mat on the lakeside and harbor side. Another possibility of why the sand mat 

 was not seen, if it does exist, may be due to the self-tuning characteristics 

 of the EG&G side-scan sonar. To provide a usable image of the highly reflec- 

 tive rubble structure, the instrument gain may have been turned down to such a 

 degree that the sand mat next to the structure was not visible. To conclu- 

 sively answer the question of the existence of the sand mat would require that 

 a sediment sampling program (preferably cores) be conducted. In fact, limited 

 bottom sampling will provide the necessary ground truth measurements needed to 

 interpret bottom characteristics seen on many side-scan sonar images. 



Crescent City, California, Breakwater 



45. The outer breakwater at Crescent City, California (Figure 25), was 

 rehabilitated in 1974 with 42-ton dolosse. Since that time the structure has 

 experienced damage regularly, due to the severe wave climate. To assist the 

 US Army Engineer District, San Francisco, in evaluating the extent of the 

 damage and as part of the monitoring phase of the "Measurement of Prototype 

 Forces on Dolosse at Crescent City, California," work unit, the Coastal 

 Engineering Research Center (CERC) conducted a series of side-scan sonar 

 surveys. 



46. The first survey was conducted in October 1985. An EG&G digital 

 side-scan sonar, model 260, was used along with a 100-kHz towfish. A 26-ft 

 sport fishing boat was used as the towing vessel, with tow speeds ranging 

 between 3 and 5 knots depending on the range used on the recorder. 



47. Environmental conditions were excellent considering the normal wave 

 climate along the northern California coast (a 0.8-ft swell with a 2-ft wave). 

 The combination of wave direction, size, and vessel size, caused the vessel to 

 slide off the swell resulting in some distortion of the record. An offset 

 distance of approximately 100 ft from the center line of the structure was 

 maintained. 



48. Apparently the high wave energy of this region had swept the bottom 

 fairly clean, leaving a featureless sand veneer. There was little evidence of 

 exposed dolosse pieces or other debris in the record (Figure 26). A large 

 offset in the toe of the structure (point A on both this figure and Figure 27) 

 can be seen at the transition between dolos cover and stone cover. This 

 offset does not correspond to any failure above the waterline; however, there 



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