interior waves they regenerate are about 3 feet in height, provided that these interior waves 

 do not in turn cause excessive waves in the berthing areas. Moreover, as high overtopping 

 waves will occur infrequently, the slight damage or inconvenience they cause will usually be 

 minor in comparison witli the extra cost of building the breakwater to nonovertopping 

 dimensions. On the other hand, the overtopping waves will subject the breakwater to 

 disruptive forces that would not occur under normal conditions, and the structure must, of 

 course, be designed to withstand such forces. 



If the breakwater is in shallow water, large waves may break before reaching it. Then it is 

 only necessary to design the structure to resist the highest wave that can reach it without 

 breaking. As a first approximation, waves break when the depth of the water is about 1.3 

 times the wave height. For design purposes, the wave should be determined by the method 

 described in the Shore Protection Manual (U.S. Army, Corps of Engineers, Coastal 

 Engineering Research Center, 1973) which indicates that the height of a breaking wave is a 

 function of wave period, slope of the seabed, and the water depth at the structure. The ratio 

 of the water depth to breaking wave height can vary from 0.7 to 1.5 depending on the above 

 factors. 



If the breakwater is in deeper water, records of the measured deepwater waves during the 

 highest wave episode ever recorded may be used to determine the design wave height. If no 

 records are available, wave hindcasts for the site based on recorded wind speeds or analysis 

 of barometric pressure patterns in the wave -generating area can be used. In this event, a 

 general rule for rubble-mound design is to use as the design wave height the significant 

 height of the one-tenth-percent-occurrence wave episode, i.e., will not be exceeded in wave 

 height (for any direction within a 90° sector centered on the perpendicular to the 

 breakwater's axis) more than one-thousandth of the time, or about 9 hours each year. Some 

 displacement of armor units may occur during the exceedance-design wave episodes, but the 

 cost of their replacement is usually small in comparison to the extra initial expense of 

 designing against the highest wave that might occur. 



The integrity of a rubble breakwater is largely dependent on the stability of the stones 

 and armor units of which it is built. Large stones, if placed directly on a soft bottom, may 

 sink and lose their usefulness. This can be avoided by first covering the entire base with 

 filter cloth and then covering the cloth with a bedding layer of spalls or quarry waste to a 

 thickness of about IVa times the average dimension of the average stone in the bedding layer. 

 Filter cloth made of woven monofilament plastic yarns have proved to be the best for 

 breakwaters, jetties, and shore-protective structures (Barrett, 1966; and Calhoun, 1972). 

 Tests have shown that these filter cloths are superior to perforated plastic membranes, cloth 

 made of woven multifilament plastic yarns, and glass fiber filters. Several methods of placing 

 the cloth under water have been devised, each suitable for a specific bottom condition or 

 type of water agitation. The filter manufacturers can furnish information as to methods of 

 placement. 



49 



