Two liighs and two lows during each tidal day can be recognized in curves B and C for 

 Key West, Florida, and Point Townsend, Washington, respectively. The amphtude of 

 consecutive tide waves are very unequal, except for short periods near 15 and 30 January in 

 curve B. Tidal curves of tliis type are called mixed tides. Curve D for St. Petersburg, Florida, 

 is intermediate between the mixed and diurnal types. Two distinct, unequal lows and Iiighs 

 are recognizable on most days, but there are several days when one tide wave vanishes, 

 however, the tide appears to stand for a time at an intermediate value between the daily 

 Iiigh and low, although only one distinct high and low can be identified. Tliis phenomenon 

 is often called a vanishing tide. The tidal range is generally low where tliese tides occur in 

 tlie eastern United States, and there has been some confusion about a proper method for 

 treating tidal datums where a vanishing tide occurs (discussed in Sec. IV). 



The prediction procedure discussed in Section HI will predict the variety of tidal curves 

 shown in Figure 2. The theoretical discussion provides an explanation for this behavior. 

 Discussions of tidal tlieory have sliown that both water level observations and theoretical 

 considerations are essential for satisfactory tide predictions. Water level records, even in the 

 ocean, are affected by phenomena other than the astronomical tide. These other effects 

 must be identified in the records (which also show astronomical tides) and removed from 

 the water level record to define the astronomical tide. Often tliese other phenomena alone 

 are of significant engineering interest. 

 2. Water Level Records. 



Tracings from several U.S. Coast and Geodetic Survey (USC&GS) (now NOS) standard 

 tide gage records are shown in Figure 3. The Portsmouth, Virginia, trace, obtained from a 

 harbor well inland from the open sea, is relatively smooth, but a nontidal perturbation, 

 presumably due to the wind, is indicated by an arrow above tlie curve near 0000, 13 August 

 1955. The trace from Atlantic City, New Jersey, was obtained as a tropical storm passed to 

 the east of the station. The predicted tide has been added for comparison. The tide gage at 

 Atlantic City is located near the end of the Steel Pier in the open ocean. Thus, with an 

 exceptionally open exposure, and in spite of the use of a stilling well, wind waves make a 

 significant contribution to the record. Several small oscillations with periods of 5 to 30 

 minutes are also clearly apparent. These short-period oscillations, but not the wind waves, 

 are prominent in the trace obtained at Little Creek, Virginia. 



Figure 4 presents a different type of tide disturbance. All three of these records include 

 water level disturbances, commorUy called tsunamis, which are produced by undersea 

 earthquakes. Note that the large amphtude tsunamis have periods of less than 1 hour, but 

 the larger storm surges have periods of several hours. Disturbances with periods of 10 to 60 

 minutes and ranges of less than 1 foot may be produced by either tsunamis or 

 meteorological phenomena. 



Figures 3 and 4 were taken from analog records of the tides to show examples of 

 perturbations which actually occur in tide records. Tides, tsunamis, and storm surges cover 



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