operate on the method of recording pressure fluctuations at the sea bottom 

 and transposing these values to a surface wave height (Figure 2), The 

 fundamental principle of the method is that surface waves induce pressure 

 fluctuations in the entire column of water between the surface and the sea 

 bottom,, For a particular depth of water and wave height, the amplitude of 

 these fluctuations depends on the wave period , These fluctuations are 

 filtered out by hydrodynamic action for the very short period waves , and 

 only the fluctuations for the longer period waves are recorded The in- 

 strument consists simply of an underwater unit to pick up and convert 

 pressure fluctuations into electrical signals which are transmitted through 

 a cable to a strip chart recorder on shore , Two types of units have been 

 employed „ In one of these instruments the pressure fluctuations at the 

 sea bottom actuate a small potentiometer through a system of bellows , and 

 these signals are transmitted to a bridge and recorder unit on shore „ A 

 "slow leak" device in the underwater unit eliminates the effect of tides 

 and other very long waves „ In the second type of instrument the underwater 

 pressure unit consists of an air-filled rubber bellows which contains a 

 thermopile o The changes in pressure due to the passage of a wave causes 

 a change in temperature of the air in the bellows , This change in 

 temperature is related to the wave period and the height of the wave at the 

 surface and is recorded on a instrument located on shore „ Programming 

 switches usually are provided so that the recorders will operate at high 

 speed for 20 minutes in each 6-hour period. This high speed record permits 

 the determination of both height and period of the waves | whereas , the 

 low-speed record permits only height determinations From the high-speed 

 section of the recorder charts the average height of the highest one-third 

 of the waves is determined and recorded as the significant waves occurr- 

 ing at that time (Figure 3)° 



For recording wave characteristics on relatively small bodies of 

 water, such as lakes and bays, the same basic principle of using an 

 underwater unit to screen out the relatively small waves is employed. 

 The details of the units differ due to the relatively small-pressure 

 fluctuations which exist and due to the response characteristics of the 

 recorders. 



In addition to the use of wave recorders , statistical information on 

 wave conditions at a given locality can be assembled from established 

 relationships between the characteristics of the waves and the fetch and 

 generating wind. These relationships were developed by The Scripps 

 Institution of Oceanography of the University of California and have been 

 presented in graphical form. These graphs permit the determination of 

 the height and period of the wave which will result from a wind of given 

 speed and duration blowing over a given fetch. The wave height given by 

 these relationships is the significant wave height as previously mentioned; 

 that is, it is the average height of the highest one-third of all waves 

 present. These curves originally appeared in the U, S„ Navy Hydrographic 

 Office publication, "Wind Waves and Swell, Principles in forecasting", 

 Misc. Report No, 11,275, As additional data on wave conditions were ob- 

 tained from shore recorders the curves were revised and presented in 

 Wave Report No, 73 of the Scripps Institution of Oceanography in March 

 1948, Also see the Bulletin of the Baach Erosion Board, Special Issue No, 

 1, July 1, 1948, 



