Swell and Wind Waves 



The heights and periods of swell 

 and wind waves are used to establish the 

 sea-surface roughness. This is es- 

 pecially important during sound- 

 transmission studies, as these waves 

 reflect and scatter acoustic energy. 



For the measurement of swell, a 

 permanent tower-mounted swell recorder 

 is monitored 15 minutes daily. This 

 provides information on seasonal variation 

 in swell. 



Information on wind waves is pro- 

 vided by a vertically directed acoustic 

 transducer which is mounted in a gimbaled 

 tripod on the sea floor. Tower recordings 

 of the strong sound reflections received 

 from the sea surface delineate all types 

 of waves as well as any midwater reflec- 

 tors. 



The higher surface waves are 

 associated with local and distant storms. 

 The highest wave observed was 18 feet 

 and occurred during a local storm. These 

 waves travel shoreward and are refracted 

 by the shoaling continental shelf. The 

 average swell period is about 12. 5 sec- 

 onds in summer and 11.5 seconds in 

 winter. The average height of significant 

 waves is greater in the summer than in 

 winter. 



To measure the height of swells, a Snod- 

 grass Mark IX pressure sensor is housed 2 

 feet out from the NW tower leg, 37 feet 

 below the mean sea surface. Pressure is 

 electrically transmitted to a specially 

 adapted recorder in the instrument house. 

 The mounting provides a favorite habitat for 

 marine organi sms. 



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