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problems a more complete description of the sea surface may be required. For 

 example, the study of the motion of a ship on the sea surface requires a three- 

 dimensional description of the surface; the study of transformation of waves 

 from short-crested swells offshore to long-crested breakers on the beach also 

 requires more information than given by a single wave recorder. 



Two basic types of wave recorders are in use today: (1) the surface- 

 contact type wave recorder and (2) the sub-surface pressure type wave recorder. 

 The first type gage has the advantage that the surface elevation is recorded di- 

 rectly, leaving no doubt as to the time history of the surfaces. The surface- 

 contact type gage has the disadvantage of requiring a supporting structure. Sub- 

 surface pressure type gages do not require supporting structures, except for a 

 tripod mount on the bottom, but have the disadvantage that the surface elevation 

 time history cannot be determined exactly. High frequency components of the 

 surface variations are not recorded by pressure type gages, hence some infor- 

 mation is lost. At times, however, it is advantageous not to record the high 

 frequency components since they tend to mask low frequency components, such 

 as storm swells. 



The mechanical design of wave gages varies widely, depending upon ex- 

 act purpose of the gage, the type of recorder with which the gage is to be used, 

 and various preferences of the designer. Brief descriptions of several gages 

 therefore will be given as examples of the many gages that have been, or might 

 be, designed. Of the gages described, the Beach Erosion Board Step Resis- 

 tance Gage, the Woods Hole Shore Wave Recorder (now being built by the Beach 

 Erosion Board) and the University of California Mark IX Shore Wave Recorder 

 are the most extensively used today in the United States. 



Beach Erosion Board Step Resistance Gage - The step resistance gage is com- 

 prised of a series of electrical contact points (modified spark plugs) installed 

 along a sealed pipe at 0.2-foot intervals (Beach Erosion Board, 1948). The 

 spark plugs are connected to a resistance circuit within the pipe. The gage is 

 attached vertically to a supporting structure (such as a pier) with the bottom of 

 the gage below the lowest expected wave trough; the top of the gage must be 

 above the highest wave crest. The normal length of a step resistance gage is 

 25 feet. 



A constant voltage transformer (115-volt AC) supplies power to the gage; 

 its primary is connected through a timing switch to provide automatic programi- 

 ming. Alternating current is supplied to prevent polarization, and is converted 

 (through a selenium bridge rectifier) to a proportional DC current, which in turn 

 drives the recording unit mechanism. A Brush magnetic pen recorder is used, 

 which has a high frequency response and can record the shortest period wave. 



The values of the resistors, connected to the contact points of the gage, 

 are adjusted so that the variation in the current is proportional to the submerged 

 length of the gage. By recording the variation of the gage current, a record of 

 the rise and fall of the sea surface is obtained which includes tide variations as 

 well as wave action. Wind chop and wave form are reproduced accurately, pro- 

 viding the response of the recorder is sufficiently rapid. 



a. Series-type step resistance gage: In this type, the resistors are 

 connected in a series circuit, with the junctions between resistors tied to the 

 contact points. As the sea rises, the water shorts all resistors tied to sub- 

 merged contact points, which causes an increase in current proportional to the 

 number of contacts below the surface. 



This instrument is susceptible to current leakage because of the relative- 



