all the armor units were carefully placed at random so that each armor 

 unit within the trench formed by the A- 3 rocks at MLLW on the harbor- 

 side of the structure was placed in such a way as to be nested with a 

 firm support. This procedure had previously been necessary to obtain 

 stability of breakwaters with armor iinits randomly placed over the top 

 and down the back side of the crest of structures and subjected to appre- 

 ciable wave overtopping. The stability of the armor units was observed 

 visually, and the wave heights on the harborside, generated by wave 

 transmission and overtopping, were measured at distances of one-quarter 

 and one-half the wavelength, measured from the centerline of the test 

 section. 



9^ Summary of Test Results . During the stability tests 

 no damage occurred to the test section for any of the 17-second waves. 

 The selected prototype design wave (15-second, 18-foot waves) caused one 

 tribar armor unit to be displaced from its nested position on the break- 

 water crown and deposited on the harborside slope. The 15-second, 21- 

 foot waves caused three additional tribars to be displaced in a similar 

 manner. No damage occurred to the oceanside of the structure during any 

 of the stability tests. The test structure was reconstructed twice and 

 repeat tests were conducted. Results of the repeat tests verified the 

 results of the original tests. The results of the transmission tests 

 (see Fig. 6-26) show that the transmitted wave heights are from about 

 25 to 40 percent of the incident wave heights for waves that do not over- 

 top the structure and about 40 to 52 percent of the incident wave heights 

 for waves that overtop the structure. 



(2) Rubble-Mound Jfttty Head; Breaking Waves (Example of an Un- 

 usual and Extensive Study) . 



California. 



(a) Project . Proposed jetty-head repair, Humboldt Bay, 



(b) Reference . Davidson (1971b). 



(c) Laboratory . WES. 



(d) Test Period . June 1968 to December 1970. 



(e) Problem . Humboldt Bay is located on the California 

 coast about 280 miles north of San Francisco. The harbor elements con- 

 cerned in this model study were the seaward ends (heads) of the 7,500- 

 foot-long north jetty and the 9,000-foot-long south jetty at the entrance 

 to the bay (Fig. 6-27). Design of the repair sections was difficult be- 

 cause of the extremely large breaking waves that attack the structures, 

 the nearly continuous rough seas, and the necessity of using the existing 

 underwater rubble-mound slope of about 1 on 5 (remains of previous jetty 

 heads that had been severely damaged by storm wave action) as the base 



of the design repair sections. The 1 on 5 slope made it impractical to 

 carry the protective cover layer of armor units down to the ocean bottom 

 to obtain a firm toe condition for the armor-unit section because of the 



385 



