreline adjustment behind the structure should be artificially placed to 

 reduce starvation of the downdrift beach. Beginning construction at the 

 downdrift end of the project will result in a more uniform accretion of the 

 shoreline. 



Construction capability plays a major role in determining the water depth 

 in which the structure is placed. Land-based equipment can operate in depths 

 up to 1 meter, and floating construction vessels usually can operate no closer 

 to shore than the 2-meter (6-foot) contour. Wave activity and tidal range can 



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