The optimum tether length for these conditions is about one-tenth of the dom- 

 inant wavelength. However, it would be practical on occasion to use tethers 

 that are somewhat longer than optimum in order to have a bottom-resting system 

 instead of a floating system, thus eliminating moorings. Conversion of any 

 floating system to a bottom-resting system would require additional ballasting 

 mass. The reduction of effectiveness due to tethers that are too long is 

 small if the increase in length is small. However, with the floats fixed at 

 about the water surface elevation at low tide, some effectiveness during high 

 tide is lost due to excessive submergence of the floats. In shallow water, 

 where a bottom-resting system with tethers shorter than the deepwater optimum 

 may be necessary, performance may be partially or fully restored by use of 

 cylindrical floats, increased density of the floats, or closer spacing of the 

 floats in the direction normal to the direction of wave propagation. 



The data in Figure 120 (Jones, 1978) summarize the performance of optimum 

 floating systems in fully developed, local wind-generated seas, as represented 

 by the Pierson-Moskowitz spectrum. This figure shows the number of rows 

 required to reduce wave heights to levels associated with sea-state 3, and 

 pertains to 5-foot-diameter spheres, each of which weighs less than 400 pounds 

 (relative density less than 10 percent). These data also pertain to an 

 approximate tether length of 30 feet (±20 percent). Variation within this 

 range has little effect on performance for spectral peak periods between 6 and 

 9 seconds. However, as the tether length varies farther from this range, 

 additional rows of floats would be required to maintain a 7-second breakwater 

 capability. 



A 150-foot length of marina-scale tethered-f loat breakwater was installed 

 in San Diego Bay in 1976 to obtain verification data from a field experiment. 

 The breakwater was subjected to ship and boat wakes generated in the main 

 entrance channel to San Diego Bay, and to limited-fetch wind waves from the 

 south. The breakwater was protected from wind-generated ocean waves because 

 it was located on the lee side of Point Loma. Significant south wind activity 

 was observed at the field site only twice in the 8-month span of the experi- 

 ment. In both instances the wind rose from calm to a maximum of about 22 

 knots within a 2-hour period, with the direction essentially constant from the 

 south. A total of 26 experiments were recorded during the duration of these 

 two storms. These spectra were quite broad, bearing little resemblance to the 

 sharply peaked spectra characteristics of waves generated on the open ocean 

 (Seymour and Hanes, 1979). The wave attenuation aspects of the tethered-f loat 

 breakwater were satisfactorily demonstrated in this limited-fetch application 

 (Fig. 121). Three particular designs of open-ocean tethered-float breakwaters 

 with significant differences in float and ballast properties (two floating 

 systems and a bottom-resting system) were described by Jones (1978). 



a. Concrete Barge-Type (Floating) Ballast Tethered-Float Breakwater. NCEL 

 undertook a conceptual-design study in 1975 to develop a ship-transportable 

 ballast module, and several concepts were investigated for a 5-foot-diameter 

 spherical float system mounted on a ballast structure of reinforced concrete. 

 The type of construction envisioned was similar to that developed for a con- 

 crete landing craft during World War II, although the structural integrity of 

 landing craft may not be required for the breakwater ballast. Each module was 

 designed to carry a four by seven array of floats, with both columns and rows 

 of floats spaced 10 feet apart. Five rows of modules contain 35 rows of 

 floats, almost the number required for a 7-second breakwater. Sets of 15 



174 



