SS eS 
surface wave records. The records differ, however, in that the 
short period waves are not registered to the same degree as the 
long period waves by pressure recorders due to the hydrodynamic 
pressure attenuation of the water. As a result, many of the 
shorter period waves may not appear on the pressure record. 
"If the technique of measuring the periods of only the 
larger, well-defined waves of the record is followed (as de- 
scribed in the above section), the measured period will be 
approximately the same as would be obtained if the record were 
made with a surface type gage. For locations on the exposed 
coast, the short period waves, not recorded by pressure, generally 
are generated by local wind. Irregular and of small amplitude, 
these waves are neglected in the analysis of surface records. 
"In several cases, attempts have been made to utilize the 
hydrodynamic attenuation of short period waves by installing 
gages in deep water (about 600 feet) so that only the waves of 
long periods (the characteristic forerunners of storms) will 
be recorded. These long period waves are recorded by pressure 
heads installed in shallow water, but are "lost" in the record 
of shorter period waves. Installations of this type of instru- 
ment have been made, but due to instrument difficulties no 
satisfactory records have been obtained. 
"To obtain the surface wave heights from the pressure 
record, two factors are required; (1) the calibration of the 
instrument and (2) the pressure response factor relating the 
subsurface pressure fluctuations to the surface wave. Thus, 
hie 
H = wave height at the surface (in feet); 
Cy = calibration factor of the instrument (expressed 
in feet of water pressure variation per chart 
division); 
K = pressure response factor based on the depth of 
the instrument, the depth of the water and the 
length (or period) of the wave being recorded; 
R; = reading of the instrument; 
the following equation is used to obtain the surface wave height: 
H = C4/K (Ry) e e e e e e e e e e e e (1) 
"The calibration factor for most instruments in use today 
is a constant independent of wave period and depth of the 
instrument. The instrument provides a record of the pressure 
variations at the instrument which is accurate in amplitude 
and wave form. 
56 
